Engine Knock

Wayne R

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Jul 19, 2022
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Virginia
Out of ideas and need help. I have a 1992 Wrangler YJ 4.0 that after sitting for about 7 years, I cannot find the engine knock cause. It sounds like a knock coming out of #2 cylinder. The cause is not from push rods, rocker arms, lifters, rod bearing, main bearing, no vacuum leak, bad sensor, and properly lubricates. It does not knock when first started in cold weather for about 30 seconds. It knocks 90% of the time- bad knock. While troubleshooting I removed the valve cover to see if there might be a flattened cam lobe or bad valve, but when I took the cover off I could not hear it and everything appeared to be operating correctly. Nothing seems to be striking the valve cover. I feel like I have a either a piston slap or need a valve job. At this point if I have a valve job, it would be cheaper to buy a rebuilt head. While in there I could replace the pistons, but if I am wrong I just threw away money if it doesn't fix the problem. I have found a rebuilt engine that after returning the core would cost $1500 not counting installation costs. That's too much on my budget, but can't afford to be wrong. Has anyone else ran into this problem or have any ideas? Don't know enough about floating valves, but my dad used it to pull out a tractor that may have caused the problem.
 

Gilaguy23

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Apr 12, 2021
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Q town New Mexico
Broken skirt on a piston perhaps or a wrist pin knocking?. Have you dropped the pan to have a looksie? Somebody do a hack job rebuild replacing only 1 piston with some of amazons finest china shit parts?
 
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Thrasher4219

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Not a bad idea to drop the oil pan, of you haven't already, and take a peak up. Please keep us updated! Good luck!

 

ytradio

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Aug 24, 2021
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Just my opinion, but I’m not sure how you ruled out the rod and main bearings but that sounds to me like the most likely culprits. If it were mine, and remember it’s not, I would put a heavier oil in it than whatever you are running now and see if it takes a little longer for the knock to start. That would indicate (to me) to start on the bottom end ( crank shaft). Again just my opinion.
 
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Wayne R

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Jul 19, 2022
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Thanks for all the replies! I ruled out rod and main bearing as being the cause due to replacing them. From the way everything acted, it seemed to be the likely cause. The one main bearing had a deep dig (#5 position), but did no damage to the crank shaft. For the most part the rod bearings looked good when I changed them, a little wear. One strange thing happened when I tried to roll the #2 piston/position main bearing (same piston that I believe has the loud knock) the new bearing got stuck on something and would not roll in. I rolled the old bearing back in with no problem. The crank shaft looked fine, and I used plastigage to measure tolerance and all was correct. I did use heavier oil, but it had no effect. When I had the oil pan off to replace the bearings, I did look at the pistons as good as possible, and there didn't seem to be damage. When the rod bearings were off I tried to feel if there was any play in the pistons and there didn't seem to be. Also, I agree with the piston slap going away as the engine warms, that's why I am at a loss. When it is cold it takes awhile to start knocking, and when I took it into the woods where it was cool it wasn't as loud. I feel like it is broken piston skirt or valve problem, but once again don't want to make expensive repairs without picking people's brain for input because I have hit a wall. By the way... the engine only has 72K on it, but it seems like someone replaced it before I bought it which was 19 years ago. Not a hint of problems up until now. The engine does lack power. There was a small amount of shavings in the oil pan.
 

Gilaguy23

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Apr 12, 2021
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I have a 5.3 silverado with the infamous piston slap. I find it odd it starts as quiet as can be and runs for 15 seconds or so then the slap sets in, gets louder and after maybe 60 secs its gone. If you found shavings in the pan id say something is going south it would seem. Hows the oil pressure,hot cold and at idle?
 
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Wayne R

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Jul 19, 2022
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Virginia
That's what has me confused. It starts with no knock, then it begins after a few seconds, and it's worse in the warm weather. The oil pressure is fine. It seems like I'll have to get a valve job done and while I'm in the replace the pistons. Then I have to ask myself if I should replace the cam because it might be a flattened cam lobe. Oh, by the way, it's not too expensive to replace the timing chain which appears to be loose. I start adding things up and it goes back to the original question of just cutting my losses and buying a rebuilt engine. Anyway, this one has got me stumped as to which way to turn on a limited budget. I hoped that someone here may have ran into the same situation. Thanks!
 
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T-Rex Jeepster

The Unoffendable
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Sep 13, 2021
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226
Location
Middle Tennessee
Thanks for all the replies! I ruled out rod and main bearing as being the cause due to replacing them. From the way everything acted, it seemed to be the likely cause. The one main bearing had a deep dig (#5 position), but did no damage to the crank shaft. For the most part the rod bearings looked good when I changed them, a little wear. One strange thing happened when I tried to roll the #2 piston/position main bearing (same piston that I believe has the loud knock) the new bearing got stuck on something and would not roll in. I rolled the old bearing back in with no problem. The crank shaft looked fine, and I used plastigage to measure tolerance and all was correct. I did use heavier oil, but it had no effect. When I had the oil pan off to replace the bearings, I did look at the pistons as good as possible, and there didn't seem to be damage. When the rod bearings were off I tried to feel if there was any play in the pistons and there didn't seem to be. Also, I agree with the piston slap going away as the engine warms, that's why I am at a loss. When it is cold it takes awhile to start knocking, and when I took it into the woods where it was cool it wasn't as loud. I feel like it is broken piston skirt or valve problem, but once again don't want to make expensive repairs without picking people's brain for input because I have hit a wall. By the way... the engine only has 72K on it, but it seems like someone replaced it before I bought it which was 19 years ago. Not a hint of problems up until now. The engine does lack power. There was a small amount of shavings in the oil pan.
This is 180 degrees out of what thermal reaction should be....