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Hello all!

Hoping you folks can help me figure out my problem with a 4.6 stroker that I've built.  After the first assembly there were no issues with the break-in process which followed the cam manufacturers directions using comp cams break-in oil and lube.  I did the normal easy break-in driving for about 50-75 miles around town and then drove it to work, 50 miles one way.  When I started it up after work it was knocking.  The knock doesn't go away with time or heat, it gets quieter during running but slowing to idle it's there every time.  I got it home and immediately tore it back down to find the #1 piston slapping and scoring the one side of the cylinder wall.  I pulled the piston and found it somewhat tight at the wrist pin.  I ordered a new piston due to the damage and honed the cylinder wall to clean up the scoring.  The second assembly was fine and I did an abbreviated break-in again and all seemed well.  I drove around for about 100 miles around town for a few days with no issues.  Starting it back up the next morning though it was knocking again immediately.  Tearing it down once again I found the exact same thing with #1, slightly tight wrist pin and all.  None of the other pistons show any issues.  I've replaced the piston once more, installed new rings and new rod bearings but I haven't reassembled the rest of the engine.  The first rod bearing set had the oil hole but the second set did not.  This last set didn't either, but I cut a groove using a dremel and cleaned it up just to get the extra oil.  I went to a different machine shop following some recommendations that the shop installing the piston was causing possible issues.  The new shop pin fit the new piston.  The new shop is an old school engine machine shop and he offered me two possible causes.....air bubble trapped around the cylinder causing elevated temperatures, and too much fuel causing elevated temperatures and possible fuel wash into the cylinder.  I went home and pressure tested the fuel rail.  The injectors are holding and the spray pattern is good.  I also run a wideband O2 sensor and I'm running between 14.5 and 15.1 on the gauge.  He recommended pulling the thermostat on the initial running to clear out any possible air trapped around the cylinder, before reinstalling it.

What am I missing....?


1990 4.2 crank turned down .010/.010 with all new bearings, thrust is at .002, plastigauged all bearings at .002 - .0025

1994 Cherokee 4.0 block, 4.0 rods, block bored .030 over and magnafluxed/tanked

Quench is .062 using .040 head gasket

New Icon ICC 944 pistons .030 over, verified at 3.902, new rings, down .022 in the bore

Comp cams 68-231-4 cam and lifters

Bosch 24lb fuel injectors, cleaned, checked and verified to be working normally

Cylinder bores are 3.9045 and pistons are 3.902, .0025 clearance per manufacturer

190 degree thermostat

Thanks in advance for any help everyone!

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Hi, Raggedyman...First thought was lubrication or coolant, local overheat as suspected by the 2nd machine shop.  Assuming that the piston-to-deck height is correct (the right pistons for the rods and deck height), I thought of a local overheat in the #1 cylinder.

You do not talk about the engine's drive belt configuration.  Do you have a water pump that is rotating in the wrong direction for its impeller design?  Serpentine belt OE layout calls for a "reverse rotation" impeller on the water pump.  V-belt drive calls for a traditional rotation impeller.

#1 cylinder is the hottest running cylinder on an inline six (contrary to what you might imagine).  The cylinder receives the hottest circulating coolant.  If the water pump's impeller is incorrect, there may be sufficient cooling for the other cylinders and overall temperature control, yet there could be a local overheat at #1 cylinder.  (An infrared block surface temp test when the engine is warm is helpful for measuring individual cylinder and head temperatures.)  Block obstructions or the wrong head gasket/coolant port alignment can be another issue.  4.0L and 4.2L heads have different coolant passageways.  Are you using a 4.0L head and head gasket on the 4.0L block?

Also, piston-to-wall cylinder clearance must match the piston manufacturer's requirements, which you share that it does.  Forged pistons call for more clearance than cast or hypereutectic pistons.  Verify actual piston clearance at #1 cylinder.  Keep in mind that pistons are cam ground, not concentric, but actually wider diameter at the piston pin bosses.  Measure clearance 90-degrees from the pin bosses, this area/diameter expands with engine heat.  Even if the cooling system works properly, #1 cylinder would naturally be more sensitive to piston expansion than the other cylinders, and you share that there were signs of skirt drag.  You may have the wrong piston-to-wall clearance.  Measure the piston diameter 90-degrees from the piston pin axis. 

Note:  The rods must be hung correctly on the piston and installed in the correct block direction.  On engines with a connecting rod oil squirt hole at the big end or through the shank to the piston pin, the crankshaft's rod bearing oil hole must align properly.  The insert bearing's hole must match the oil hole in the rod; the rod and piston must each face the correct way in the block.

The other concern is piston ring gap.  Some ring sets, in particular chrome rings, require end gapping.  This involves a ring file (hand operated design or power driven).  Check the piston ring gaps in the bore before installing rings on the piston.  The oil expander must be carefully installed so that its rails have a normal end gap.  Ring gap specifications should be available from the piston and/or ring manufacturer.

This is the place to start looking...We can go from here.


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Thanks for taking the time to respond, I really appreciate it!  I was running the cherokee water pump, which is reverse rotation, and using the same serpentine belt configuration normal for the engine.  I also had the fan in place offset from the center of the engine and running a separate electric fan using the fan control relay and wiring.  I checked the actuation of the fan controls and checked to be certain that it was a puller fan and not pushing....it's mounted on the engine side of the radiator.  It's the standard cherokee 4.0 engine/head and I'm using a fel pro 26211pt head gasket.  I compared it to the original gasket back when I first assembled the engine and the passages were the same.  On this rebuild, however, I've switched back to the wrangler water pump which is also reverse rotation.  I found a pulley from a grand cherokee that extends further towards the block and accommodates the longer shaft of the wrangler pump.

I've double checked the piston/wall clearance and piston diameter and they are within spec.  All pistons are correct in the bore and the oil hole is in the new rod bearing and aligned with the hole in the rod.

I installed new rings on this go around and checked the gaps for each and they are within spec.

I pulled the thermostat for the initial running and I'll put it back in as soon as I can, I just wanna make sure I don't develop an air pocket at that point.  I checked the angle of the engine and I'm sitting at 4 degrees high to the front, all things considered.  I was thinking of getting the rear of the engine higher than the front and pulling the coolant sensor in the back of the head to bleed off any air bubbles.

The engine is currently in a 1990 yj and I'm running all the stock gauges.  I rewired the engine harness and connector plug to match up to the yj bulkhead connector, and swapped in the yj oil pressure switch to run with the gauge.  I also rewired the emission malfunction indicator light to be my check engine light.  All things considered, when she runs she runs great.  All the gauges function normally, although my oil gauge is a little sketchy at times and requires a tap on the face.

I'll use my heat gun to check the block during the initial running to see if I can find any issues.

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Good concern, Belvedere, especially with skirt drag.  Rod throws should mic and Plastigage correctly as a final check.  In addition to checking rod bearing clearance, Plastigage used as specified can indicate rod tilt (i.e., non-parallel surfaces) between the crank journal and rod bearing.

Raggedyman, you have been very thorough and taken a sound approach.  The air pocket is a long shot, the YJ does not generally present a problem or require "burping" like the early XJ Cherokee 4.0L did!  I would, however, reverse flush the heater core as a precaution; this could eliminate a prime source for blockage and air blocks.

I look forward to your update on local heat readings with the infrared surface thermometer.  Iron is a good surface for accurate readings, you'll find that hose readings can be way off.  Stick to the metal points.  Compare each cylinder and its cylinder head counterpart.  Check oil pan temperature for possible signs of engine stress. Check the inlet versus outlet tube temperatures at the heater core.

Was this block bored with a torque plate in place?  Cylinder bores must be 90-degrees from the crankshaft center line; these blocks can distort without torque plate boring.  On that note, the 4.0L blocks during these particular years did have issues with core shift.  A 0.030" re-bore is not considered excessive, the engine reman industry uses this as the common re-bore size for cores.  For future reference, though, it is often valuable to have the cylinder walls sonic tested during machine work to assure adequate cylinder jacket casting thickness and cooling efficiency.




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1 hour ago, Moses Ludel said:

Raggedyman, you have been very thorough and taken a sound approach.  The air pocket is a long shot, the YJ does not generally present a problem or require "burping" like the early XJ Cherokee 4.0L did!  I would, however, reverse flush the heater core as a precaution; this could eliminate a prime source for blockage and air blocks.

Hmmm, how early?  My build uses a 1994 cherokee block and head.

As far as I know the machine shop did use a torque plate, although I didn't specifically ask.  Thanks for all the good info, I'll definitely keep it all in mind.

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I think you pulling number one piston and rod assembly twice and having a stiff piston on pin situation both times is an indication of a problem right there. On top of that you have scuffing in that same bore as well. Scuffing is a form of micro welding that occurs with friction (heat) between two metal components in the absence of lubrication.

On a jeep six the oil pump is in the middle putting cylinders 1 and 6 furthest away, for some reason number one is not getting enough oil, question is why?

First up, what is the oil pressure when the engine is hot and at cruising rpm? if its 10-15 psi we've got a problem. Get a proper mechanical gauge mounted to monitor the pump pressure.

Second is there a restriction at number 1 is there a issue with the main bearing oil hole and upper bearing shell alignment. is the clearance on this bearing excessive?

Third maybe a bad cam bearing at that end dumping oil that should be going to the # 1 main.

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GoatBoy4570, good analysis and points!  Any oil restriction or starvation problem, as you describe, would show up as a damaged #1 cylinder rod bearing and/or scoring at the piston pin and cylinder wall.  Here is the "official" explanation of oil flow in a 4.0L inline six Jeep engine:

"OPERATION [Oiling and Lubrication System]

The pump draws oil through the screen and inlet
tube from the sump at the rear of the oil pan. The oil
is driven between the drive and idler gears and
pump body, then forced through the outlet to the
block. An oil gallery in the block channels the oil to
the inlet side of the full flow oil filter. After passing
through the filter element, the oil passes from the
center outlet of the filter through an oil gallery that
channels the oil up to the main gallery which
extends the entire length of the block.

Galleries extend downward from the main oil gallery
to the upper shell of each main bearing. The
crankshaft is drilled internally to pass oil from the
main bearing journals (except number 4 main bearing
journal) to the connecting rod journals. Each connecting
rod bearing cap has a small squirt hole, oil
passes through the squirt hole and is thrown off as
the rod rotates. This oil throwoff lubricates the camshaft
lobes, distributor drive gear, cylinder walls, and
piston pins.

The hydraulic valve tappets receive oil directly
from the main oil gallery. Oil is provided to the camshaft
bearing through galleries. The front camshaft
bearing journal passes oil through the camshaft
sprocket to the timing chain. Oil drains back to the
oil pan under the number one main bearing cap.
The oil supply for the rocker arms and bridged
pivot assemblies is provided by the hydraulic valve
tappets which pass oil through hollow push rods to a
hole in the corresponding rocker arm. Oil from the
rocker arm lubricates the valve train components,
then passes down through the push rod guide holes
in the cylinder head past the valve tappet area, and
returns to the oil pan."

Note my red highlighting of the oil flow from the main gallery to the crankshaft, rods, mains and camshaft bearings.  Scoring on the rod bearing and/or piston pin scoring or seizure would be a clear sign of oil starvation or restriction.  A rod bearing or main bearing oil hole restriction or misalignment could inhibit oil pressure locally—like at #1 rod if that's the only trouble spot.  

Also note the connecting rod bearing cap squirt hole reference to throw-off lube, which lubricates the camshaft lobes, distributor drive gear, cylinder walls and piston pins.  There must be proper rod bearing alignment, as well as an unrestricted oil hole in the rod bearing shell, to allow pressurized oil from the rod journal to splash against the cylinder wall and piston pin!

First, as GoatBoy4570 notes, there must be sufficient oil volume and pressure at each main bearing and also through the crankshaft passageways to the rod journals and bearings.  Then the rod bearings must deliver throw-off oil to the cylinder walls, piston pins and camshaft lobes.  Also, if main bearing holes do not align with the gallery feeds, the crankshaft will not receive adequate oil flow to each connecting rod.

I also believe it's worthwhile to confirm, as GoatBoy4570 has suggested, that there is sufficient oil pressure near the front of the engine.  Use of a mechanical gauge, tapped into the OEM gauge port, would provide a reasonable read.  As GoatBoy4570 notes, oil pressure and volume loss might first occur at the front of the engine when the oil pump location is at the middle or rear of the engine block.  Is the oil pickup screen setting near the floor of the oil pan?  If the screen sets too high, the pump can dry sump (starve) or create cavitation, especially at higher engine speeds.  The front end of the engine could be the first area to suffer.  You need to use a 1991-up 4.0L oil filter adapter pipe and oil filter type.

I'm guessing that you're running the stock 4.0L oil filter adapter and filter?  In the '94 4.0L, the 4.0L oil filter's built-in bypass takes the place of the older block-mounted bypass mechanism found in 1990-back 4.2L/258 and 232 engines.  The 4.0L's filter attaches to the block via a pipe adapter.

As for core shift, your engine year is among the models that can have the issue, though this does not mean you have the problem.  Let's not "borrow trouble" for now, especially with a moderate 0.030" common re-bore.


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The lack of lubrication was my initial diagnosis as well.  With the oil rings scraping oil from the cylinder walls to lubricate the wrist pin I immediately started there.  The inside of the piston shows a dark spot indicative of oil burning, but I blamed that on the heat from the scoring since it was only on that side of the piston.  The second set of rod bearings did not have the oil hole but the general consensus was that it was not needed and some bearing manufacturers have stopped doing it.  I have added that oil hole on this set of bearings.

I am running the stock 4.0 oil filter adapter and filter, but I swapped in a melling high volume oil pump.  Due to the size of the pump, I had to clearance the oil pan.  I used a 3/8" drill bit coated with grease when I was fitting the pan to get a good amount of clearance around the pickup screen.  I had also read of others having issues with the fitment of the pump to the block so I made sure that wasn't happening.  I'm using the stock yj gauge and my idle pressure is 40-42psi and running is 58-60psi.  However, the gauge can get sketchy as times so I'll try getting a mechanical gauge to double check.

My rod bearings don't show any bad wear on any of the sets I pulled, and each time my clearance was between .002-.0025" using plastigauge strips.  I don't see any indication of heat around the cam bearings and I did double check their alignment in the block.  However, I did not pull the main bearing to check it on any of the teardowns.  I'll try that as well just to make sure.

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Everything you describe about the oiling system and clearanses says solid build, good to go. The Melling pump spec`s perfect regardless of sketchy gauge.

This is starting ta point big time to running lean on that first hole, that burnt spot on the back side of the piston is evidence, this piston is soaking up so much heat oil can't cling to it.

Get out your non contact thermal temp gun and compare temps across your cylinders shootin the header tubes close to the port, try different rpms, open and closed loop, hot and cold, the injector or the wire to it, something has to be amiss. If number 1 tube is hotter than the rest, that's lean. Try swapping that injector to another cylinder. or if you have a set from another motor swap em all out. is that intake runner loose at the head, is air getting in somewhere to make that hole go lean? Is number 1 injector at the end of the line on the fuel rail, do you enough fuel pressure?

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Thanks for all the info, I'll definitely check into the fuel side as I put it all back together.  The injectors were bought off ebay, but from a supposedly good site that cleans and tests each set.  I was thinking on swapping the injector but if it is bad then I'm just going to be repairing another piston....but at least it'll tell me what's wrong.  I was also going to rig up a noid tester using some led's to check the signals coming from the ecu.  I did pressurize the fuel rail and check the injectors and the spray patterns look good with no leaks.

Number 1 injector is at the feed/return side of the fuel rail and I was initially running the fuel pressure at 30psi.  If I remove the vacuum hose to the fuel regulator I can get the pressure up to 40psi.

Perhaps that's part of my problem from the start.  Maybe I'm running too low of fuel pressure.  Current build theory, from what all I've read, needs a higher fuel pressure for the 24lb injectors.  It just doesn't make sense that I'm only having this issue on one cylinder.

My heat gun maxes out at 500 degrees so I'll try to find a different one that reads higher.

The ecu is just a junkyard ecu, so I could have issues stemming from that even though I get no codes.  I have a second ecu but I think it was from a grand cherokee with the 4.0.  I'll check the pinouts and see what I can find out.

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Retinking my last post, trying to nail down a lean cylinder under load is pretty tricky, you've got exhaust temp and reading the plugs to go by. reading exhaust temp you'd have to rig up a pyrometer and move from pipe to pipe, mount the gauge on the hood, bomb down the same stretch of road at the same speed, you get my drift (: not the most practical.

Im thinking a lean injector may not show up idling or revved up in the driveway but if you have a cooler pipe at idle that could be an indication.

If you unplug (electrically) each injector and see if number 1 makes little difference that could be an indicator.

I had a TPI once were i had a couple cooler cylinders and i traced it to bad connections at the injector, one as i recall, i had to solder a new connector on it was so bad.

The ECU generally by design are pretty bullet prof, but they do go bad, had em where the motor starting and running had issues and swapping out the ecu fixed it.....or at least made it better so other issues could be found too.

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Raggedyman...I'm unclear what exhaust manifold or header you're running.  A key concern is the location of the oxygen sensor from the cylinder head ports and where the O2 sensor picks up the A/F (oxygen content) reading.  The type of O2 sensor, its orientation on the header or manifold, and whether it is a heated or non-heated type would each affect the A/F.  Fuel pressure and fuel volume at the rail is also critical to flow/volume as you note.

The idea of using an IF heat probe at the exhaust manifold or, ideally, at header ports/tubes is excellent.  As GoatBoy4570 notes, this could mimic the results of a pyrometer for comparing per cylinder heat and the approximate A/F ratio.


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I'm hoping to use a noid tester to check the firing of the injectors at the connectors.  The wiring harness was also taken from a cherokee and various pieces were added/removed to make it work with my YJ.  That said, I really didn't touch the injector section so I didn't take a close look at the connectors other than making sure the grounds were good.  I also plan on using a stethoscope to listen to the injectors as they fire...maybe I can catch something.

The header is a stock wrangler header with a stock O2 sensor in the normal position.  I drilled and added another bung about 4 inches further down the pipe and I'm running an older AEM wideband.  In truth it's about 8 years old but the readings seem to follow along pretty good with WOT and release so I'm only assuming it's reading fine.  I did check the temps with my handheld heat gun but they climb higher than 500 degrees at the first bend and my heat gun won't go any higher.  I did not, however, check the other side of the block.  I'll be sure to check that this time.

My plugs never showed any signs of detonation or lean.....they were all a nice tan/brown coloring.  

I was thinking of testing the oiling by submerging the pickup in a bath and priming the system again with the oil pan off.  I know it'll make a mess but I should be able to rig up a catch basin.  Think that would show me anything on the bottom side?

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Raggedyman...Sounds like the O2 sensor works okay, and I like your analysis of the spark plug coloration, an old failsafe diagnostic step.

On the oiling, keep in mind that there needs to be good seal from the block mating point to the pickup screen, the pump relies on low static pressure to draw oil without cavitation.  I've never had an issue with a Melling High Volume pump and always use one, even on the Jeep/Buick V-6 engines and AMC V-8s with timing cover pump housings.  You're not experiencing any issues with bearing scuff or scoring, and that's a good sign of available oil.

See my notes in red regarding oil from the rod squirt hole to the cylinder wall, piston pin and camshaft lobes.  This oil is both lubrication and cooling for these areas.  Most of the engine is oil cooled, the only real "coolant" relief is around the exhaust valve seats and the cylinder jackets.  Oil cooling is the only method for the crankshaft, bearings, rods, pins, pistons, camshaft/valve train and timing chain set.  That's a bunch!


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  • 2 weeks later...


The engine is back together and I have approximately 30 minutes of idle time in the garage and about 25 miles on it so far.  It still runs very strong and I have no doubt it'll blow my '01 4.0l away easily.  

I have installed the thermostat and checked, and rechecked, the coolant level and so far so good.  I replaced the cherokee water pump with a stock '94 wrangler water pump and used a belt pulley from an '00 grand cherokee to run the belt correctly since the wrangler pump sticks further out and the stock wrangler pulley doesn't go far enough back towards the engine to safely run the belt.  I didn't have a wrangler mechanical fan but I do have the cherokee mechanical fan, and it bolts up just fine.  It pulls plenty of air and fits inside the shroud easily since it's a couple of inches smaller in diameter.  I feel much more comfortable using the mechanical fan in it's normal position instead of offset like the cherokee does.

The temperatures on the passenger side of the block run about 5-8 degrees cooler around #1 than they do towards the middle 3-4-5.  #6 also seems to run a few degrees cooler than the middle.  I'm also a few degrees cooler on the head right at the plug on #1.  The exhaust header runs also seems to run much cooler on #1....approximately 10-40 degrees cooler in various spots.

I also replaced my sketchy oil pressure gauge and now it's steady around 65 while running and 45 while idling when it gets warmed up.  I'm running the 195 degree tstat but when my temp climbs up to nearly 210, it drops back to 195 very quickly and pretty much stays there.

More to come as the miles start adding on....

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Raggedyman...Recheck the coolant level for a few days, wait for a complete engine cool-down each time to allow siphoning from the recovery tank to the radiator.  Do not open the system until completely cooled down.

As the system purges, you may need to add more coolant at the radiator or recovery tank.  See if that stabilizes the temp—keeping it closer to thermostat temp and not the flare to 210 F.


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Mechanically your all in, solid oil pressure, small temp variations look normal. if your header tube temps are fairly consistent you can assume its fueling correctly. thermostat opens fully when she's good and warm. An engine is more efficient thermally when its run at 195.

So now you break er in a bit a keep your ears tuned to every nuance of how this thing is running.

Third times the charm!

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  • 8 months later...

i see this is old, but one other thing you could try would be to back the rear end in the ditch and jump on the bumper. I had a mustang with a 428 in it that liked this kind of attention. Worked best with it running. Thing needed burped.lol 

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