96' 4.0 Cherokee Misfire Help

TJ Starting

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As the title says, I'm fighting a pretty bad random cylinder misfire on my Cherokee. 1996 4.0 auto with high mileage.

Getting up on the highway the other evening on the way home, giving her the beans and got up to about 5k rpms. Oops. Couple seconds later the whole vehicle just kinda started trembling. So, had a exit for a small town coming up, pulled her over and popped the hood. Only visible issue was the dipstick was unseated from the tube and it looked like it had puked some oil out. So I reseated it and babied it the last couple miles home.

Check engine light had been on because the driveshaft had somehow caught the wires for the downstream o2 sensor and I was waiting on new one in the mail. Plugged the obd scanner in and besides the existing o2 code, we've got random cylinder misfire and then random cylinder misfire for all the cylinders. Symptoms are, high idles at about 1500 rpms in drive. Once I put it in drive it will almost stall until you give it gas and start moving. Coming up to a stop sign, almost stall when you stop. When driving and its low in the rpm range, a lot of shaking. Also a loss of power, I think you get the idea.

So both o2 sensors are now replaced with NTK. Also things that have been replaced lately are, spark plugs, distributer cap and rotor, and ignition coil. Also removed the exhaust manifold and welded the cracks up since this has happened.

Any ideas??
 
Well, I'd rule out a cracked head and anything serious like that with a compression and leak down test.
 
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Well, I'd rule out a cracked head and anything serious like that with a compression and leak down test.
Ok, I had wondered about that to and I'm planning on doing that this evening if I have time. But would I have some issues with overheating, coolant loss and such if that was the case? Or not necessarily? I have been running it some just trying to figure it out and I haven't had issues with temp or coolant.
 
Ok, I had wondered about that to and I'm planning on doing that this evening if I have time. But would I have some issues with overheating, coolant loss and such if that was the case? Or not necessarily? I have been running it some just trying to figure it out and I haven't had issues with temp or coolant.
Not all head or head gasket issue impact the cooling system. Depends on the location of the crack or where the head gasket failed. With the dipstick blown out of the tube you could be seeing a lot of crankcase pressure from some bad rings or a damaged piston. Compression test will narrow things down. You can rent (borrow) a tester from most auto part stores.
 
Ok, just doing the compression test and bad news. We've got zero on #6. 😶

Otherwise got...
#1 160
#2 170
#3 175
#4 170
#5 175
This was all just dry test.
 
20210126_213154.jpg

Spark plug from 6. This is about a month old autolite XP985.
 
View attachment 221389
Spark plug from 6. This is about a month old autolite XP985.

Time to pull the head and take a look around to see what went wrong. Sounds like you lost a piston or a valve. Maybe both. You can rig up a nipple and put compressed air into number 6 while at TDC to see where the compression is going if you want to do that first. If it comes out of the intake, its the intake valve, exhaust its the exhaust valve, dipstick tube its the piston. More or less anyway.
 
Time to pull the head and take a look around to see what went wrong. Sounds like you lost a piston or a valve. Maybe both. You can rig up a nipple and put compressed air into number 6 while at TDC to see where the compression is going if you want to do that first. If it comes out of the intake, its the intake valve, exhaust its the exhaust valve, dipstick tube its the piston. More or less anyway.
Good idea and thanks for the reply. Sounds like I have another project for tomorrow evening.
 
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Time to pull the head and take a look around to see what went wrong. Sounds like you lost a piston or a valve. Maybe both. You can rig up a nipple and put compressed air into number 6 while at TDC to see where the compression is going if you want to do that first. If it comes out of the intake, its the intake valve, exhaust its the exhaust valve, dipstick tube its the piston. More or less anyway.
This^

My bet is with the dipstick tube coming out it's a piston issue.
 
Alright, this one really got me weirded. I hooked the air compressor up and put 110psi to it. Holds pressure just fine. Even unhooked the air and cranked it a second then hooked it up again trying to get the piston at a different spot. Did that multiple times. No dice. So hooked the compression tester back up and no compression.

Any thoughts? Give it more pressure or something else? Or just pull the head?
 
Ok, forget my last post, found a little bugger skewing the test results.
20210127_184418.jpg

Air coming out of dipstick tube and PCV.
 
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Ok, forget my last post, found a little bugger skewing the test results.
View attachment 221566
Air coming out of dipstick tube and PCV.
Either a ring broke, the piston skirt came apart, or the rare chance you burned a hole in the piston. I’d pull the head just to see what happened and start shopping for an engine.

Sucks to hear it. Keep us posted.
 
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Planning on pulling the head when I get a chance but not rebuildable?
Maybe, have to see what is in there. See what the cylinder wall and head looks like and decide what to do next. Most people seem to grab a known good used engine, or a reman over machining and rebuilding. It cost a lot to do it right. Reman engines are not much more money and carry a warranty.
 
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Planning on pulling the head when I get a chance but not rebuildable?
Depends what is damaged. I know a guy locally who had a ZJ 4.0L with a broken piston. It didn't damage the bore somehow (was knocking bad but no scoring) so he pulled the piston out, threw a new one in and drove it for another 50k miles until the transmission died.
 
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Well I was planning on selling it for sure by the end of this year. Its just my dairy with high miles. Only about 5 miles round trip to work, so not really terribly worried with the longevity if I get it rebuilt.
 
Thanks again guys.
My first time doing engine work this in depth, but I replaced all six pistons and rings this last weekend finally. Also did the rear main seal while I was at it. Valve cover gasket, head gasket, and oil pan gasket. So no oil leaks anymore either at the moment.

Its had a lifter tick on cold startup ever since I've had so retorqueing the rockers when reinstalling them solved that as well. Killing 10 birds with one stone, my oh my. Runs better then it did before the incident.
Note...mechanic my dad knows was over when I had the head off and said that cylinder walls look to be in amazing shape still so maybe itll be good for another 100k.
 
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