Help Diagnosing Engine Knock

bromel said:
All I can say is that I've now owned three Jeeps with the 4.0—a 95 XJ, a 99 WJ, and a 00 TJ—and each one makes the sound in varying amounts. I've always assumed it was the lifters bleeding down since the noise returns after a hot start, which in my mind eliminates a piston slap diagnosis. But, honestly, who freaking knows. One day I hope someone figures it out.
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Lol, for real. Some day!

Ok, maybe a dumb question for some, but what exactly does "lifters bleeding down" mean? I've seen that mentioned a few times but I honestly have no idea what that actually means, lol. What would I look for to see if that is happening in my TJ?

Thanks in advance!
 
The Goblin said:
Dang! All that and STILL got the noise? Sorry!

I checked the clearance of my bearings too and just like yours, they were all fine. Piston slap is really starting to be my main suspicion. It's super hard to diagnose. Have you checked your exhaust manifold for cracks by chance? I'm still suspicious of that for mine because of the way the noise quiets down as things heat up.
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I don't think a cracked exhaust manifold is causing this noise.

I just replaced my exhaust manifold with a genuine Mopar unit and the cold idle noise is still there and is the same as before. And it still goes away after the engine warms up. The new exhaust manifold *did* however make the ticking noise under acceleration completely go away.
 
The Goblin said:
Ok, maybe a dumb question for some, but what exactly does "lifters bleeding down"
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https://en.wikipedia.org/wiki/Hydraulic_tappet
Under Disadvantages, "There are a number of potential problems with hydraulic lifters. Frequently, the valvetrain will rattle loudly on startup due to oil draining from the lifters when the vehicle is parked. This is not considered a significant issue provided the noise disappears within a couple of minutes; typically it usually lasts only a second or two. A rattle that does not go away can indicate a blocked oil feed, or that one or more of the lifters has collapsed due to wear and is no longer opening its valve fully. The affected lifter should be replaced in the latter situation."

The part in this description that says oil draining from the lifters is what people mean by bleed down.
 
pagrey said:
https://en.wikipedia.org/wiki/Hydraulic_tappet
Under Disadvantages, "There are a number of potential problems with hydraulic lifters. Frequently, the valvetrain will rattle loudly on startup due to oil draining from the lifters when the vehicle is parked. This is not considered a significant issue provided the noise disappears within a couple of minutes; typically it usually lasts only a second or two. A rattle that does not go away can indicate a blocked oil feed, or that one or more of the lifters has collapsed due to wear and is no longer opening its valve fully. The affected lifter should be replaced in the latter situation."

The part in this description that says oil draining from the lifters is what people mean by bleed down.
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Oh dang! This was SUPER helpful!

Now I really need to check my lifters... Thanks a ton!
 
The Goblin said:
Hey everyone! I am brand new to your forum here and pretty new to the Jeep world overall (just a few months). I picked up my 2002 TJ 4.0L automatic earlier this year and have spent quite a bit of money and time working on it. It only has just over 78,000 miles.

The engine had a ticking noise from the front/top of the engine when I bought it, but after treating the oil with Sea Foam, changing the oil and adding Lucas, changing the alternator and serpentine belt, the ticking went away.

BUT, after clearing up that ticking, I was able to hear a new noise that I couldn't hear before: a knocking coming from underneath the engine. It sounds like it is coming from the back of the oil pan/flex plate area. I have done a ton of research online and posted some things over on Jeep Forum, but I wanted to post here to see if my fellow TJ owners would have some more specific thoughts. I've read a ton of posts, many here, but I'm still not confident on what this sound could be.

Various shops have looked at it while doing other things for me (changing gears, assessing a transmission problem I'm working on) and they all seem to have different opinions like piston slap, main bearings, rod bearings.

The sound is not there when I first turn on the engine, but it shows up after about 60 seconds. It goes away when the engine is under load or when the RPMs get up to normal driving range. The oil pressure seems to stay constant.

If it was a main bearing or rod bearing, wouldn't the oil pressure be low?

I have checked, removed, and re-installed the torque-converter bolts and that didn't change things.

I'm really leaning towards a cracked flex-plate, but for some reason these shops I've been to don't seem to think that's it. I've watched a ton of videos on youtube about rod knock and they all seem to be very loud and audible from the top of the engine; mine you can't even hardly hear unless you stick your head underneath the Jeep (where it's pretty noticeable). I found a series of videos from a guy with an older Cherokee and the sound in his video I swear sounds a lot like mine and he ended up changing his flex plate.

I did notice a possible exhaust leak when I was looking around with a flashlight under the engine. Looks like it was just above the cats.

So, any advice guys? Any way to rule out bearing knocks or flex plate issues? Could it actually be an exhaust leak? Anyone else experience this?

This is it:
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Sounds like a loose timing chain
 
I found someone on YouTube (who I think is on this thread) with this exact noise and directed the comments to this thread.

I have this exact same noise on my '01 TJ 4.0L 3 Spd and I posted about it on here a while ago: https://wranglertjforum.com/threads...er-2001-tj-123k-miles-32rh.55755/#post-982077

From what I have learned from local mechanics, this is likely a lifter or piston slap and to basically just ignore it til the engine needs to rebuild. I think I am going to try an oil change with Engine Restore and see how it goes. @ 125k by the way.

Has anyone had any luck with that?
 
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Gibsonfirebird12 said:
I found someone on YouTube (who I think is on this thread) with this exact noise and directed the comments to this thread.

I have this exact same noise on my '01 TJ 4.0L 3 Spd and I posted about it on here a while ago: https://wranglertjforum.com/threads...er-2001-tj-123k-miles-32rh.55755/#post-982077

From what I have learned from local mechanics, this is likely a lifter or piston slap and to basically just ignore it til the engine needs to rebuild. I think I am going to try an oil change with Engine Restore and see how it goes. @ 125k by the way.

Has anyone had any luck with that?
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Piston slap seems about right. Im a couple thousand miles shy of 200k, and nothing that Ive tried has done anything to change the sound. It does get much quieter, almost silent, when its hot.

Drive it till it dies. The engines are cheap enough and in supply, so there's no need to worry.
 
I'm not so sure it's piston slap. My understanding of that phenomenon is that the cold piston rattles around in the cylinder until the metals heat up, expand, and take up the tolerances. Then the noise disappears. The noise on this thread, which every 4.0 I've ever owned has to some degree, starts out silent. It appears after 30 seconds or so and then goes away when the engine is fully warmed up. If I turn the engine off and restart it a few minutes later, the knocking noise is back for 30 seconds or so and then it goes away. The pistons at this point are still hot, so piston slap makes no sense. I still think it's the lifters bleeding down.
 
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bromel said:
I'm not so sure it's piston slap. My understanding of that phenomenon is that the cold piston rattles around in the cylinder until the metals heat up, expand, and take up the tolerances. Then the noise disappears. The noise on this thread, which every 4.0 I've ever owned has to some degree, starts out silent. It appears after 30 seconds or so and then goes away when the engine is fully warmed up. If I turn the engine off and restart it a few minutes later, the knocking noise is back for 30 seconds or so and then it goes away. The pistons at this point are still hot, so piston slap makes no sense. I still think it's the lifters bleeding down.
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I still hear the noise even when hot, for what it's worth. I barely hear it while driving, but if I am at a stop it is still very present. I follow your logic though.

I am also hearing the noise coming up through my auto shifter. I thought it might've been my transmission but my mechanic confirmed that I'm hearing harmonics from the engine. So, I hear the noise very loudly when I am in gear and at a stop.
 
Honestly I have no idea. Some guy on this thread replaced all the lifters and it still makes the sound. The OP replaced the bearings and I think the timing chain. Still no difference. I've also heard cracked piston skirt and loose wrist pins. As others have said, I just stop thinking about it and hope for the best.
 
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bromel said:
Honestly I have no idea. Some guy on this thread replaced all the lifters and it still makes the sound. The OP replaced the bearings and I think the timing chain. Still no difference. I've also heard cracked piston skirt and loose wrist pins. As others have said, I just stop thinking about it and hope for the best.
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It's probably me you are referring to. I've changed the cam, lifters, push rods, rockers, timing set, oil pump and the sound remains (I didn'tchange this stuff chasing the cause of the noise but was hoping it would alleviate it). I've checked main bearing clearance and they're all in specification. I did not inspect connecting rod bearing clearance, if I recall correctly.

You've explained the issue nearly exactly how it is with my Jeep. I drive the Jeep everyday and the sound hasn't changed for the better or worse. I do have a sound when depressing the clutch to shift up that sounds like a power steering pump whine. The previous owner did have a steering gear leak and frequently ran the pump low on fluid so it may very well be the cause of the noise but sometimes I wonder if that noise is related to the startup noise.
 
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After a complete rebuild at 126k because of this sound, it had a cracked #5 piston and all the main crank bearings completely worn out. And still running strong.
But much longer, it would have been a boom.
Trust me, drop the oil pan and inspect.
Best case scenario it looks good and you get a new oil pan seal. Probably rear main if needed lol.
Just drop it. If you can't do it, have a mechanic do it.
 
I was going through pictures and videos of my Jeep and found a video showing the timing chain slop. It had 1/2" of movement. The service manual reads to replace the set if there is 1/4" or more of play. I did change the timing set and the noise did not get any better. Just wanted to share the video.

 
My noise did go away and I have a video too. It's easy to check, saying it didn't fix your problem doesn't mean it wont fix another Jeep. You can test it in two minutes, not checking because of either of the videos is silly.

 
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pagrey said:
My noise did go away and I have a video too. It's easy to check, saying it didn't fix your problem doesn't mean it wont fix another Jeep. You can test it in two minutes, not checking because of either of the videos is silly.

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I think the OP changed his, and the noise remained.

Harcust says he changed the cam, lifters, push rods, rockers, timing set, and oil pump on his Jeep and the noise remained.

MANY others have said theirs made this noise at 50K miles, and still runs great at 150K miles - and the noise didn't change.

That is what makes this noise a mystery.
 
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