4.0L Jeep Valve Spring Shims and Leakdown Testing

[SomeBuckaroo]

What is the proper valve spring shim OD / ID for an all-stock '98 TJ 4.0L (0630 head)? I believe the stock springs are approx 1.390 OD / 0.970 ID. The closest shims I can find are Comp Cams 4736-16 at 1.250 OD / 0.814 ID; I'm not sure if that ID will clear the valve seal boss. I could disassemble and measure, but I'd rather ask here first so I can get the proper shim ahead of time. Thanks!

[Moses Ludel]

SomeBuckaroo...If you had exact dimensions, including the I.D. for guide boss clearance, I would suggest the reman industry supplier Dura-Bond.  They have shims by size, including the thickness, which is important for establishing your spring pressures or free lengths.  Here's access to the Dura-Bond catalog: 

https://www.dura-bondbearing.com/

Finding dimensions is difficult.  I drilled down further and dusted off my Pioneer Automotive catalog. They show a "Jeep" (not by head casting number) shim size listing at 1.360" (O.D.) x 0.640" (I.D.).  Like you, I'm wary of the I.D. measurement due to the various valve guide boss designs and sizes of stock machined casting guides and replacement valve guides.

If there is a shoulder boss, the I.D. will likely be larger than this 0.640".  Regardless, the Pioneer Type A-200 is 0.060" height for springs "in service" (meaning reused).  Type B-200 is 0.030" for new valve springs when there is wear at the OE cylinder head spring seat.  Type C-200HP is 0.015", a "balancing" spring for establishing correct spring pressures and heights at smaller increments.  You would need a spring pressure tester for detailed shimming.  Spring standing height is a reasonable approach for a non-racing engine.  the A-200 full part number is Pioneer A-200-RS-500.

What is apparent within the reman industry is the use of larger I.D. shims that are more "universal".  The shim is centered by the spring's pocket/seat in the head.  The I.D., as you note, needs to be sufficiently inboard of the spring I.D. while the O.D. centers the shim.  (You need a reasonable pocket in the head casting for the shim to seat.) I prefer your approach, which is to have the shim I.D. close to the guide boss O.D.  Summit Racing and others carry Pioneer shims in various sizes and thicknesses.  Dura-Bond shims can be sourced by the exact sizing you want.  Neither Dura-Bond nor Pioneer lists shims by engine application. For either brand, you do need to know the guide boss diameter where the shim(s) seat.

If you're striving to restore spring pressure, be careful not to compress the spring length too much, or you could create spring/coil bind.  Free length of your OE springs is approximately 1.876".  Valve closed pressure of 61-69 lbf. is at 1.64" spring length/height.  Valve open spring tension jumps to 184-196 lbf at a 1.216" spring length.  I would measure the installed spring lengths and adjust shims to reach these specifications.  If you are running a high lift camshaft, be clear about the valve lift and whether it will work with stock length/height springs. You may need the aftermarket "kit" that includes valve springs.

The OE spring official inside diameter of the coil is 0.827" to 0.847".  I would take the time to measure the actual spring seat O.D. and guide boss or guide shoulder diameter, depending upon how the shim will seat in your 0630 head. You may need both the "A" type shim plus thinner "C" type shims if the valve seats are cut deep or the valves require much refacing.  Either would raise the valve stem and keeper height. 

I'm not clear whether you are doing a valve "clean-up" and polish or actually cutting new 3-angle seats.  Are you replacing valves? For a simple decarbonizing clean-up and valve/seat lapping with minimal seat material removal, 0.030" shimming may be enough if the valve faces are in top condition and spring free length is close to OE specification.  In any case, this requires measuring the spring lengths, so ordering the shims in advance might not work well.

If you need a small quantity of shims, either Summit Racing or a local automotive machine shop would be sources.  Shops have boxes of assorted spring shims on hand and, if accommodating, could sell you the number of correct size and thickness shims that you need.  Make sure the O.D. fits just within the pocket and beyond the spring diameter.  The I.D. needs to set sufficiently inside the spring coil for stability.  The shims should not move excessively service, which would prematurely wear into the head pocket.

Looking forward to your findings...

Moses   

 

 

 

 

[SomeBuckaroo]

Thank you for the detailed response! I'll share my story to give a picture of my situation:

I am "fine-tuning" the 4.0L that I rebuilt two years ago. The original had over 300k miles. Compression and leak-down showed approx 20% worst-case difference. The symptoms that annoyed me most were a slightly rough idle (slight miss) that had developed over the years, valvetrain noise, what sounded like an exhaust leak (even though extensive shop-vac & soap-spray tests showed no leaks), and the recent development of a low-RPM ping (e.g. audible in 1st gear in a parking deck). This is not my first rodeo, but rather my 2nd. I blew & replaced the headgasket in my XR4Ti 20 years ago (with marginal success). My Jeep's engine, on the bleeding-edge of 1960s technology, is considerably less complicated. How hard could it be? 🙂

I found a deal on a pristine 30-over '92 short block that had been sitting, assembled, for a decade. It lacked head alignment dowel-holes but otherwise directly fit my '98 TJ. I had my original head serviced at a small-town shop. He decked 10 thou and cut the valves & seats. The valve work was done with air tools that looked 50+ years old. I got the feeling his specialty was farm engines.

I followed your advice and went with a 252H cam, from Comp Cams, including lifters. I degree'd the cam to verify my new timing chain (Cloyes 9-3127-5) was installed correctly. I measured piston, cylinder, gasket (Fel-Pro 530 SD), and head volumes for a static CR of 8.8 (I lucked out: the reman block and pistons had been decked for a standard build, vs high CR). I followed your lifter pre-load posts and calculated 0.040 rocker shims would make up for the material decked - and then verified preload at approx 0.050 with shims installed. The rocker ratio and pushrod angle sure does make a difficult job of "calculating" preload. I lucked out there too, as rocker shims don't seem to come in many flavors of thickness. I installed new rockers but used original push-rods, retainers, keepers, and valve springs.

I put the engine back in the truck, turned the key, and the thing started right up. Owing to that 252H cam (and the fresh block), torque is way up as evidenced by use of 4th gear in my commute where I previously had to use 3rd. HOWEVER, the slightly-rough idle, valvetrain noise, mysterious "exhaust leak", and (worryingly) the low-RPM ping, all remained! I thoroughly pursued the usual suspects: vacuum leaks, plugs/wires/cap, all sensors & ECU (swapped from known-good donor), etc - to no avail. I even viewed sensor, injector, and ignition waveforms on my 4-channel 'scope, without finding any clear smoking gun (to my untrained eye).

That brings me to my most-recent effort to find the root cause of my annoyances. I speculated that my original valve springs were tired, so I replaced them with new OE-spec parts from Melling. That got rid of the low-RPM ping for good, and reduced valvetrain noise - but only for several days. The rough idle and exhaust noises persist. Based on that evidence, I further speculated that a combination of 300k miles and Bubba's Finest-Cheapest head work caused valve-seat recession, excessive spring installed-height, and thus too low valve seated-force. From what I read (correct me if wrong), the usual/cheap/easy tools provide only a relative measure of seated-force across the valves. Expensive setups are required to measure absolute seated force with accuracy and precision.

Instead of seated-force, I decided to measure spring installed-height, using a tool that fits in place of the spring. I transferred each measurement to a mic as I did not trust the cheap tool's scale. I found the installed-heights to be generally about 0.020 over stock 1.640 spec, with several at 0.030 and #1 exhaust at 0.053 over! My math shows 0.053 over translates to approx 15% too low of seated-force, far outside spec. Incidentally, the sources I found show stock seated-force for a '98 at 70-79lbs, vs the 61-69lbs figure you quoted. And yes, I had the springs out but neglected to measure spring pocket dimensions (it's my 2nd rodeo).

At this time, my theory is that low valve seated-force (especially on #1 exhaust) can be overcome, under some circumstances, by lifter pump-up - resulting in a valve that does not close fully, or not at the intended point. However, I don't know how sensitive my 4.0L's valvetrain is to valve seated-force, and if the symptoms I'm observing are reasonable indicators of a problem in that area. With the holidays approaching, I'll have time to pull a valve spring and measure the pocket dimensions. I'll post my measurements here to benefit the next cowboy. And thank you for your recommendation on shims.

Thanks for listening!

 

 

 

 

 

 

 

[Moses Ludel]

SomeBuckaroo...I am listening with great interest.  You have taken a thorough and thoughtful approach to this project!  To begin, my specifications for valve spring tensions came straight out of the 1998 Mopar™ Jeep® Wrangler Service Manual.  I'll share here before replying to your comments and observations below.  I use red highlighting to make it easier to follow our exchange:

This set of specs is also found in the XJ Cherokee manual for 1998.  Unless there is an FSM discrepancy worth noting, either these specs or yours should work as long as they  allow for full valve spring compression without spring bind.

Let's go through your experience and the legacy of this project.  The end game is to see if the trouble symptoms can be eliminated:

On 12/6/2022 at 8:08 AM, SomeBuckaroo said:

Thank you for the detailed response! I'll share my story to give a picture of my situation:

I am "fine-tuning" the 4.0L that I rebuilt two years ago. The original had over 300k miles. Compression and leak-down showed approx 20% worst-case difference. The symptoms that annoyed me most were a slightly rough idle (slight miss) that had developed over the years, valvetrain noise, what sounded like an exhaust leak (even though extensive shop-vac & soap-spray tests showed no leaks), and the recent development of a low-RPM ping (e.g. audible in 1st gear in a parking deck). This is not my first rodeo, but rather my 2nd. I blew & replaced the headgasket in my XR4Ti 20 years ago (with marginal success). My Jeep's engine, on the bleeding-edge of 1960s technology, is considerably less complicated. How hard could it be?

Yes, you wisely picked a 1964 engine design with modern EFI that is rugged and lower tech, a good place to expand your troubleshooting skills.  AMC's seven-main bearing, cast iron OHV 232 and 258 engines were workhorses that withstood punishment in I-H and Jeep trucks.  Some estimates suggest there were 5-million 4.0L derivative engines built between 1987 and 2006. An OHV inline six with 12 valves, pushrods and hydraulic lifters is a solid learning tool.

A maximum of 20% is acceptable for leakdown percentages.  The cranking compression should be no more than 10% difference between the highest and lowest cylinder readings.  These days, a starter motor/amperage draw "compression test" with your 4-channel scope is useful and quick—and even more valuable if you test the cranking compression of #1 cylinder and use that as a reference baseline.

I found a deal on a pristine 30-over '92 short block that had been sitting, assembled, for a decade. It lacked head alignment dowel-holes but otherwise directly fit my '98 TJ. I had my original head serviced at a small-town shop. He decked 10 thou and cut the valves & seats. The valve work was done with air tools that looked 50+ years old. I got the feeling his specialty was farm engines.

Age of the equipment is less important than the actual work strategy and finished product.  More on that down below...

I followed your advice and went with a 252H cam, from Comp Cams, including lifters. I degree'd the cam to verify my new timing chain (Cloyes 9-3127-5) was installed correctly. I measured piston, cylinder, gasket (Fel-Pro 530 SD), and head volumes for a static CR of 8.8 (I lucked out: the reman block and pistons had been decked for a standard build, vs high CR). I followed your lifter pre-load posts and calculated 0.040 rocker shims would make up for the material decked - and then verified preload at approx 0.050 with shims installed. The rocker ratio and pushrod angle sure does make a difficult job of "calculating" preload. I lucked out there too, as rocker shims don't seem to come in many flavors of thickness. I installed new rockers but used original push-rods, retainers, keepers, and valve springs.

Compression ratio is reasonable, and 0.050" preload on the lifters is within range...

I put the engine back in the truck, turned the key, and the thing started right up. Owing to that 252H cam (and the fresh block), torque is way up as evidenced by use of 4th gear in my commute where I previously had to use 3rd. HOWEVER, the slightly-rough idle, valvetrain noise, mysterious "exhaust leak", and (worryingly) the low-RPM ping, all remained! I thoroughly pursued the usual suspects: vacuum leaks, plugs/wires/cap, all sensors & ECU (swapped from known-good donor), etc - to no avail. I even viewed sensor, injector, and ignition waveforms on my 4-channel 'scope, without finding any clear smoking gun (to my untrained eye).

Here are some questions that would help clarify your troubleshooting progress:

1)  Have you pinpointed the valvetrain noise with a stethoscope, sounding tube or Steelman ChassisEar?  Did you install the camshaft thrust bolt, spring and pin during the camshaft installation?  This keeps the camshaft from walking forward, which can be an issue with these engines.  No longer an OE part, the pieces were Mopar Camshaft Sprocket Bolt Kit #83502890.  There is also the rubber timing cover chain slipper on engines through 1998.  Did your timing cover and valvetrain have these pieces?  This is the thrust bolt/spring/washer:

2)  How many miles are on the refreshed engine now?  Did you allow time for the PCM to "relearn" before the trouble symptoms returned? 

3)  Have you listened closely (with a hollow tube, stethoscope or ChassisEar) for the exhaust leak noise?  Is the noise pinpointed to a specific area?

4)  How old are the O2 sensor and injectors?  Are your injectors the original Mopar units?  Have you checked the fuel trim at an idle while the symptoms appear?  Have you followed the fuel trim on this engine with a scan tool or live data stream at various engine speeds? 

Point of interest:  I bought an injector cleaning machine to test flow and performance of my '99 XJ's injectors at higher mileage.  Installing new injector filters and ultrasonic cleaning the injectors made a significant difference.  Clean, flow tested OEM injectors perform much better than inexpensive offshore replacement injectors that are often mismatched.  For details on my tune-up and fuel injector service:  https://4wdmechanix.com/jeep-4-0l-ignition-tune-up-and-injector-cleaning/.

5)  TPS voltage, MAP and other sensors checked okay with a scan tool?  What test method?  The TPS has been replaced twice on our XJ in 191K miles.  The upstream O2 sensor is a perishable as well, also replaced twice.  The idle air control valve is another perishable that you need to remove and inspect.  They get sooty and create idle and other issues.  There are inexpensive offshore replacements at Amazon.  Read the reviews.

6)  Was the oil pan on the engine when you acquired it?  Were you at any point able to measure the piston-to-cylinder wall clearance?  I'm not suggesting that you dive that deeply unless necessary, but piston skirt clearance is an issue on Jeep 4.0L and 2.5L engines with coated skirts. 

7)  Pistons were new, but the wall clearance, piston skirt clearance and piston pin fit would be impossible to assess without dropping the pan and removing the head.  Checking piston pin fit, rod truing and whether the bores are square to the crankshaft's centerline would require removal of the head, rods and pistons.  If your engine was a higher caliber "reman", this should all have been done properly during the machining and fitting of parts.  Did you get any history on the origins of the reman short engine?  The 1991-95 blocks did not have the stiffer 'NVH' block webbing at the main areas nor did they have the NVH girdle between main caps.  You lost that when your 1998 block was not reused.  Some pre-'96 blocks allegedly had detrimental core shift (not toward the thrust side of the cylinders), which would make the engine noisier—especially with a rebore.  When building a 4.0L block from the 1991-95 era, I would sonic test the block's cylinder walls before and after boring.  But let's not leap to this kind of trouble yet.  0.030" is a reasonable bore size.

😎 Given the usual block decking, head surfacing (0.010" as you note), valve seat depths in the head, valve face machining and margins plus the valve stem lengths, you were very wise to consider valve spring fitted lengths!  At this point, that's the only way to measure the valve keeper height above the valve spring seats.  Whether the valve stem tips were machined or not, you compensated for valve stem height when you adjusted the lifter preloads with rocker arm shims—presumably for each of the 12 lifters, right?  Customarily, valve lifter preload is set by correct pushrod lengths.  Your shim approach could raise concerns about the rocker arm contact angles or binding at the valve stem tips—worth checking but not likely an issue.  Make sure the rocker arm contact points align correctly with the valve stem tips.  Also measure between the valve keepers and valve stem tips to see whether the stem tips were machined, which shortens that distance.  Valve stem tip grinding is usually apparent.

That brings me to my most-recent effort to find the root cause of my annoyances. I speculated that my original valve springs were tired, so I replaced them with new OE-spec parts from Melling. That got rid of the low-RPM ping for good, and reduced valvetrain noise - but only for several days. The rough idle and exhaust noises persist. Based on that evidence, I further speculated that a combination of 300k miles and Bubba's Finest-Cheapest head work caused valve-seat recession, excessive spring installed-height, and thus too low valve seated-force. From what I read (correct me if wrong), the usual/cheap/easy tools provide only a relative measure of seated-force across the valves. Expensive setups are required to measure absolute seated force with accuracy and precision.

Your conclusions are each possible.  Good that you're working with new Melling valve springs.  Once you establish correct installed lengths, you should have confidence that the valves are seating properly.  What does concern me, based on your scepticism about the valve job, is that valve guide clearance may be incorrect.  (If guides were knurled, very old school, they will not hold up.  If guides were not serviced at all, there could be excessive guide wear.  When there is too much stem clearance, the valves will seat erratically.  Loose guides can also be noisy and cause knocking that sounds like the rocker/valvetrain area or possibly a ping-like noise at the combustion chamber.  This is not true "ping", knock or detonation, which would be the piston rocking and vibrating.

Traditionally, Jeep engines with cast-in guides required oversized valve stems and reaming the guides to correct for wear.  For decades now, the aftermarket and common machine shop solution for guide wear is silicon-bronze guide liners that require machining the original cast-in guide and pressing the silicon-bronze guide liner sleeve into place.  Finish ball driving is required for an exact fit.  The ball driver sizes the guide liner bore while also securing the guide liner in place.  Finished size is for a stock diameter valve stem.

I'm not sure whether you ran the engine long enough to create valve seat recession.  If the original valves were reused, you'd have to see 1) how well the seats are centered on the valve faces, 2) whether three angles were cut to narrow the seat to proper width, and 3) the thickness of valve head outer margins if old valves were ground and reused.  The seat valve contact widths and centering would be the main concerns here. 

Your description of the spring installed heights points to a more accessible problem worth pursuing.  As you note, the valves may not be seating with enough spring tension/pressure.  A simple vacuum gauge test at the intake manifold can show guide wear and valve seating issues, including weak valve springs.  I watch for a wavering gauge needle.  This is a test to run before you perform any other work.

After a manifold vacuum test, here's what I would do without removing the cylinder head:

1)  Remove the coil lead at the distributor cap.  Do not crank the engine with the starter motor.  Rotate the crankshaft by hand at the crankshaft damper bolt.  Bring #1 piston to TDC on its compression stroke.  (Work on only one cylinder at a time.)  With the piston at or near TDC, a valve(s) cannot accidentally drop into the cylinder.

2) Use an air-hold at the spark plug hole with your compressor air line pressure keeping the valves in place.  With the two valves closed, this should be easy to accomplish.  Now you can loosen and remove the two rocker arms for the #1 cylinder.  The valves will stay in position with the air pressure applied in the cylinder.  (Sometimes the piston will drop in the cylinder if the piston was on either side of TDC.  Air should still keep the valves in place.) 

Note:  You can buy an inexpensive air hold fitting at Amazon, Summit Racing, etc.  Here is an example:

3) Use an over-the-top valve spring compressor to carefully remove the two valve springs.  This is an OTC 4573, there are other compressors like this:

 

4)  The valve stem seals will likely keep the valves in place.  You can use clothes pins or soft-jawed small clamps on the valve stems as a backup.  (You should not need to fuss with the stem seals unless they look damaged.)  Temporarily disconnect or turn off the air supply to the air hold.  This will release seating pressure on valves.  Drop each valve down slightly to unseat it, and wiggle or measure for valve guide wear.  (If you have a magnetic stand and dial indicator, you can take an accurate lateral stem movement reading.)  If stem movement is excessive (well beyond the 0.003" FSM maximum; my limit would be 0.0025"), this could be your valvetrain noise.  It could also cause rough valve seating and an erratic idle.

5)  If valve guides are okay and not the source of trouble, re-apply the air hold pressure to hold the valves in position.  Install the shims to establish the correct closed valve spring lengths/heights then install the valve springs, retainers and keepers.  Be careful not to damage the stem seals...Now the spring tension is correct.

6)  With #1 piston still at TDC and both valves seated (rockers not re-installed), run a cylinder leakdown test with the new spring heights.  (If leakdown percentage looks high, there may be loosened carbon at the valve face.  You may need to install the rocker arms and rotate the crankshaft to unseat the valves with the pressurized air applied.  The air will clear debris.)  I would like to see 12% leak on a fresh engine but would consider an engine still tune worthy up to 20-25% leakage.  Uniform leak percentages are desirable here.

7)  With #1 cylinder's rocker arms back in place securely, rotate the crankshaft until the next cylinder's piston is at TDC on its compression stroke.  (You may need to view the ignition distributor rotor angle with the cap removed to find TDC on the compression stroke for each cylinder.)  Repeat the sequenced steps performed at #1 cylinder.  Once completed, you can move to the next cylinder(s) and perform the same steps.  It may be easier to do this work in the firing order for the cylinders.  If so, the cylinder sequence will be 1-5-3-6-2-4.

Instead of seated-force, I decided to measure spring installed-height, using a tool that fits in place of the spring. I transferred each measurement to a mic as I did not trust the cheap tool's scale. I found the installed-heights to be generally about 0.020 over stock 1.640 spec, with several at 0.030 and #1 exhaust at 0.053 over! My math shows 0.053 over translates to approx 15% too low of seated-force, far outside spec. Incidentally, the sources I found show stock seated-force for a '98 at 70-79lbs, vs the 61-69lbs figure you quoted. And yes, I had the springs out but neglected to measure spring pocket dimensions (it's my 2nd rodeo).

The current installed spring lengths/heights raise some concern.  Usually the symptom would be valve float at higher rpm, but idle seat pressure could have an effect, too.  (Again, demystify this with a simple vacuum gauge test at the intake manifold with the engine idling.  Listen for roughness that coincides with needle waver.)  From there, rule out whether the valve guides are loose.  If the valve work was with old valves and slight stem wear, with either no guide work or the knurling of cast-in guides, the valve guides could be a problem.  It comes down to valve seal and seating at the speeds you hear noise or experience engine roughness.

Note:  If you have the machine shop invoice, look down the parts list for valve guide liners.  Also see whether the shop installed new valves or refinished the original valves.  Sometimes a shop will install new exhaust valves and refinish the intakes.  Sometimes they do all new valves.  Other times, they use no new valves.  Major reman shops use all new valves.  Volume cost for 4.0L valves makes it cheaper to replace the full set rather than waste labor time refacing old valves.

At this time, my theory is that low valve seated-force (especially on #1 exhaust) can be overcome, under some circumstances, by lifter pump-up - resulting in a valve that does not close fully, or not at the intended point. However, I don't know how sensitive my 4.0L's valvetrain is to valve seated-force, and if the symptoms I'm observing are reasonable indicators of a problem in that area. With the holidays approaching, I'll have time to pull a valve spring and measure the pocket dimensions. I'll post my measurements here to benefit the next cowboy. And thank you for your recommendation on shims.

Again, I'm not fixed on the spring pressures though they need to be corrected while you're inspecting for guide wear...From your comment on pocket dimensions, are you already using an air hold and topside valve spring compressor?

Check the valve guides for wear and also note whether the machinist installed silicon bronze guide inserts.  If not, the cast guides were either not serviced or may have been "old school" knurled.  Knurling rolls a spiral down the original cast guide that raises the material and essentially shrinks the bore diameter.  Once sized, the rolled spiral has only half the valve stem contact surface.  Knurled guides, for this reason, wear or deteriorate much faster than the original guide surface or a silicon bronze guide liner.  To your point, the practice is still used on slow rpm engines like older tractors.  Review the invoice.

Some footnotes and takeaways:  If you installed the thrust pin and spring at the timing sprocket bolt, I would rule out camshaft fore/aft walk (another knock cause) or lobe-to-lifter wear issues, especially with a new cam, lifters and timing set if properly lubricated during installation.  The concern around piston skirt clearance cannot be verified without dropping the pan.  If you're confident about the short-block's history, originality and build quality, the piston fit and bore machining should not be an issue.  0.030" oversize is standard for the reman industry, it's the common, least expensive replacement piston size.  Even with a core-shifted or poorly webbed block and no girdle, noise should not be at the "warranty claim" level.  The bore size would be unlikely to exaggerate the OEM noise level.  This short engine, by nature, is noisier but not to the excess you have experienced...Systematically separate the mechanical issues from electronic fuel-and-spark management related possibilities.

Thanks for listening!

Happy to listen...Your methodical troubleshooting deserves attention. 

Keep us posted!

Moses

 

On 12/6/2022 at 8:08 AM, SomeBuckaroo said:

 

 

 

 

 

 

 

 

[SomeBuckaroo]

Moses, thank you very much for this amazing engagement. I verified your valve spring figures (61-69lbf) are (of course) correct! I was looking at the wrong FSM - apologies for the distraction.

Generally regarding your response:
Leakdown: I built my own tool using a 0.040 orifice 0.250 long, drilled myself. My "percentages" are the ratio of air-pressure on either side of this orifice while regulating input to 90psi. This is clearly a relative if not subjective metric. Do you have any guidance on improving this approach?

Compression via starter-current: thank you for this clever trick! Monitoring the battery voltage alone during cranking might even be enough, if we can assume the battery's internal resistance remains constant while cranking through several rotations.

Replies to your items:
1.) Valvetrain: By ear alone, the noise is coming from the top-front of the engine. I have not used any kind of stethoscope tool. I did install the camshaft thrust bolt/spring/pin, and replaced the timing chain slipper.

2.) Currently ~12,000 miles on my rebuild. I used a CompCams zinc additive for the first 100 miles, drained, and re-filled with Mobil1 full synthetic 5w30.

3.) Exhaust leak noise: Only listened by ear alone. Strangely, it is loudest (in the passenger cabin) while decelerating (0% throttle) in-gear. 

4.) O2, Injectors, fuel trim, scan tool: Replaced both O2 with new NTK units at rebuild. I can provide a scope trace of the upstream sensor at idle, and potentially under load as well. Injectors are original. At rebuild time, I pulled and cleaned them by forcing carb-spray through them, in both directions, by briefly applying 12v from a bench supply. I also cleaned the fuel rail with carb spray, until no more particulate matter came out. I also replaced the fuel pump with a Bosch unit. Interesting note: Bosch no longer makes the pump assembly for 97-04 TJ, but the 97-01 XJ is still made, is a direct fit - except for the wiring harness - and produces the same fuel pressure. The fuel level sensor reads about the same as original. (I know all this because I did it). Fuel trims: Have not checked recently. Scan-tool: I use a cheap Bluetooth OBD2 adapter, combined with the "Torque Pro" Android App. No recurring CELs or codes, but one of the "Mode 6" tests (I believe an idle O2 response test) does fail. It seems that the OBD2 measurements update at only about 1Hz, so I only use them for values that would not change very quickly. I directly 'scope the sensors for far higher sampling rate.

5.) TPS/MAP/IAC/Cam/Crank sensors: I swapped all with Mopar units from a known-good donor, or brand-new Mopar units. Verified their readings by 'scope or OBD2. Replaced the ignition coil. Thoroughly cleaned IAC cavity. TPS is indeed notorious; years ago I had strange issues stemming from an intermittent TPS unit that failed when it heated up.

6.) Oil pan: Was *not* present on reman engine. However, I unfortunately did not measure piston clearances.

7.) Reman short-block: I can dig up the paperwork I got with the reman. It was done by a what looks to be a large shop in the Atlanta area. I retrofitted my crank-bearing girdle onto the '92 block - but you are right, I am missing out on additional NVH improvements. Regardless, I don't notice much NVH difference between the original and the reman (and I don't mind the NVH; it's a Jeep!).

8.) I did indeed shim all 12 rockers with 0.040 shims. I am almost certain that my valve step tips were not machined, nor were my valve guides serviced at all. I did provide new FelPro valve-guide _seals_ to the shop; he removed the old ones and installed them. I have indeed already used an air-hold, on all cylinders, to measure spring seated-force. I used a top-side spring compressor of my own concoction; I'll post a picture for everyone to enjoy.

Going forward: I will get a video of manifold vacuum at idle, and under other operating conditions as is feasible. I will also follow up with more details on OBD2 measurements (fuel trim, sensor readings, etc) as well as from the reman paperwork. Thank you again!

[Moses Ludel]

Hi, SomeBuckaroo!  Let's walk through your reply...

On 12/16/2022 at 7:22 AM, SomeBuckaroo said:

Moses, thank you very much for this amazing engagement. I verified your valve spring figures (61-69lbf) are (of course) correct! I was looking at the wrong FSM - apologies for the distraction.

You're welcome...I'm enjoying your thoughtful approach...

Generally regarding your response:
Leakdown: I built my own tool using a 0.040 orifice 0.250 long, drilled myself. My "percentages" are the ratio of air-pressure on either side of this orifice while regulating input to 90psi. This is clearly a relative if not subjective metric. Do you have any guidance on improving this approach?

As you share, this is a dicey method.  A leakdown tester, in practical terms, is measuring the bleed-off of the combustion chamber plus the small volume considered "sweep area".  For your purposes, it's important to know how much of the air that fills this volumetric area is bleeding off past the valves, piston rings, head gasket or a casting crack.  Pressurized air flows into that space while a percentage of air (volume) leaves that space.  A leakdown tester "measures" the leak percentage as the pressure flow within the filled space.  There is an inflow of air from the shop's air supply at a fixed air pressure/volume. If the cylinder had a 100% seal while pressurized, there would be no measurable volume or percentage of air moving out of the combustion chamber and swept area.  Leakage would be 0%.  When the cylinder does leak (an 8-12% minimum on a newer production engine), that leakage reflects as the air flow (volume or percentage) moving through the gauge set.  Since this has nothing to do with the volume of the combustion chamber, as long as the inflow air pressure and volume are sufficient, the leakdown tester will be accurate for any automotive combustion chamber volume.

Many leakdown testers call for low air input pressures like 60 psi, including my Snap-On unit from the eighties.  I discovered early on that 60 psi seldom works because the piston compression rings, by design, need topside pressure to expand properly against the cylinder wall and seal.  In this static test, I run 90-plus psi as you are doing.  The OTC 5609 ($85 at several online sources, you may find a better price) and the Harbor Freight Maddox ($80, even less on sale) gauges are affordable leakdown testers.  This valuable tool will pay for itself if you take the plunge.  It can pinpoint valve and ring issues, casting cracks and head gasket seepage.

Compression via starter-current: thank you for this clever trick! Monitoring the battery voltage alone during cranking might even be enough, if we can assume the battery's internal resistance remains constant while cranking through several rotations.

Unfortunately, battery voltage reading with a VOM is only an indicator of overall starter load and the battery draw down.  The meter reads too slowly for compression stroke load comparisons.  You need to go a step further with your 4-channel scope and an amp clamp.  A lab scope (oscilloscope) can read time/voltage events in milliseconds.  This will create a pattern of the overall starter draw (voltage and amperage loads) and also the resistance created as each piston rises to its firing position on the compression stroke during cranking.  The ignition coil is disconnected for this cranking compression test.

Use another scope channel to pick up #1 cylinder in the firing order.  Measured in milliseconds, you'll see each piston spike the starter load on the cylinder's compression stroke.  Follow the firing order to see how the cylinders compare with each other.  The starter motor amperage draw will display a "relative" cranking compression pattern.  The starter motor and battery must be in good condition for this test to be valid.

A question remains:  What is the baseline compression?  If all cylinders were resisting uniformly, we still need to measure the seal and actual compression of at least one cylinder to know the engine's condition...What if all cylinders were at a low 100 psi?  The amp clamp/oscilloscope reading is a dynamic but relative compression test. But the lab scope is not done yet.  We can take this a step further with the use of an in-cylinder pressure transducer.  This will read the cylinder psi of a running engine. The transducer, fitted into a spark plug hole, measures pressure as a voltage signal that the oscilloscope can read. The in-cylinder transducer can pinpoint compression, valve event and valve timing issues. Worn valve guides and springs (poor valve seating) would show up with the engine running.  You would need to run the test on each cylinder for a thorough analysis.

For your immediate purposes, short of investing in an in-cylinder pressure transducer, a leakdown tester will do what you want.  Used properly, this is the best mechanical diagnostic tool for assessing engine condition and seal.  You are testing seal with the piston at TDC, the highest cylinder taper point, which will produce the highest percentage of leakage.  A common compression gauge, working with moving pistons, is often unable to pinpoint cylinder taper or telltale wear.  Currently, your engine is way too new for cylinder taper issues.

Replies to your items:
1.) Valvetrain: By ear alone, the noise is coming from the top-front of the engine. I have not used any kind of stethoscope tool. I did install the camshaft thrust bolt/spring/pin, and replaced the timing chain slipper.

With the engine idling and making the noise, use an insulated spark plug wire/boot remover to carefully pull each spark wire lead—methodically, one at a time.  See if you can diminish or eliminate the noise by momentarily relieving each cylinder of its firing load.  Try to pinpoint a cylinder that causes the noise.  If there is no sound difference, the upper valvetrain is likely the issue. 

You mentioned no block/head dowels, but there isn't enough head shift to create head gasket interference with the pistons and that kind of noise.  These engines ran with head bolt (only) alignment forever.  Head gasket interference is more common with largely oversized bores.  A stock 4.0L head gasket has wide enough margins to compensate for minor head alignment issues. 

You have new lifters with the cam kit.  Are the pushrods new, too?  If not, did you check them for straightness?  This can be done by rolling them on flat glass.  Straight auto window glass is good for this purpose.  Any quality glass will work.  Check for signs of pushrod rubbing.

2.) Currently ~12,000 miles on my rebuild. I used a CompCams zinc additive for the first 100 miles, drained, and re-filled with Mobil1 full synthetic 5w30.

Good that you used the zinc additive, a must for break-in, some use it ongoingly.  I would have run a non-synthetic oil for 250-500 miles to assure ring seating, but with the moly rings in a reman engine, you should be fine if the engine is not using oil.  I use zinc additive for at least the first 500-1,000 miles.

3.) Exhaust leak noise: Only listened by ear alone. Strangely, it is loudest (in the passenger cabin) while decelerating (0% throttle) in-gear.

I would watch the fuel trim under this driving mode (deceleration).  You may have a rich mix.  Otherwise, make sure the exhaust system does not have a restriction at the muffler or a clogged cat. 

4.) O2, Injectors, fuel trim, scan tool: Replaced both O2 with new NTK units at rebuild. I can provide a scope trace of the upstream sensor at idle, and potentially under load as well. Injectors are original. At rebuild time, I pulled and cleaned them by forcing carb-spray through them, in both directions, by briefly applying 12v from a bench supply. I also cleaned the fuel rail with carb spray, until no more particulate matter came out. I also replaced the fuel pump with a Bosch unit. Interesting note: Bosch no longer makes the pump assembly for 97-04 TJ, but the 97-01 XJ is still made, is a direct fit - except for the wiring harness - and produces the same fuel pressure. The fuel level sensor reads about the same as original. (I know all this because I did it). Fuel trims: Have not checked recently. Scan-tool: I use a cheap Bluetooth OBD2 adapter, combined with the "Torque Pro" Android App. No recurring CELs or codes, but one of the "Mode 6" tests (I believe an idle O2 response test) does fail. It seems that the OBD2 measurements update at only about 1Hz, so I only use them for values that would not change very quickly. I directly 'scope the sensors for far higher sampling rate.

Sounds thorough...The scan tool is limited to the PCM's data stream.  Your 4-channel scope works independently of that data.  Both tools help.  As for manifold issues or exhaust flow, I picked up an inexpensive vacuum/pressure transducer that checks pulses at the exhaust pipe or intake manifold.  This is telling for manifold vacuum, exhaust restrictions or valve event and timing problems.

5.) TPS/MAP/IAC/Cam/Crank sensors: I swapped all with Mopar units from a known-good donor, or brand-new Mopar units. Verified their readings by 'scope or OBD2. Replaced the ignition coil. Thoroughly cleaned IAC cavity. TPS is indeed notorious; years ago I had strange issues stemming from an intermittent TPS unit that failed when it heated up.

All good...I consider the TPS a perishable.  It is a moving, mechanical potentiometer, which means it will wear out.  Often sooner than later.  I use a quality TPS.  NTK is good if you don't want the Mopar® price bump.

6.) Oil pan: Was *not* present on reman engine. However, I unfortunately did not measure piston clearances.

7.) Reman short-block: I can dig up the paperwork I got with the reman. It was done by a what looks to be a large shop in the Atlanta area. I retrofitted my crank-bearing girdle onto the '92 block - but you are right, I am missing out on additional NVH improvements. Regardless, I don't notice much NVH difference between the original and the reman (and I don't mind the NVH; it's a Jeep!).

The reman procedure for large shops is systematic.  If the core was in good shape, you likely have a 0.010"-0.010" or 0.020"-0.020" maximum crankshaft, the 0.030" oversize bore you describe, line boring, square-to-crankshaft cylinder boring with the use of a torque plate in better shops, the 0.010" standard block decking, etc., etc.  The wild card with your engine is the cylinder head, which is also the bridge between the old and reman engine.

8.) I did indeed shim all 12 rockers with 0.040 shims. I am almost certain that my valve step tips were not machined, nor were my valve guides serviced at all. I did provide new FelPro valve-guide _seals_ to the shop; he removed the old ones and installed them. I have indeed already used an air-hold, on all cylinders, to measure spring seated-force. I used a top-side spring compressor of my own concoction; I'll post a picture for everyone to enjoy.

If this problem is mechanical, and it points there, you definitely have valve guide and stem wear if the original valves were reused.  This could create "valve guide/stem knock" exaggerated by worn valve springs.  Logic points there, as the rest of the engine is "new".  Likely the noise came with the old cylinder head.  Your initial thought about the valve springs was probably spot on.  The spring lengths need adjusting as you planned.  I've not done the rocker shimming, though it's common with block decking and head surfacing.  I do selective fit pushrods instead.  Since the noise existed before and after the short block installation, the head (valve guides, stems, etc.) is more likely the issue than pushrods or rocker arm shims.  If those are the original valve guides (no liners) and valve stems, that's 300K miles of wear on a pushrod engine!

Going forward: I will get a video of manifold vacuum at idle, and under other operating conditions as is feasible. I will also follow up with more details on OBD2 measurements (fuel trim, sensor readings, etc) as well as from the reman paperwork. Thank you again!

You're welcome...The manifold vacuum at idle may reveal a valve guide/spring issue.  You'll see a wavering vacuum needle.  If so, this may resolve with a quality head rebuild if the current head is restorable.  Reman shops replace all 12 valves with new and do guide liners.  Valves are relatively inexpensive for a Jeep 4.0L.  If the seats were cut properly, the surfacing good, you'd likely get by with new valves and guide liners.  Sublet the guide liners installation to a quality shop.  Lap the valves to confirm seating.  This is a basic head that you can service.  

Try to confirm shaky valves and loose guides before a teardown...We're looking forward to your updates and findings.

As a footnote, we had discussed valve spring replacement (cylinder head in place) nine years ago.  I searched for the link here at the forums.  Some added information for those interested:

Moses

 

[SomeBuckaroo]

Happy New Year, Moses.

1.) Video of manifold vacuum at idle & warmed up: https://youtube.com/shorts/ABvIyk5CheE
I recorded the video in 60fps to best capture the oscillation, but I think YouTube reduced the framerate. The indicator oscillated somewhat more violently than the video represents.
I have to admit that I conducted this test several times over the years, but only now did I realize the meter I used had a significantly damped response (unsure if by design or malfunction). Accordingly, its indicator hardly oscillated at all. The meter in the video above is a cheap replacement with very fast response (to judge response I quickly released a vacuum and watched the indicator's rate).

2.) My short block's paperwork indicates it was remanufactured in 2009 by Automotive Precision Machinery of Forest Park, GA. Bore: 030, Mains 010, Rods 010, Oil Pressure 56, Amps 7, "Other: E.P3" It was sold in 2013, and I believe sat unused until I purchased it in 2020. Additional documentation indicates the pistons are EngineTech P1593(6).

3.) I had my head serviced in 2020 by Engine Works of Bowdon, GA. The paperwork they gave to me lists only "Rework Head" (for which they charged $200). Zero additional details of any kind.

4.) The Torque Pro app, via my cheap Bluetooth ODBII adapter, reports:
Fuel Trim Bank 1 Long Term: 1.56%
Fuel Trim Bank 1 Short Term: 2.34%
Fuel trim Bank 1 Sensor 1: 0.78%

 

[Moses Ludel]

SomeBuckaroo...Thanks, and have a productive 2023! 

As for the vacuum test, there's something amiss.  An MPI engine should have uniform vacuum.  You're showing a rhythmic fluctuation, not extreme, but related to the engine's firing order.  This could be valve seating issues from either your valve spring concern or poor valve face-to-seat sealing at one or more cylinders.  The short block's cylinder seal (basically the piston rings) should be uniform, which you can confirm with a precise leakdown test.  If so, the fluctuation would be a valve(s) and its relationship to cylinder seal with the engine running.

Your reman short engine is textbook for a 4.0L core in reasonable condition.  This is the typical "reman kit" sizes that shops order for good cores.  You have a thorough rework of a short engine/core on its first rebuild.  A production reman shop would recondition the rods, deck the block, usually line bore the mains, replace cam bearings and freeze plugs and fit pistons during the final honing of the cylinders.  Some shops include balancing, most charge extra for this service or build the cost into the package price.  An inline six will benefit from balancing, though this is not as crucial as it would be for a 90-degree V6 engine without a counter-balance shaft.

Head work would require a conversation with the shop to determine their "standard" head rebuild.  (At this stage, you may not want to bother.)  The charge is not high, which does suggest that the original valves and springs were reused.  The machining likely included surfacing the head, seat work and valve refacing.  If that's the case, correcting the pushrod lengths would be necessary due to the block decking and head surfacing.  The cost left little or no room for replacement valve seats or new exhaust valves, which many shops replace routinely.  As I've shared, reman shops usually replace all valves, which they buy in bulk at discounts.  This saves labor time.  New valves also help eliminate valve stem height and spring pressure issues.

Fuel trim looks reasonable, not indicating an issue with fuel flow or mixture.  However, I would do a routine intake manifold and gasket leak test—leaks can cause manifold vacuum fluctuations.  Use a lower volatility spray like WD-40, and with the engine idling, spray around the intake manifold flanges at their cylinder head junction.  (Avoid spraying on the hot exhaust manifold!)  Listen for rpm changes.  This would indicate seepage of air into the intake stream, which affects A/F ratio but also causes variations in manifold vacuum.

Vacuum hose or brake booster leaks can cause fluctuations, too.  Check the brake booster's check valve (an inexpensive item) and its grommet for leaks.  A clogged cat or muffler can create fluctuations.  Bent pushrods, too...Rule these out as well.

This is a process of elimination...

Moses  

[SomeBuckaroo]

Thank you again for the wealth of information and advice. I think I will individually test each of the following, to increase my confidence that the issue is indeed in the head:

1.) Remove & cap all vacuum lines, to eliminate vacuum hose leaks as the cause
2.) WD-40 manifold/gasket leak test as you described.
3.) 'Scope simultaneous traces of my MAP sensor and #1 cylinder ignition secondary (via capacitive probe). Intent is to narrow the issue to one (or several, but hopefully not all) cylinders. Do you think the stock MAP sensor is fast enough to respond to vacuum changes within the firing order, at idle? Do you have any other thoughts on this approach? Scope traces of the issue and its resolution would be a very satisfying conclusion!
4.) Precise leakdown test using OTC 5609 leakdown tester. I understand the mechanics of a leakdown test, but I don't understand how to differentiate between piston-ring leaks and valve leaks. Would you clarify that for me?

Are there any other fairly lightweight tests you'd recommend to rule the head in (or out) as the cause? As much as I'd like to keep my original 0630 head, I'm starting to think that - should it be the cause - replacing it with a fully assembled reputable reman would be my preferred exit strategy! Thank you again for your time.

[Moses Ludel]

SomeBuckaroo...You're welcome...I like your approach...Since you have a distributor ignition, I would do the scope test you describe with the capacitive probe placed at each cylinder in the firing sequence:  1-5-3-6-2-4.  See if that shows distinctions.  #6 is opposite #1 in firing sequence, and even those two probe points would be handy.  Assuming the ignition spark wires, cap and rotor are in top shape, spark firing patterns would be revealing and provide the comparison you need to isolate given cylinder(s) weaknesses.  A reading like this (spark firing lines) would come from the high tension coil lead if your scope can catch this.  

You'll need to experiment with you real time MAP readings.  You're right, vacuum to the MAP would likely reflect averages rather than pinpoint a given cylinder's vacuum pulses.  We're working with milliseconds here.  My Autel MP408 oscilloscope and a back probe might read the MAP sensor's electrical/electronic signal to the PCM as a fluctuating voltage reading.  Here, however, the MAP sensor has converted the engine's vacuum signal into a quantity of voltage, much like a potentiometer. 

What would work, though, is my 4-channel MP408 MaxiScope plus the automotive pressure/vacuum pulse sensor that I picked up for under $75 at eBay from a British Columbia manufacturer. The pulse sensor probe (hose) can be placed in the tailpipe.  Exhaust pressure pulse readings reflect fluctuations or losses of pressure in discernable milliseconds. 

When we hook up the MaxiScope with a channel/probe to reference #1 cylinder, with another channel/probe showing all six spark firing lines, we can pinpoint pressure losses at each cylinder with the engine running.  The pressure pulse and other reference signals would show precisely which cylinder(s) have a drop in pressure.  This is why I get excited about the oscilloscope.  The MP408 and OAK (optional accessories kit) have become my go to diagnostics tools for mechanical and electrical testing of anything that produces voltage over time.  I don't minimize the value of a scan tool, it can be a great device for peering at PCM data in real time or tracking signals to and from a PCM/ECU/ECM.

Here is how I would use the OTC 5609 leakdown tester.  Tests are done with all spark plugs removed: 

1)  Tune-up related diagnostics—First-off you want comparisons of the leakdown rate/percentage per cylinder.  This is cylinder "balance" and a great tune-related test.  Since the leakdown test has the engine/piston static at TDC on the compression stroke, the valves are seated as well as they can be.  (Run the test with the piston at TDC on the compression stroke as intended.  This has the piston at the maximum cylinder taper point for leakage.)  If valves are closed when applying tester pressure, there is less risk of debris blowing between the valve face and seat, which would cause a false reading.  On an older engine with carbon buildup around the valves, this is a precaution when running the leakdown test.  You're blowing compressed air into the cylinder through the spark plug hole.

2)  Pinpointing piston ring and piston issues—Think of this like a standard compression test only much more precise.  With the air pressure applied, leakage past the rings or a worn/damaged piston will create a distinct air flow noise in the crankcase.  This can be heard by ear or with a stethoscope at the dipstick tube—or simply by removing the oil filler cap.  Compressed air goes into the cylinder, a percentage of it blows past the piston rings (i.e. the term "blow-by") and enters the crankcase.  You can hear the air movement.

3)  Pinpointing valve seepage—Again, although you don't have the running engine to rock the valves (weak valve springs or uneven seat/valve face relationship could be more pronounced with a running engine and firing loads), you have the optimal vantage for pinpointing valve seat and face pitting, uneven seat cuts or warped valves.  The compressed air is leaking past the intake and/or the exhaust valves.  If leaking past the intake valve, you will hear the air exiting from the throttle body throat (open the throttle for a clearer read) or air cleaner box.  If leaking past the exhaust valve, you will hear noise at the tailpipe, either by ear, with a stethoscope or using a sounding tube.

4)  Blown head gasket or casting crack and warpage—A blown or seeping head gasket will also turn up.  Remove the radiator cap and look for bubbles with the leakdown tester applying pressure.  (If you suspect a blown or seeping head gasket, remove the radiator cap before running the leakdown test.  You might otherwise damage the radiator from the air pressure.  Set the leakdown tester's pressure low.)  If there is a valve seat crack, cylinder wall crack or blown head gasket, bubbles will appear in the radiator.  If a casting crack is lower in the cylinder, lower the piston enough to expose the crack but not open a valve.

Note:  Like with a compression gauge check, a blown head gasket may be between two cylinders.  Listen at the adjacent cylinder's spark plug hole while performing the leak test.  The leakdown test is an audible test for flowing air. 

Since you could have a combination of leak points (rings, intake valve or exhaust valve), you may identify air flowing at several points.  Distinguish the noise sources and volume.  If just ring seepage is suspected, you can try the traditional use of a few squirts of clean motor oil into the cylinder then run the test again.  There should be at least a brief point where the leakage/percentage decreases significantly.  Always make sure you are at TDC on the compression stroke to be sure that both valves are seated.

Valves seal thoroughly and should not leak air.  However, conventional automotive rings have end gaps.  With end gaps, there will always be a certain amount of air seepage past the rings, which is normal.  This establishes the baseline percentages for "normal leakage".  8-10 percent leak is excellent for a production engine in new or "broken-in" state.  12-percent is still tops for modern engines with high compression ratios. 

I consider an engine in "good operating condition" to as high as 20-percent leak as long as the percentages between the highest and lowest cylinder are not far apart.  Higher percentages reflect general wear, but at 20-percent leak, the engine may still have balanced, acceptable performance.  Beyond this, the engine is declining, may be using oil or should be considered "worn" to the point that a tune-up may not make much difference.

The value of a leakdown tester is its ability to diagnose current and pending troubles.  Here is a test that shows valves building carbon or starting to seep between the faces and seats.  Or rings that are losing seal but are still "functional" for now.  I use my leakdown tester to gauge the approximate cylinder taper...Lower the piston in the cylinder without opening a valve.  As the piston moves down, the cylinder wall taper decreases.  If the percentage of ring blow-by/leakage decreases with the piston lower in the cylinder, there is a likelihood of cylinder taper or upper cylinder scratches.  I have a bore scope to look closely at the upper cylinder walls and taper area.

A leakdown tester can even measure valve chain/belt wear, camshaft lobe wear or valve opening events (valve timing).  With a degree wheel on the crankshaft and the tester applying air at TDC on the compression stroke, slowly rotate the crankshaft in its normal direction of rotation to where the cylinder's exhaust valve just unseats and begins to "leak".  Do not pass this point and rotate the crankshaft backward to compensate.  Rotate the crankshaft in one direction.

Note the degree on the timing wheel where the exhaust valve begins opening.  Compute valve timing from there.  (This is not the usual 0.050" ramp camshaft lobe/lifter height test.  Therefore, these results will not match the camshaft manufacturer's advertised specifications.)  Based on the camshaft profile, the intake valve timing can be computed from here.  This would be a test for wear and damage in the valvetrain.

The leakdown tester enables testing a static engine in a vehicle, laying on the shop floor, chained to a cherry picker (near the floor or with a safety stand supporting the engine), resting in a pickup bed, at a recycling yard or on an engine stand—anywhere compressed air is available.  Your leakdown tester can be used for checking the cylinder seal of any I.C. gasoline engine, from a freshly built automotive long block to a high mileage used engine, from lawn mower engines to motorcycle, passenger car or truck engines.

After running the leak test, as a follow-up, you can use an inexpensive bore scope to inspect for carbon buildup, cylinder wall damage or signs of casting cracks.  Creative thinking will open up even more uses for the leakdown tester.  This is a quick way to understand engine dynamics and an engine's overall mechanical condition with the cylinder head in place.  In addition to the leakdown tests, you can consider:  1) the engine's oil pressure and bearing condition, 2) the precise valve lift and 3) the valve timing.  Then you have the whole picture.  

Let us know what you find...

Moses