Help Diagnosing Engine Knock


The Goblin

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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:
 

bromel

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Extremely common 4.0 noise. It's the lifters. Replace them if you really want to, or just turn up the radio. My Cherokee made that noise from day one... now it has 255k.

People like to call that piston slap, but they don't know what piston slap is. You'll notice the noise goes away after a few minutes, once the engine is hot. If you turn the engine off, wait a few minutes, and restart, you'll hear the noise come back in earnest. That's because the hydraulic lifters have lost some oil pressure.
 
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pagrey

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It doesn't sound good. The oil pressure gauge on a Jeep is fake so you might have to hook up an external gauge. You'll probably have to pull the pan. Doesn't sound like my lifters but who knows? Good luck with it.

Edit: I've never heard a bad flex plate in person, it's possible but I really have no experience with that
 
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The Goblin

The Goblin

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Change your oil and see what it looks like.

Can do. Looking for anything in particular? It's only been a couple hundred miles since I last changed it and added the Lucas though.

Extremely common 4.0 noise. It's the lifters. Replace them if you really want to, or just turn up the radio. My Cherokee made that noise from day one... now it has 255k.

People like to call that piston slap, but they don't know what piston slap is. You'll notice the noise goes away after a few minutes, once the engine is hot. If you turn the engine off, wait a few minutes, and restart, you'll hear the noise come back in earnest. That's because the hydraulic lifters have lost some oil pressure.

Oh you think it may be the lifters? That's good to know. I haven't really done much research on that. I'll get it started and let it warm up like you said and see what happens with the sound. I'm pretty sure I've heard it after driving it around for a while though, but I will check it out. Thanks!

It doesn't sound good. The oil pressure gauge on a Jeep is fake so you might have to hook up an external gauge. You'll probably have to pull the pan. Doesn't sound like my lifters but who knows? Good luck with it.

Edit: I've never heard a bad flex plate in person, it's possible but I really have no experience with that

Lol! It's fake?? That's also good to know!! Frustrating, but good to know! I'm thinking about pulling the pan anyway and seeing what I can find out. I found a pretty good tutorial online about how to check and see if it's a rod bearing by pulling a spark plug and pushing down on the piston when it's sitting in its downward stroke (with the engine off, obviously). If the rod bearing is bad, the piston will drop a little bit if you push down on it with a socket extension, or something like that. Worth a try before pulling the pan

Thanks guys. I really appreciate your input and your advice. This Jeep community is really pretty dang awesome.

Any opinions on how to rule in/out a flex plate??

I will keep you posted on what I find!
 

Skylinessuck

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Lucas in the oil? That might be the problem right there lol. That is (mostly) in jest. I would avoid that junk in the future. It could certainly cause a lifter to tick. Seeing as how all of this started when you changed your oil, it seems like a possibility. I said change your oil because I would look for sparkly bits floating in it. That would tell you if you have a serious engine issue (like a spun bearing). The later oil pressure gauges like yours are just glorified idiot lights that show in the middle or not. Early gauges are functional but watching it fluctuate with RPMs freaks some people out so Jeep removed it. Or so the story goes.
 

bromel

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Oh you think it may be the lifters? That's good to know. I haven't really done much research on that. I'll get it started and let it warm up like you said and see what happens with the sound. I'm pretty sure I've heard it after driving it around for a while though, but I will check it out. Thanks!
I will keep you posted on what I find!

Well, I should rephrase to avoid sounding overly confident. That noise to me sounds exactly like the typical 4.0 lifter chatter, although I could certainly be wrong. The fact that the noise comes and goes, especially with RPMs, is consistent with leaky lifters. Piston slap occurs when the engine is completely cold, and the pistons bang around against the cylinders for a few seconds before they heat up and expand. Rod knock or piston skirt issues typically would be more consistent, and get worse with RPM.

A cracked flex plate certainly does make a ticking noise too, so you'll want to check that out. But my money is on the lifters. Some people throw in some Marvel Mystery Oil before an oil change, which seems to work well diminishing that chatter.
 
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The Goblin

The Goblin

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Lucas in the oil? That might be the problem right there lol. That is (mostly) in jest. I would avoid that junk in the future. It could certainly cause a lifter to tick. Seeing as how all of this started when you changed your oil, it seems like a possibility. I said change your oil because I would look for sparkly bits floating in it. That would tell you if you have a serious engine issue (like a spun bearing). The later oil pressure gauges like yours are just glorified idiot lights that show in the middle or not. Early gauges are functional but watching it fluctuate with RPMs freaks some people out so Jeep removed it. Or so the story goes.

Lol. Not a Lucas fan, huh? ;) I've never used it before now. To be honest, I'm not sure when the sound started because I couldn't hear it before. I've only owned the TJ for a few months and most of that has been at various shops. It had a pretty loud "ticking" noise on top when I first bought it so I treated it with Sea Foam, drove it for 100 miles or so, then drained the oil as the instructions say to do (it was pretty black, but I didn't notice any metal). Then I added 5 qts of 5w-30 and 1 of Lucas and the ticking was gone when I turned it back on. I didn't even notice this other noise until I leaned down to check under the engine for oil leaks after I changed it. That's when it hit me. You can kind of tell in the video that when I'm standing up next to the Jeep you can't even hear the sound, it's only when I'm underneath that it's audible. I will drain the oil again and see if there are any metal particles in it. I wasn't looking for them last time, so I may have missed something. Thanks for the tip. (Crazy that the oil gauge basically is just an on/off needle! May as well just made a light go on or off! lol)

Well, I should rephrase to avoid sounding overly confident. That noise to me sounds exactly like the typical 4.0 lifter chatter, although I could certainly be wrong. The fact that the noise comes and goes, especially with RPMs, is consistent with leaky lifters. Piston slap occurs when the engine is completely cold, and the pistons bang around against the cylinders for a few seconds before they heat up and expand. Rod knock or piston skirt issues typically would be more consistent, and get worse with RPM.

A cracked flex plate certainly does make a ticking noise too, so you'll want to check that out. But my money is on the lifters. Some people throw in some Marvel Mystery Oil before an oil change, which seems to work well diminishing that chatter.

I was thinking the same thing with the piston slap, wouldn't it be really consistent? This sound is not a steady tap, tap, tapping sound; rather it's definitely more of an inconsistent "rattle" type sound. You can hear it in the video, it's really sporadic. I would think if it was a bearing or a piston it would be very consistent with each revolution of that piston, right? Whereas this just seems to be all over the place, like something is loose or rattling around. It's really alluding me. As far as what you said about piston slap showing up when it's cold, this is the other way around; it's NOT there when the engine is really cold and it only starts rattling after about 60 seconds or so.

Also, with a piston or bearing, would the sound get louder with higher RPMs? This goes away the faster the engine revs.
 

Skylinessuck

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Lucas and several other oil additives are just thick oil (which can bandaid low oil pressure and reduce smoking/blowby), some seal swelling additives, and extra detergents. It's like you are putting 20w-50 in when the factory calls for 10w-30. I'm not saying it can't be beneficial in certain situations, but they sell it as a snake oil that everyone should run at all times and it's not. If that were the case then the regular oil companies would just make their oil like that. About the Seafoam, does it say to drive with it in the oil? How much do they say to put it? I'd personally be scared it would thin my oil too much to do anything but let it idle, but I guess if the instructions say so and they aren't out of business from getting sued, it must be relatively safe.

I am hesitant to guess on the noise without hearing it in person, but it didn't really sound like lifter noise to me. Could be though. Rod bearings usually make more racket with rpms, but main bearings can act weird. That's why I suggested changing the oil. You will see the sparkles quickly if it's a bearing. Best to cut open the oil filter to check, but they would almost certainly be obvious just floating in the oil. Hopefully nothing is in there and it's a flex plate or something like you thought. Can you pull the starter and look in there?
 

Flivver250

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Hard to say what your issue is with certainty. An exhaust leak in that area can sound like a mechanical malfunction. That would be my suspicion.
 

Mjv

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Try loosening the serpentine belt tensioner with the engine running to take the load off the crank. This will tell you if it's a crank bearing. Be very carefully doing this. I have done this before and found a bad bearing and replaced it on a 4100 Cadillac engine by dropping the oil pan.
 
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Bill M.

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Hard to tell from your video but it sounds more like a rattle than a lifter tick or engine issue. I would use a stethoscope to better locate the noise. My guess is either flex plate or bad cat.
 
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The Goblin

The Goblin

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Lucas and several other oil additives are just thick oil (which can bandaid low oil pressure and reduce smoking/blowby), some seal swelling additives, and extra detergents. It's like you are putting 20w-50 in when the factory calls for 10w-30. I'm not saying it can't be beneficial in certain situations, but they sell it as a snake oil that everyone should run at all times and it's not. If that were the case then the regular oil companies would just make their oil like that. About the Seafoam, does it say to drive with it in the oil? How much do they say to put it? I'd personally be scared it would thin my oil too much to do anything but let it idle, but I guess if the instructions say so and they aren't out of business from getting sued, it must be relatively safe.

I am hesitant to guess on the noise without hearing it in person, but it didn't really sound like lifter noise to me. Could be though. Rod bearings usually make more racket with rpms, but main bearings can act weird. That's why I suggested changing the oil. You will see the sparkles quickly if it's a bearing. Best to cut open the oil filter to check, but they would almost certainly be obvious just floating in the oil. Hopefully nothing is in there and it's a flex plate or something like you thought. Can you pull the starter and look in there?

Thanks for the info on the Lucas, it's new to me so I'm learning a ton here. I followed the instructions on the Sea Foam listed on the can. It says to add it to the oil a little bit before an oil change so you can drive it for a bit and then change the oil to get out all the deposits it supposedly burns off.

I still have the old oil and filter I just changed a few weeks ago, so I can check those too for metal shavings. I have inspected the flex plate by taking off the cover on the bottom of the bell housing. Checked, removed, and re-installed the torque-converter bolts twice (hoping it was just a loose bolt), to no avail. From my understanding, it sounds like most flex plates crack around the bolts where it connects to the crank so you can't see the cracks unless you pull the transmission. I am at a point where I may just do that just to finally rule in or out the dang flex plate.

Hard to say what your issue is with certainty. An exhaust leak in that area can sound like a mechanical malfunction. That would be my suspicion.

Hmmm. Ok. Thanks a ton for this. I've seen this mentioned in a few other posts, same thing you said, that exhaust leaks can actually sound like a mechanical issue. I'm so new to this that it seems really crazy to me, but I trust you guys who have more experience than I do. I will see if I can get a video of the leak so you can see where it's at. From what I remember, it looked like it was just at the top of the cat. I would be thrilled if this was the answer!

Try loosening the serpentine belt tensioner with the engine running to take the load off the crank. This will tell you if it's a crank bearing. Be very carefully doing this. I have done this before and found a bad bearing and replaced it on a 4100 Cadillac engine by dropping the oil pan.

Ok, thanks. I'm assuming that if it were a bad bearing, by taking the load off the crank, the sound would go away, correct? Or at least get quieter? Am I understanding that right?

Hard to tell from your video but it sounds more like a rattle than a lifter tick or engine issue. I would use a stethoscope to better locate the noise. My guess is either flex plate or bad cat.

It sounds like a rattle to me too! I actually picked up a stethoscope a little while ago and tried it out. The problem is, I'm not sure where to place it in order to find the problem. I can definitely hear the sound clearly when I place the stethoscope on the bottom of the bell housing (assuming a flex plate), there isn't much to hear when I place it on the bottom of the oil pan (no surprise since it's mostly hollow), but I do hear a loud sound as well when I put the scope on what I think is the base of the engine (right next to where the oil pan bolts onto the engine). Any pointers on where to listen? It's so hard with knocking noises because they can radiate to other places as the sound vibrates through the other parts of the engine. So I'm just not sure where I should actually be listening. Thanks!


Thank you everyone. I just have to say that I am so grateful for you guys. It's so nice to know there are good people who are so willing to help. You guys freaking rock! Thank you!
 

sawmillman

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Just went down this same road. You start from cold, no rattle for a minute or so, then it starts and continues until it gets warmed up and goes away. It sounds like it's coming from the bell housing, but you can also hear it on the back of the oil pan. Sounds like the same piston rattle I just had. I ended up doing an in Jeep engine rebuild. The pistons were just loose enough at the bottom of the stroke to rock back and forth. Cylinders looked good with minor scouring on the piston skirts.
 
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The Goblin

The Goblin

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Just went down this same road. You start from cold, no rattle for a minute or so, then it starts and continues until it gets warmed up and goes away. It sounds like it's coming from the bell housing, but you can also hear it on the back of the oil pan. Sounds like the same piston rattle I just had. I ended up doing an in Jeep engine rebuild. The pistons were just loose enough at the bottom of the stroke to rock back and forth. Cylinders looked good with minor scouring on the piston skirts.

For real? Dang. I was hoping it WASN'T the pistons or bearings, lol. But it does seem like your symptoms are very similar to mine. Guess it may be time to pull the oil pan and see what I can see.

I may still pull the transmission back to see if I can get a look at the flexplate. I'm not convinced yet that that isn't the problem.

I have been looking for exhaust leaks, as mentioned above, but I'm not actually seeing the one I thought I saw earlier on. And when I put the stethoscope on the cats, I don't hear any banging, so I'm thinking it's probably not that.

Did you replace any bearing while you were in there? Or was it just your pistons? What all did you do for the rebuild?
 

fourpointzero

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Just went down this same road. You start from cold, no rattle for a minute or so, then it starts and continues until it gets warmed up and goes away. It sounds like it's coming from the bell housing, but you can also hear it on the back of the oil pan. Sounds like the same piston rattle I just had. I ended up doing an in Jeep engine rebuild. The pistons were just loose enough at the bottom of the stroke to rock back and forth. Cylinders looked good with minor scouring on the piston skirts.
Just to clarify.....your noise goes away once warmed up? Im only asking because my TJ has a little rattle under the engine as well, but only after it warms up. It starts up quietly and within a couple of minutes, maybe after the compression comes up, I can hear it. Some days seem to be slightly noisier than others.

If Im not mistaken, Im at 190k on odometer now. Bought it over 5 years ago w 134k. I suspect it was like that since owning it, but my NV manual at the time was so so loud that it’s hard to say for sure.
 

sawmillman

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For real? Dang. I was hoping it WASN'T the pistons or bearings, lol. But it does seem like your symptoms are very similar to mine. Guess it may be time to pull the oil pan and see what I can see.

I may still pull the transmission back to see if I can get a look at the flexplate. I'm not convinced yet that that isn't the problem.

I have been looking for exhaust leaks, as mentioned above, but I'm not actually seeing the one I thought I saw earlier on. And when I put the stethoscope on the cats, I don't hear any banging, so I'm thinking it's probably not that.

Did you replace any bearing while you were in there? Or was it just your pistons? What all did you do for the rebuild?
If it goes away once it warms up it's probably not the flexplate. First off I was hoping it was lifters (even thought I knew it wasn't). I pulled the head put in a new cam and lifters. Started it and it still rattled. Tore it back down and put in new pistons, good cylinder hone, oil pump, and rod bearings. I had put a timing set in a year ago. My crank looked real good and the rod bearings did too, but I replaced them anyway since I was in there. I looked at the mains but they looked good so I didn't replace them. If you pull the pan off put the pistons on the bottom of the stroke and take a hammer handle or something like that and see how much back and forth rock you can get by pushing on one piston skirt and then the opposite one. There will be a little even if all is ok, but if there is too much it will make the rattle noise until it warms up and the piston expands some. After tearing it all down and getting a good look I know mine would have ran for thousands of miles like it was, but the rattle drove me nuts. It ran perfect, and once it warmed up the noise pretty much went away.
 

sawmillman

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Just to clarify.....your noise goes away once warmed up? Im only asking because my TJ has a little rattle under the engine as well, but only after it warms up. It starts up quietly and within a couple of minutes, maybe after the compression comes up, I can hear it. Some days seem to be slightly noisier than others.

If Im not mistaken, Im at 190k on odometer now. Bought it over 5 years ago w 134k. I suspect it was like that since owning it, but my NV manual at the time was so so loud that it’s hard to say for sure.[/QUOTE
Started up quiet and then the rattle started after a minute and got quiet again after the temps came up to normal. 148000 on mine.
 
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The Goblin

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Just to clarify.....your noise goes away once warmed up? Im only asking because my TJ has a little rattle under the engine as well, but only after it warms up. It starts up quietly and within a couple of minutes, maybe after the compression comes up, I can hear it. Some days seem to be slightly noisier than others.

If Im not mistaken, Im at 190k on odometer now. Bought it over 5 years ago w 134k. I suspect it was like that since owning it, but my NV manual at the time was so so loud that it’s hard to say for sure.

It doesn't go away, per se, but sometimes it seems quieter after it's been running for a bit. The reason I am thinking flexplate is that the sound goes away under load or with higher RPMs, which I've heard is typical of a cracked flexplate, but I'm not sure. Wouldn't bad pistons or bearings get louder or at least stay the same with acceleration?


If it goes away once it warms up it's probably not the flexplate. First off I was hoping it was lifters (even thought I knew it wasn't). I pulled the head put in a new cam and lifters. Started it and it still rattled. Tore it back down and put in new pistons, good cylinder hone, oil pump, and rod bearings. I had put a timing set in a year ago. My crank looked real good and the rod bearings did too, but I replaced them anyway since I was in there. I looked at the mains but they looked good so I didn't replace them. If you pull the pan off put the pistons on the bottom of the stroke and take a hammer handle or something like that and see how much back and forth rock you can get by pushing on one piston skirt and then the opposite one. There will be a little even if all is ok, but if there is too much it will make the rattle noise until it warms up and the piston expands some. After tearing it all down and getting a good look I know mine would have ran for thousands of miles like it was, but the rattle drove me nuts. It ran perfect, and once it warmed up the noise pretty much went away.

Whoa! You did all that with the engine IN the Jeep? I was looking at the oil pan the other day and it looks like I will have to remove the exhaust to be able to drop it, is that correct? anything else you needed to remove that I can't see yet?


Here's a quick video of the sound while revving the engine: