Scary noise coming from my 4.0L

88 MJ, 190k+ mile 4.0L

It's sounds like a cross between a bolt being shaken around in a coffee can and a spoon in a garbage disposal. It doesn't have a particular rhythm to it and it isn't obvious as to precisely where it is being emitted from. I've pulled the oil pan and valve cover and everything looks fine. Nothing broken or loose as far as I can tell and no metal shavings in the pan. The worst part is that it's intermittent. During the first occurrence it was persistent at all rpms, the second time only above idle, now it usually only happens for a short time, sometimes only a single thwack. It sorta sounds like the time when my water pump allowed the impeller to eat away at the housing, but I just now went out and pulled the belt off and it still does it. I'm stumped. :dunno: Short of stripping down the block, any ideas? Anything else you guys need to know? The truck runs fine before, during and after the noise. It doesn't stutter or anything.
I need the truck this weekend for the second annual Comanche Club PowWow. I'm flat-towing it to The Badlands, so I only need it to perform when we're there. But I also don't want to have the block blow up on me in the middle of a trail. :explosion: I can't fix anything if I can't find a problem! :looser:
Jeep on!
--Pete


Some more back story if you want it:
It happened a month ago at a local wheeling event. I hadn't gotten a 1/4 mile away from my tent before the noise started. I pulled over but couldn't precisely locate the source of the noise so I told everyone in my group to carry on without me while I limped back to camp to diagnose the problem. I was thinking the starter gear was maybe touching the flywheel so I pulled it. Everything looked fine so I put it back together to try something else, but the noise had now vanished! I took it for a test drive and everything checked out, so I hooked up with the last group. The truck ran great all day. A couple weeks later (maybe 3 miles later as far as the truck knew since I flat towed it home) the noise came back while in an Autozone parking lot. The shmucks inside came out and were stumped (big shocker there). It drove ok last time the noise happened so I figured I could limp the 1/2 mile home. Before the noise was omnipresent, now it only happened above idle so I tried to keep the rpms as low as possible. About half way home the noise suddenly stopped with a quick squealing sound. Now another week has gone by and I've pulled the valve cover and the oil pan. Everything looks nice and tight. The noise happens only occasionally now, but without finding a problem, I can't fix it or predict whether it'll get worse again.

Hmmm... it automatic? Hard to understand exactly what it sound like over the net but the flexplate could be loose or the torque converter could be causing this. Someone (old man?) recently posted a pic of an AW4 transmission with a chunk of bellhousing missing causing similar problems.

Edit: You need to try and locate the noise. Try using a piece of neoprene tube as a stethoscope.

Manual trans. 99 TJ AX-15 to be precise. I'd love to use my mechanics stethescope or even a length of hose, but now the noise is so intermittant that it doesn't last long enough for my typical methods to be of any use.
Jeep on!
--Pete

P.S. What, you don't know what a caffee-can-bolt-garbage-disposal sounds like? :laugh3: If I had a way to digitize the sound I would do it in a heartbeat. Everyone that has been able to hear it has been shocked, stumped or both.

mine is doing the same thing but i have a 2001 with the automatic. sounds like it is coming from the oil pan or tranny. but at higher rpm's just revving it, it goes away. and no other side effects besides the rattling..

Pete M said:

Manual trans. 99 TJ AX-15 to be precise. I'd love to use my mechanics stethescope or even a length of hose, but now the noise is so intermittant that it doesn't last long enough for my typical methods to be of any use.
Jeep on!
--Pete

P.S. What, you don't know what a caffee-can-bolt-garbage-disposal sounds like? :laugh3: If I had a way to digitize the sound I would do it in a heartbeat. Everyone that has been able to hear it has been shocked, stumped or both.

Click to expand...

Throwout bearing/clutch springs.

Check the crank pulley. If the outer part is beginning to separate from the inner part, it could be going inwards and eating into the timing cover.

Could you have something being sucked into the mechanical fan's cowl or even a the cowl itself being drawn onto the fan when the fan engages?

No shroud anymore. It died when the water pump went last spring after getting doused with hot antifreeze and then getting smacked around with remnents of the belt that wound around the fan. I went out and examined it and the crank pulley looks ok. No evidence of it separating or contacting the timing chain cover. Would the throwout bearing noise change with the clutch in and out? It's not a rattling sound, more like a thwacking sound from impacts. Kinda like if someone was rapping on the inside with a hammer. And it can be dang loud too.
Of course, right now the truck won't make the noise. I think I'll take her out for a drive. Maybe swing by a mechanic if the noise starts again. Further updates as events warrent.
Jeep on!
--Pete

Well, two days of running errands in it and the noise hasn't popped up. It's hard to diagnose anything when it won't do it. :dunno: Too late to worry about it now though, the Jeep is all hooked up to the Durango and I'll be leaving shortly for the MJ PowWow. Fingers crossed that it stays away for just a couple more days!
Jeep on!
--Pete

I'm guessing vibration damper hitting the timing cover.

langer1 said:

Throwout bearing/clutch springs.

Click to expand...

Ditto........Hans

I've seen something similar on two different engines. It was caused by an air bubble in the cooling system. The bubble would get to the water pump and the noise was caused by alternating between pumping water then air then water. It causes a significant noise/rattle.

Thats actually much better that its manual transmission. You can more easily diagnose the problem. You can control when the clutch is engaged and how and the engine RPM, so the noise doesn't seem to have anything to do with the clutch? I say it could be the harmonic dampner or something else on the front end. It would be so helpful to know which end of the engine this comes from, you should carry around the stethoscope, who cares if youre in the bank parking lot with a piece of hose under the engine.