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Hey Guys.  I have enclosed a link to the noise this engine has.  I have not been able to figure this one out.  Here's a little history and the diagnostic procedures used so far.


This engine is a "refresh".  It's in a 98 TJ that blew its engine.  I pulled this engine from a pull-n-pay out of a Dakota.  When I tore it down it was clear the chain tensioner had destroyed itself and gone through a couple bearings.  I did a complete refresh on the Dakota engine.   Took it down to the block and thoroughly cleaned it with electrolysis, warm solvent soak, pressure wash, soap and water cleanup of all passages. Cam bearings, dingle hone cylinders, all new bearings throughout, new rings-properly clocked, new lifters, new pushrods, Cleaned and lapped the heads (valves). New intake and exhaust valves on #4 cylinder.  New oil pump.  The old cam and crank were fine so I used them.

The engine runs great and has good "pop" for a 4-banger.  It runs smooth with no misses.  I broke it in and drove it around a bit so the computer could relearn the fuel trims and to get some back-pressure on the rings with some semi-hard slow pulls.

The noise is deep in the block and toward the rear.  Specifically it seems to be mid-block and around the #3 or #4 cylinder.  The noise is its worst at idle.  With even a slight throttle change it disappears.  It doesn't knock at anything but idle.  I cut-out (killed) each cylinder and it did not change the sound.  I really suspected a lifter was not charging.  Indicators were the location of the noise and the fact that it went away with 'any' acceleration (thus charging the lifter with a touch more oil pressure).  I suspected a weak lifter and ruled-out everything else, so I replaced the lifters.  Same sound.  I took the pan off.  No metal in the pan.  Pulled the break-in filter apart.  No metal in the filter.  It has 40 psi consistently at idle (rises with acceleration).  (10W30) With the pan off, I checked each rod on all four quadrants=no movement.  I pulled the caps off #3 and #4 rods and the bearings are perfect.  I suspected piston slap.  Moved the pistons through their range, examining each.  At the top of the stroke (rotating the engine back and forth) I watched the piston skirts for movement [because the force axis on the piston changes].  There was no lateral movement.  The #4 piston had "maybe" a 16th of an inch lateral movement at the bottom of the skirt.  (It was barely perceptible).  I don't believe this is enough to cause piston slap.  The engine has no noise upon startup, and begins this knocking after about 30 seconds usually.  It doesn't seem to change with engine temperature.  I went back to the top of the engine and pulled the valve cover.  All the rockers were tight.  I ran the engine and pressed down on each valve.  There was no "clacking" or any indication there was any play in any valve.  I can take/post more videos if it would be helpful.

I think that about covers it.  Hopefully I've included all the variables.  I'm out of ideas!  Any help would be much appreciated!  Thank-you!

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I value your thoroughness and detailed description of the work performed, Wayne.  You're taking a conscientious and professional approach.  Here are two suggestions:

1)  You may have a lean, individual cylinder knock at idle.  An injector pulse tool like the Waekon 76462 could indicate an injector that is "misfiring" or, better yet, an inexpensive diagnostic tool for injectors could test the actual flow behavior: 



https://www.amazon.com/Professional-Injector-Kawish-Automotive-Diagnostic/dp/B07V43G3H3/ref=psdc_15727741_t1_B0021V0FRE  [This is just one example at Amazon, there may be other devices that you will prefer, including OTC.  Each is relatively inexpensive in this tool class.]

2)  The noise is in the area of the distributor and oil pump.  Use a stethoscope, sounding rod or, ideally, a Steelman Chassis/Engine EAR tester to see whether the distributor shaft or drive gear, or even the new oil pump, is creating a knocking noise.  I have a Chassis/EAR that does wonders with its multi-channel sound comparisons:


The Steelman tool is also useful for comparing and contrasting the origins of a knocking sound, separating the six channels will indicate the intensity and decibel contrast.

Despite your fuel trim checks, there may be a specific injector(s) getting a very lean idle mix and creating knock.  As you hint, knock or ping is piston rock.  If your calculation is right about piston skirt clearance, and you've ruled out "slap", this may simply be a cylinder/piston knock from a lean idle mixture.  Also use a timing light to see how much advance you have at idle, significant advance can contribute to knock if you're running low octane fuel.  These engines do not use a knock sensor to retard timing and stop pre-ignition/knock.  Sometimes, a simple change in octane will stop knock, though the fuel price may not be desired.

I'm going to spend 2021 sharing in-depth diagnostics and troubleshooting at the magazine site.  There are lab scope procedures for associating noises with injector firing and also in-cylinder transducer pressure tests that indicate cylinder imbalance and low compression while cranking.  These techniques will help demystify electronic fuel-and-spark issues and basic engine conditions like lower compression in a given cylinder.  Two- and four-channel lab scopes move well beyond most scan tools for diagnostics.

Here is a current video series I did on engine knock analysis and injector cleaning to resolve knock caused by poor injector firing.  This could be helpful to your current issue and illustrates the use of the Steelman Chassis/EAR:

1) https://www.4wdmechanix.com/how-to-diagnose-an-engine-noise-or-knock/

2) https://www.4wdmechanix.com/curing-an-engine-knock-with-surr-tools-and-sea-foam/

This is a place to start.  Let us know what you find.  If necessary, we can go from there.


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  • Moses Ludel changed the title to Jeep TJ Wrangler Refreshed 2.5L Engine Has Knocking Noise

Hello Sir.


Thanks for responding.  You have certainly given me some alternate direction.  I will try to get out to the shop within the next day or two and will let you know what I find.


I have used a stethoscope on this engine quite a bit, but I'll keep at it.


Thanks again!



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Wayne...I have one additional focus area to suggest.  Had the Dakota engine been previously rebuilt?  You lapped the valves and did not resurface the head or block deck.  Did you use the original pushrods when determining pushrod length?  Pushrod length is crucial on these engines, and you can check the needed pushrod length (per valve) with an inexpensive CompCams gauge for the 2.5L engine's length range. 

We have discussed correct length pushrods at the forums.  4.2L, 4.0L and 2.5L AMC engines require measuring and choosing the right length pushrods.  These topics will cast light: 

1)  https://forums.4wdmechanix.com/topic/1168-new-lifters-and-valvetrain-noise/?tab=comments#comment-8187

2)  https://forums.4wdmechanix.com/topic/1155-42l-re-build-77-cj-7-project/?tab=comments#comment-8020  [This exchange includes photos and guidelines for the CompCams gauge test of lifter length.]

Especially with a used camshaft and new head gasket, I would check the pushrod lengths.  This can be done with the cylinder head in place.  Pushrod access is with the valve cover removed.  Review our discussions on pushrod length and tests.  You may need different length pushrods at one, some or all valves.

A further comment on fuel knock...Our 4.0L '99 inline six had a knock at idle.  When I cleaned the injectors via the fuel rail (see the link in previous reply), the noise disappeared.  This suggested a lean condition on a given cylinder(s) at an idle. 

These AMC/Jeep engines set spark timing and fuel flow to a preset determined by the sensor signals and PCM programming.  One example is the oxygen sensor that looks at the exhaust stream for its combined, all cylinders oxygen content, not the O2 reading for individual cylinders.  Likewise, spark timing is fixed by sensor feedback and PCM programming.  Timing is not per cylinder advance but rather a preset number of degrees for all cylinders determined by the engine mode and operating conditions.

No two cylinders of any production engine have precisely the same timing advance needs.  The best ignition designs work in concert with a knock sensor.  They set spark advance individually for each cylinder.  In the early nineties, I played with SafeGuard ignition control by J&S Electronics (http://www.jandssafeguard.com/).  These devices can retard timing per cylinder until a knock disappears in the culprit cylinder(s).  The remaining cylinders each run at their peak tolerable advance, which is synonymous with peak power.

With SafeGuard, you can set a fixed conventional distributor to a slightly exaggerated base timing advance.  SafeGuard pulls per cylinder timing down to a knock-free level in nanoseconds.  You never hear ping from detonation.  Each cylinder runs at its individual peak timing advance—just below the cylinder's knock point.  This process is continuous, adjusting under all engine speeds and load conditions.

Without a device like the SafeGuard or a factory ignition system capable of sensing knock in individual cylinders and retarding just the culprit cylinder(s), an engine may depend upon higher octane fuel to stop knocking.  The Mopar EFI Conversion for 4.2L engines specified the use of 91-92 octane gasoline.  The MPI conversion was stock 1995 YJ Wrangler inline 4.0L six design, which has a cylinder head (combustion chambers) that is less vulnerable to knock.  The Mopar MPI used from 1991-up on 2.5L and 4.0L engines does not require a knock sensor because the combustion process is less prone to pre-ignition.

Food for thought...I would take a cranking compression test to see whether compression is high in one or more cylinders.  (This could cause knock.)  For overall condition, I use a leak down test to confirm cylinder seal.


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Oops.  I left a lengthy message a couple days ago but I don't believe I was logged-in.

I ordered the comp cams pushrod length checker:


I had a bad computer day when I posted this response previously.  I also had this tool in que and did not finalize the order so it will be a bit delayed.

I checked the secondary ignition with a PICO scope.  The secondary looked good with no misfires or lean cylinders.  I listened to each injector with a stethoscope and the pintles all sounded ok.  After these checks I leaned away from a lean cylinder so I didn't scope the injectors.

However...I think you may be on to something with the pushrods.  Given the variables you described, it makes sense.  I also replaced both intake and exhaust valve in cylinder #4 with new units.  I did not measure the length of the new valves prior to installation.  While waiting for the comp cams length checker, I was considering this approach.

From another source:  "...install the bridge, pivots, rockers, and cap screws over the push rods. [with the valves closed] Slowly turn each push rod while slowly turning down the cap screws until resistance is felt in the push rod turning.

Mark the cap screw with index marks as shown in the pic. Alternate tightening the cap screws until they bottom out, then torque them to 21 ft/lbs. Observe the index mark position. This will show you the preload on the lifters."



This is a method used by crane cams.  Do you have any opinions on this method?


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Hi, Wayne...Yep, valve stem lengths and face margins vary, although most replacement parts are equivalent to OEM/stock.  Valve seat recession occurs, and this increases preload.  A valve may originally have a borderline pushrod length that becomes unacceptable with valve seat recession.  Lapping valves (properly) should make little preload difference, you're not removing much material at all.

The capscrew degree of rotation method relies upon several factors:  1) thread pitch on the capscrews determines the amount of drop or preload, 2) the rocker arm ratio is a factor, since the capscrew is pulling down the entire rocker arm, and 3) torque is relative to whether the threads in the head casting or capscrew have any stretch.  It's certainly a ball-park quick check.  Make sure they are talking about stock rockers or the same rocker arm ratio you're using.

When you get the gauge, you'll smile big.  It will completely demystify pushrod length.  The only measurement concern is the actual length of the pushrods, although ordering new ones to fit should be precise.  Melling offers pushrods by length.  I listed part numbers for earlier (1990-back) 4.2L engines at the '77 Jeep CJ project link.  There are many length options, see the Melling full catalog.  You'll pick a pushrod(s) that bring the valve/lifter into an acceptable range:


I did this vlog some time ago to provide a framework for understanding valve stem length, valve seat depth and lifter preload concerns:



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Sorry for the delay...holidays.  I hope your Christmas went well!

I received the pushrod length checker.  I used the EOIC method to make sure I was on the base and it worked well.  I was able to obtain precise, repeatable measurements.  Each pushrod (with minor variances in a couple) measured right at 9.400.  That is zero-lash (no preload).  The pushrods I have are Melling and each measure 9.481.  Easy math...this gives .081 preload.

My initial thought was, "Perfect.  This is very high and I found my problem".  However, it is my understanding too much preload would likely cause valves to be held open.  Since my running ignition secondary patterns were good, I don't believe this to be the case.  So in a nutshell, I am not seeing any symptoms indicating this extra preload is causing issues.  I do not know how much travel the lifter plunger has total, so I'm not sure how close I am to its limits.

On another note, a compression check showed 145 psi at #1,2,4 and 125 at #3.  (5300' Elevation)  It should be noted this was not a 'proper' compression check (the engine was cold) but it gave me a reference point for relativity between the cylinders.  If this is unrelated I will look into it later.  For now, I am focusing on the knocking noise.

What do you think about that preload?  Thanks!!


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Hi, Wayne...I like a preload of 0.030" to 0.040" in a freshly rebuilt engine.  This allows for normal valvetrain wear and moderate valve and valve seat recession over time.  The lifter preload will increase accordingly, usually just a nominal amount, before rebuilding time.  If you were using the pushrod checker during a fresh build and trying to determine the correct pushrods to order, you would order 9.440" (or closest to that length) pushrods. 

Since the camshaft is also presumed original, there's no reason to suspect that the camshaft's heel is on a greater than stock radius.  If the valve stem lengths, head gasket thickness and valve seat depths are okay, the only other variable would be the head deck height.  Uniform pushrod length measurements are a good sign.  

The 0.080" preload hints that the 2.5L may have  been cylinder head decked at some point.  I'm doubting the block deck has been milled, as you did not mention oversized pistons or other signs of a previous rebuild/remanufacturing of the engine.  A valve grind with decking is possible or an exchange/rebuilt cylinder head.  The lifter plunger travel may compensate for 0.080" preload; however, if lifter plunger/preload becomes excessive, as you note, the valves will not seat. 

For a reference, if you have any of the old 2.5L lifters, drain one of oil, reassemble it, allow the plunger to extend to the circlip, then measure how much travel you have between the plunger at the top/against the circlip and the point where the plunger stops or bottoms.  Subtract 0.080" from that measurement.  The difference is the travel range left with your current lifter preload.  You don't want too little preload, and you don't want the plunger to either block the flow of oil or bottom in the lifter body.  The plunger should "float" in the correct zone.

In the lifter, the oil acts as a solid column to support the plunger at the right height or position.  Normal oil bleed down is approximately 0.004" per crankshaft revolution, and a constant flow of pressurized oil restores the plunger height each time the valve opening and closing event takes place.  To do this, a steady and sufficient flow of pressurized oil to the lifter is essential.

Lack of oil to a lifter(s) creates clearance at the rocker arm.  While the knock is occurring, with the engine idling, check for rocker arm to valve stem clearance.  Use a feeler gauge(s).  If there is noticeable clearance, a lifter is bleeding down from either lack of oil supply or a defective lifter check ball.

Lifters can also be tested for bleed down.  Bleed down is a sign of a bad lifter (highly unlikely with new lifters) or too little pressurized oil flow through the lifter(s).  Lifter leakdown can be caused by inadequate or erratic oil flow through the lifters.  If you suspect this is the cause of your knock, check for rocker arm clearance with the engine idling and noisy.

As for the compression check, I'm not a big fan of cranking compression tests.  However, since they can be a ballpark or quick check, I would now move to a cylinder leakdown test on #3 cylinder, then the others if necessary.  A leak down test is done with the piston at TDC on the compression stroke, so the piston and rings are at the maximum wear point in the cylinder.  (This is the maximum taper point on a worn wall.  I also check pushrod length in this position, as the piston being at TDC on the compression stroke assures the camshaft heel position.  Your method should be okay, too, I used it for many years on routine valve adjustments with stock profile camshafts.)  There are "leak down" or "leakdown" discussions here at the forums, use these two key words.  You can find an inexpensive tester for $30 to $60.  It will become a handy diagnostic tool if you do this kind of work.  A quick search turned up this $25 bargain close-out at Harbor Freight.  OTC makes a kit for $75.  Depends upon how often you want to use it and the degree of accuracy demanded:


I went back through your initial info.  You mention piston skirt movement of 'maybe 1/16"'.  If this is anywhere near accurate, the skirt of the piston would have way too much clearance.  It's hard to gauge rock.  I would measure actual skirt-to-wall clearance with a flat feeler gauge, using care to not get the gauge stuck between the skirt and wall.   Normal lower skirt-to-wall clearance should be 0.0013" to 0.0021" for new pistons in a properly sized bore.  You suggest maybe 0.0625"?  That would be excessive.  0.003"-0.004" might be okay on a lightly honed cylinder with a good piston—a piston with proper shape and dimensionally correct.  For reference, production pistons are cam ground and not "round".  They are slightly narrower at the pin bosses, allowing for thermal expansion in this area...The skirt measurement is the reference needed.  Use the feeler at the centerline of the skirt.

I have no desire to create extra work for anyone, but if you're right about the amount of piston skirt clearance, that needs attention.  Piston skirts "shrink" from overheat and wear.  Excess wear or galling would have been noticeable.  The easiest way to measure skirt clearance in an assembled engine is to bring the piston/skirt downward to the lower end of the cylinder bore.  Stop the skirt at or slightly below the bore.  Gently work the feeler gauge between the skirt and cylinder bore/wall.  At the thickness we're discussing, any blade of 0.004" or less thickness should readily curve to the radius of the cylinder wall.  There should be virtually no bore wear at the bottom of the cylinder, so excess clearance would be due to a worn or collapsing/collapsed piston.

The other possibility is oiling, though this should not differ widely when the engine warms.  Oiling depends upon cam bearing and rod bearing oil hole alignment and clearances.  If you installed the bearings with oil holes aligned, and if the rods/pistons are in their original order and facing correctly, there should be good oil flow at the camshaft, lifters and each connecting rod squirt hole. 

You mention cam bearings.  Did you install new cam bearings or have them installed?  Were cam bearing oil holes aligned with the block passageways?  This is a long shot, but misaligned or spun cam bearings create a range of oiling issues.  When you cleaned passages, did you inspect the cam bearing oil hole alignment?

You shared that checking the new rod bearings for shake in multiple positions of the journal showed no movement.  If rods are facing correctly and in their original positions, there should be no variables created by your work.  I do use Plastigage to check the clearance at 90-degree intervals; this provides a fairly good take on the journal's roundness and the bearing oil clearance.  Jeep® oil pressure is always high, which masks irregular clearances and oil bleed-off from the sides of the rod bearings.

Check the cheeks of the rod journals for wear.  Check the rod side clearance.  Oil will bleed off the bearing(s) if there are irregularities here.  If the crankshaft has never been reground, Plastigage will enable you to make a judgment call about use of undersize 0.001", 0.002" or even 0.003" (sometimes mixtures of these undersize shell sizes, which I can discuss) on a round, slightly worn or undersize crankshaft journal.  If you determine this need, we can discuss how to size rod (or even main) bearings for the precise oil clearances you want.

Ford 300 cubic inch inline sixes were notorious for excessive rod bearing/oil clearances when new.  I dealt with that years ago and know the official Ford warranty "fix".  On that note, when rebuilding an engine with round OEM/standard size journals, polishing the journals is acceptable if the bearings are sized and fitted for proper oil clearance.  Measurements can be done on the bench with outside and inside micrometers.  If the engine is assembled or in the chassis like in your situation, sizing can be determined or confirmed with the proper use of Plastigage.

Though dropping the pan is not much fun, measuring the piston skirts might be in order.  There is some discussion of broken pistons and skirts on late nineties and early 2000s 4.0L and 2.5L Jeep engines at other forums.  A good friend with a Jeep repair shop has described broken pistons and skirts on later AMC-design Jeep engines that have accrued mileage.  While there may not be a widespread problem here, it's worth noting that piston skirts do break on these engines.  Piston slap could logically occur as well.

My takeaway from these failed piston concerns would be replacement of the OEM pistons if the engine is torn down, whether re-boring or not.  If bores are true and can clean up with glaze breaking like you performed, the same size pistons and rings should be used.  Checking ring end gaps is sensible, though nearly all service/OEM replacement ring sets are accurately pre-gapped unless noted otherwise.  High performance rings, however, usually do require gapping with a hand or power gapper.  When performing a light overhaul as you did, ring end gaps should be checked at maximum cylinder taper point in the cylinder (just below the ridge) to confirm round and degree of cylinder/bore wear.

For decades, I have used hypereutectic United Engine "Silv-O-Lite" pistons on stock engines.  Keith Black (KB) pistons are an option.  Mahle is now making Jeep® replacement pistons.  Any of these pistons would be a far better bet than the OEM.  (As you have seen, the OEM pistons have very light skirts.)  I only use forged pistons in a high compression or race engine, they are built with clearance for expansion and can be noisy when cold.

On another note, I am very interested in your use of a PICO.  I am devoting magazine how-to videos to scanners and lab scopes this coming year after attending some very interesting trade-level webinars on advanced use of these diagnostic tools.  You mention stopping short of using the tool for injector testing.  Would it warrant doing an injector test or checking fuel trim per cylinder?  This would be a basic requirement if the injectors are not flowing fuel properly or are simply "dirty".  You did do cylinder shorting to no avail, so I understand why you ruled out a lean mix knock or detonation.  My concern is that the PCM and oxygen sensor can only generalize when adjusting A/F at idle.  The result is an average rather than actual A/F per cylinder—some cylinders may be lean while others enrichen. 




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Hello Sir:

Thanks again for all the input.  That's quite a comprehensive analysis.  I have been working on another couple Jeeps with more pressing time constraints so this is a bit delayed.  I thought I'd update on some progress.  I pulled a lifter out and measured hydraulic piston travel (plunger).  For the lifters I installed, they have .198 total travel.  My preload (although high) looks like it is within the range the lifter can handle.  From here, I will likely get the Jeep back on a lift, drop the pan, and check for piston slap again.  What you've said makes sense and gives me a more accurate way to check.  If it looks good, I will re-assemble the top side (valve cover, etc.) and run the PICO tests you've described.  If the piston is indeed is the issue, I will pull the head and examine the bore for piston fit (I recently obtained a dial bore gauge).  I will definitely stay tuned and am looking forward to your videos on the PICO!



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Hi, Wayne...When you pushed the lifter plunger down, did the plunger's base still clear the oil feed hole in the lifter?  The lifter needs a steady, unrestricted oil flow.  Confirm that the plunger does not block or restrict the oil feed at a preload height of 0.080".  (It shouldn't block the hole anywhere within the plunger's range of travel, but I would confirm this.)  If all looks good, normal wear over time (valve seat recession and valve face wear, primarily) should not create more than 0.005"-0.007" additional preload.

In the article section of the URL below, check out the illustrations of the two lifter cutaway views.  The plunger should be able to bottom without restricting the oil flow.  If the lifter plunger's base is floating 0.118" above this point, you should be okay:


Note: If the noise was a lifter bleeding down from low oil at idle, there would be measurable rocker arm-to-valve stem clearance.

Your check for piston skirt to cylinder wall clearance sounds on track...Let us know what you discover.


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