I think I've got a major problem.

It all started a couple of days ago with an odd noise. My girlfriend called me up and said her Cherokee ('93, 4.0 2WD Auto) was making an odd noise, she described it as sounding like it was "underwater." When she got home, I listened to it, indeed it sounded like she described, or maybe like hearing it running down a long tunnel, just kind of a weird, non-normal sound. You could only hear it in the cabin, it was faint enough that the engine/exhaust noise outside drowned it out. Couldn't find anything online, it still ran great, so I told her to keep driving it, it would probably go away. It didn't.

At the same time, I checked all the fluids, she told me it never overheated, the oil level was spot on, the oil gauge claimed great pressure. (3rd line when cold, 40-50psi hot)

Fast forward to day before yesterday, she calls and says the sound is a lot more aggressive than before, nothing I can do over the phone for her, so when she gets back, I look at it. The odd noise is still there, but now there's a new knock. It's not very loud (yet.) Gotta use the car for work, so she drives it yesterday. Last night it was loud. A definite knock coming from the engine bay while in the cab, and from the front of the engine (it seems) when standing in front of it with the hood open. Check fluids again, everything fine (almost better than fine, actually, my XJ is jealous of how her fluids look new and don't leak,) oil gauge reads fine. Drive it a couple blocks, the knock comes and goes seemingly randomly, then it develops a sortof squealing noise. Can't be good. Park it, call for help!

It's certainly not a tap, mine taps, tapping is normal. It sounds like someone is wailing on the block with a hammer. Still, her car performs like it did the day we bought it, has "oomph," runs smooth. All I know is that it's going to sit in the driveway until I figure it out.

You can give me the bad news now! (Though if there's a glimmer of hope, I like that too..)

Ian

 

First and foremost, check the torque convertor bolts. They have a habit of backing out and making contact with the bell housing creating a loud knock. If they are loose, remove them one at a time, clean the threads, reinstall them with some thread lock, and retorque them to proper specs. If the bolts are not loose then check the flexplate for small cracks. They may be very hard to see but will be coming out from the center of the flexplate. If there are cracks then the flexplate is most likely broken around the bolts that hold it to the crank. Repairing this will require removal of the transmission to replace the flexplate.

Loose torque convertor bolts will make the knocking noise consistently at all rpm ranges.

A broken flexplate will be more pronounced at idle with no load on the engine and will get quieter on acceleration as the broken pieces wedge together.

This is what a broken flexplate looks like. You can also see the small cracks that lead to it's eventual demise.

 

Just went and cranked it again, moving it closer to the garage door, have to add "horrible grinding sound" to the list of symptoms. It's just getting better and better!

Ian

 

Definitely sound like the flywheel (flexplate) I would check that first...Tj

 

Oh man, I'm so glad right now. The speculation is over, I've discovered the problem. Nestled the car up to the garage and popped the hood, apparently either last night on that short drive, or this morning while moving it to the garage, that idler pulley finally decided to present itself as obviously dead.

The culprit:



So glad it's nothing major, and I never would have believed a simple pulley could have created such a knock and racket, but there it is.

Ian

 

Come to think of it, I just tightened up that belt for her a few hundred miles ago since it was squealing when the A/C kicked in, that's probably what caused it. I know I didn't overtighten it, I have a tension checker. Old age just got it, I guess.

Ian

 

Thats good to hear. Put a new one on and see what happens. I would also look at your flexplate while your at it

 

I hate the torque converter bolt problem. I have it but I can't get the inspection plate off but I'll probably try again soon. The sound annoys the hell out of me.

 

I have had a hermonic balancer "crank pulley" hitting the timming cover, making a nasty engine noise before. Just another spot to check for next time.

 

ha i never get that kind of luck if its a knock i can be sure that soon a friendly rod will be making its way to daylight

 

Awesome!! i was gonna say crank balancer due to the fact you said the belt was squeeling.......