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Not your average engine knock

laredo

NAXJA Forum User
Location
Ma
Hi guys. I'm desperate for some help and hope someone can point me in the right direction. I've spent about a week on forums trying to find a solution but to no avail. About 200 YouTube videos later and I can't find a similar noise.

Here is the rundown.
99 Jeep Cherokee 4.0 with 66k miles. The Jeep still looks and smells new. It's like a time capsule. I've owned it for one year. I had an oil change when I first got it and have only put on about 3500 miles. This knock possibly didn't start until after the oil change I just did a few weeks ago.

The sound is a slight knock/rattle that only seems to primarily happen on light acceleration. The rattle does not have a pattern. The noise only occurs when it gets up to 200 degrees or so. After 20 minutes of driving it seems to go away. When I do have the noise it is around 1500 rpm. It goes away after that. I can't really hear it driving over 30mph. I can get it to do it in park as well.

I could not isolate the noise with a stethoscope so I brought it into my mechanic who primarily deals with Jeeps. They thought it was possibly lower end but then still weren't too sure. It's not the flexplate bolts,they were all tight and they didn't see any cracks. I can't see any cracks in the exhaust manifold. It does not sound like piston slap or rod knock or a noisy lifter.

Here's something else I'll throw out there. I had researched oil for hours on bobistheoilguy and decided to use Pennzoil Yellow bottle 10w-30 on my last oil change. This is supposed to run great in the 4.0's and have a good amount of detergents in it. Is it possible the oil degunked something in my engine and it's just getting clogged up? Since it only has 66k I am assuming there may be some build up from sitting. I also used a purolator pureone filter. My mechanic took it off and noticed it was rather heavy and the oils that I had just changed was pretty black. I got the Jeep back home yesterday and just changed the oil again with Pennzoil and substituted a quart for marvel mystery oil to hopefully further break everything down.

Any other ideas guys on what it may be? If it was the exhaust manifold or gasket wouldn't I be able to hear that noise loudest with the stethoscope on it? It seemed quiet but the fact that it goes away after hot would lead me more towards exhaust. The mechanic had an electronic stethocope and still couldn't pinpoint besides lower end and oil pan which most vibration will echo down to.

I'll try to get a video later if it helps. The jeep is hot now and not doing it. Thanks in advance for any sort of help and sorry for the novel.
 
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Have you inspected the flex plate and checked the flex plate bolt torque ?

A cracked exhaust manifold leak ticking noise will typically quiet down as the exhaust heats up. I would suggest you are hearing a noisy lifter or piston slap. MMO should reduce or eliminate lifter noises.
 
I personally didn't inspect it but my mechanic said the bolts were all nice and tight and they couldn't see any cracks. I know sometimes though the cracks are hard to see. I've had lifter noise in other jeeps and it didn't quite sound the same. Piston slap I think is louder. This is more of a rattling knock.
 
It does still have the factory cat. I've banged on it and don't hear any rattles. I thought that maybe something broke off in there and rattling however it seems to be in good shape. I don't hear the noise with the stethoscope on the exhaust either.
 
4.0s or: How I learned to stop worrying and love the knock

When I bought my beat up XJ with 185,000 on the clock it ran nice and quiet. I immediately set about tuning it up/cleaning it out/getting it back into top order. Somewhere amidst the oil changes/seafoams/sensor replacements it developed a knock. It was gentle at first and I didn't pay it much mind, but it got louder. I ignored it as I gradually modded things the way I wanted them, but after the lift/etc was done, interior spruced up and accessories installed, I ran out of things to do so naturally I began obsessing about the knock. It came and went which made it hard to troubleshoot.

First I ran whatever cheap 10w-30 I could buy by the case, because 4.0s supposedly run fine on whatever. No real improvement. Then, I ran Rotella T6 with a Wix filter because 4.0s supposedly run even better on that. The knock got louder (sometimes). Tightened up the flexplate. Then I had the top end rebuilt and replaced the lifters and put in some ZDDP additive... it stayed the same. I bought a digital stethoscope and listened to everything and that didn't seem to narrow anything down. I bought a new knock sensor, tried a different oil filter, no luck. Tried a little Lucas bullshit oil additive. Meh.

During all this I took many good 4x4 trips and the engine ran great every time, in fact better and better as I replaced stuff. Knock stayed. I tried combinations of different fuel, different o2 sensors, etc. Last oil change I put in 5qts Castrol GTX 10-40 high-mileage + 1qt MMO w/Mobile 1 filter. Lo and behold, the damn knock is almost gone - I think I can still detect it at idle after warmup, but it might just be valvetrain chatter it's so faint now.

All that to be said, our 4.0s are notoriously noisy and you can go nuts/broke trying to track it down. Try some different oils/filters and play with stuff, but as long as your oil pressure is good and the noise isn't a terrible catastrophic clanking I think you will be okay (especially at only 66k!). Don't stress at all if you're on pavement; you're allowed some stress if you're 10miles from cell service :laugh:
 
Thanks for the thorough response. I have pretty much only owned Jeep Cherokees. 84,89,99 and now another 99. I live up in Mass so finding a low mileage one with no rust is hard to come by. I started searching over 3 years ago and told myself if I found one I would buy it. I was extremely picky and patient since my other Jeep ran fine with 180k. This one for me was my unicorn. I drove over 6 hours and spent a lot of money on it.

It sounds like you're the same as me as far as obsessing over things and I have a hard time not having an answer. I just want to make sure I'm doing everything I can to narrow down the issue instead of regretting it later. I also just saved up another 2k to lift it finally and want to make sure I'm not going to have to do an engine rebuild. Nothing seems to point me to one issue which is making me crazier. The whole not doing it cold,doing it once it gets warm and then going away at operating temp just doesn't make sense. I've been driving around like Ace Ventura with my head out the window listening to every little noise.

I meant to get a video up today but ran out of time. Maybe you could check back tomorrow and give it a listen if you don't mind. I always come here to get intelligent answers. Facebook Jeep pages are not even worth my time asking.

Thanks again!
 
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My renix 4.0 is pretty noisey compared to most modern engines . It has 214k, pretty solid tho, runs great. My original exh manifold was cracked in about a dozen places. It didn't make that much noise. I replaced it tho. It might be time to do all exh gaskets on your rig. It's cheap and easy to do anyway. Also check out the motor mounts make sure they're not sagging and contacting metal to metal.
 
Btw: don't use pennzoil. Unless you want gunk build up. I wouldn't use pennzoil in my neighbors lawnmower.
 
Mtbxj87 I had never used Pennzoil until after researching. Most people said it was some of the best running oil in the 4.0 and the quietest. It also has a good amount of detergents in it. Do you have an opinion on why you don't like it?
 
Btw: don't use pennzoil. Unless you want gunk build up. I wouldn't use pennzoil in my neighbors lawnmower.

Not intending to sidetrack the thread, but this couldn't be more wrong. Maybe decades ago, but Pennzoil conventional is proven to have amazing results in Jeep 4.0Ls. Head over to Bobistheoilguy and look at used oil analysis results, mine included.

Everyone has opinions about oil, but rarely does anyone ever substantiate them... used oil analyses are the only way to do so.
 
eh, Pennzoil, ive seen lots of vvt type engines damaged by it. it might work fine in a 4.0 but my choice is different. that issue has been beat to death, im sorry for mentioning anything.
Any way, that sounds like flywheel to me. Valvetrain sounds fine from what I can hear.
 
Thanks for watching. Any explanation as to why it would go away after driving for a while and only at certain rpm? I'm not disagreeing with you I'm just trying to learn.
 
certain loads. sounds exactly like a friends TJ that turned out to be a very cracked flexplate. The oil thing can be googleded, as many threads and opinions as you could read haha.
 
Oil debates tend to be worse than religion and politics. Mechanic said the flex plate didn't look cracked but I may have to investigate for myself.
 
knocking noise 2 sounds like flex plate bolts to me.
 
Until you actually pull the torque converter and the flex plate, I would not rule out cracked flex plate or loose crank shaft bolts. I have seen cracks right at the center that could not be seen without pulling everything.
 
Thanks Old Man for your opinion. I'm leading more towards this but wanted some opinions before I have to lower the transmission.
 
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His jeep made almost the exact same noise which got worse and worse, I think you can see what the issue is.
 
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