ok, so my old lady's cherokee has a knock... it's one of those that you can't tell where it's coming from, when your up top, it sounds like it's in the valve cover, when you go underneath it sounds like it's in the oilpan...


i'm thinking it's a rod bearing because oil pressure starts getting low after it warms up, but it does something i have never seen another rod bearing failure do... it gets almost dead silent when you put a strain on the motor, such as going up a hill in to high of a gear.

i put a quart of marvel mystery oil in it because it was a quart low on oil when we got it and i had heard good things about it helping with knocks...it didn't help.

 

nothing? come on guys.... i guess i'll just start at the top and work my way down... it needs a valve cover gasket anyways, so i'm going to pick one up and do that, while the valve cover is off, i'm going to make sure i don't have a rocker off or some wierd crap like that, if all thats good i guess i'll pick up a timing cover gasket set and a oilpan gasket and check the rods/mains.

 

the 4.0 is known for ruining the rod bearings, it will still run with worn out rod bearings but be loud.

I would check the torque converter bolts, when they come lose they make all kinds of noise.

Try searching too, this topic has been cover a bunch

 

Mine has the exact same problem, only makes the noise at idle not during driving, and the sound travels exactly how you described. I've been searching for months and still cant find it, pretty aggravating. The good news is it still runs great, been makin the noise for over a year now. Please post if you find the problem......

 

Hey guys I had the same problem with my jeep when I bought it. My aunt who was the P/O had taken it to her mechanic and he basically told her that it was a crank or rod bearing and hat it wasn't worth fixing. Only thing is it ran just fine with no problems. I did some digging around and use a long socket extension and probed around with my ear on the end of it. Turns out the knocking was actually coming from the power steering pump. I would check that if I were you also water pumps have a knock to them as well if they're that far worn and going out. Just turn on the jeep and put a long extension on the back of the pump and put your ear to it to check if the knock is Comming from there.

 

So??????? Anything new????

 

Rod knock is caused when the crankshaft throw pulls the piston away from TDC. Combustion pressure keeps the rod pegged to the throw, and at TDC the forces flip from compressive to tensile. When this occurs you’ll hear the crank throw bang into the lower bearing shell, and this is rod knock.

When the engine is loaded the rods will stay in compressive load and then the knock goes away.

Sounds like this is your problem.

 

it quit running, so the rod knock is the least of my worries... think it needs an ICM to get it running

 

Blasto, i know what a rod knock is, ive had some unfortunately. But to knock for over a year and not even get any louder? Not to mention still run great? I aint sayin its impossible, but definitley not probable..... Dont you think??
Dougy, keep us posted...

 

Besides, dont rod knocks usually speed up with engine rpms ??

 

my 91 is the same way. but i doesnt knock when i first start it up, you can hear it when it warms up and under load you dont hear nothing..oil pressure is great and the thing runs like a champ..

 

check the torque converter, you would be surprised

 

stethoscopes are 10 bucks at autozone.

 

A lot of this sounds like a good ole broken flexplate. Knocks lick a S.O.B. when sitting still and idling at a redlight but the knock subsides when under a load. When idling it has just enough play to bang around and drive you crazy. Under a load such as acceleration it puts enough pressure on itself to keep all the parts jammed together. And heres what it looks like.

 

thats not the problem, the sound travels everywhere, you cant believe it.....