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Discussion Starter #1
Hey guys. Was just wondering how many miles should a jeep engine be able to go before a rebuild. My jeep yj 4.0 5 speed is at 179,000 miles and its runs very smooth except for one thing. There is a slight little bit of knocking coming from the engine at idle. It's got a good painted frame. Idk how long the clutch has been in but it feels okay.
 

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A knock at idle could be anything from a loose timing chain to a cracked piston skirt.
If you rev the engine does the knock become more noticeable in the time between accelerating and decelerating?

Perhaps you should have a mechanic listen to the noise and ask his opinion.
Listening to a video is difficult to discern where the noise is coming from, but if you post a video, someone may have an idea.

My 4.2 has close to 200K miles and runs quite well, considering the number of miles. It's not perfect, but it's OK for now. Some Jeepers with XJ's with 4.0 engines report 300K miles.

Good Luck, L.M.
 

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I'm at about 275k so far... been beating it REALLY hard for the last 100ish k of that.
 

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Btw your 'knocking' is likely the lifter noise than 90% of 4.0's make
 

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Hey guys. Was just wondering how many miles should a jeep engine be able to go before a rebuild. My jeep yj 4.0 5 speed is at 179,000 miles and its runs very smooth except for one thing. There is a slight little bit of knocking coming from the engine at idle. It's got a good painted frame. Idk how long the clutch has been in but it feels okay.
I am close to 150k on mine, same question as you. I have the same "knocking" noise at idle. Nothing real bad, I am sure it is not piston skirt. Runs fine and same as you on clutch. I assume mine is original.

As Gottagofast said, maybe lifters? I am not sure. Even worth putting in a new set?

Zero leaks here as well.
 

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On a 4.2 or 4.0 you need to pull the head to change the lifters.
On a 2.5 a person can change the lifters without pulling the head. Some posters recommend a zinc additive to the oil for the first few hundred miles after changing the lifters as a wear in protection for our flat tappet cams..

I had an occasional lifter clack upon a cold start. It always went away quickly as oil pressure built and the oil warmed.
A couple treatments with Seafoam cured it completely. That was 4 years ago. Regular oil and filter changes help to keep it gone. YMMV.

Good Luck, L.M.
 

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So was visiting with my local independent Jeep shop this morning and the Owner took me out back to show me a YJ he just "inherited". He got it from the original owner who bought it brand new in 1994. It has recently stopped running. All original, but you could tell, not well cared for Jeep. Come to find out, the guy NEVER, not once, NOT EVER changed the oil. He just added oil as he went. 197k on the odometer. Pretty good testament to that 4.0!!!
 

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So was visiting with my local independent Jeep shop this morning and the Owner took me out back to show me a YJ he just "inherited". He got it from the original owner who bought it brand new in 1994. It has recently stopped running. All original, but you could tell, not well cared for Jeep. Come to find out, the guy NEVER, not once, NOT EVER changed the oil. He just added oil as he went. 197k on the odometer. Pretty good testament to that 4.0!!!
A couple cans of Seafoam done over 4 oil changes and it'll be as good as new (if the mechanic can get it running).

Good Luck, L.M.
 

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Discussion Starter #11
I will post a video of the sound, but it is likely that it could just be lifter noise like you guys say. It is very noticeable at startup and kinda noticeable at idle. It goes away if you rev it usually, but there are instances when I am going uphill and I'm on the throttle for a long time that it comes back.
 

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I will post a video of the sound, but it is likely that it could just be lifter noise like you guys say. It is very noticeable at startup and kinda noticeable at idle. It goes away if you rev it usually, but there are instances when I am going uphill and I'm on the throttle for a long time that it comes back.
Exhaust leaks can sound a whole lot like lifter tap. Feel all around the exhaust manifold while it's running, especially the rear cylinder.
 

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Exhaust leaks can sound a whole lot like lifter tap. Feel all around the exhaust manifold while it's running, (as soon as you start it w a cold engine)especially the rear cylinder.
^edit

Also a rubber hose a few feet long... one end by your ear and the other moved around the engine can help pinpoint sounds...
 

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Discussion Starter #14
Thank you for the feedback! I just did the rubber hose thing and there is a super small pinhole in the exhaust manifold where the manifold attaches to the exhaust pipe. Made a me a but angry because I just put in a new muffler and there is almost no rust on the entire underside of the jeep😂
 

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Discussion Starter #15
In probably just gonna leave it be unless you guys have any easy fixes because it not really hurting anything right?
 

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When you get ready to fix it just find the best price you can on a dorman manifold.... none of the high dollar units are any better.
 
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