zacattack805

i know i'm kinda pulling up an old thread but i am having what seems to be the same problem. there is a bad clunking noise at start up but once i put it into drive and take off, it goes away, but comes back as soon as i put it in park, neutral, or in low gears. also the noise happens if i have my foot on the pedal and take it off suddenly. any suggestions?

Dcraig

So this thread has been convoluted and I'm gonna try to clear it up like 6 years after.
Lots of guys are trying to say that you need the full rebuild to solve a rattle. If you have piston slap or bad bearings it can be solved with individual parts. Hopefully you hear it before you do any real damage by driving it. Rod bearings are soft and don't do a ton of damage to the crank and pistons are a magnesium mix and won't hurt the inside of the cylinder- they turn to powder when broken. If it knocks hard all the time, it's a piston skirt broken off. If it knocks at start up and goes away then comes back it's slack in a rod bearing. You need to pull the pan and look at the bearings for play and the cylinder walls for scarring. (Also pieces of piston in the pan is a dead givaway.)

You could have the old flex plate issue or power steering but you know the sound you are hearing, lol. It's in the engine.

Buy a piston, rod bearing and head gasket set on Rock Auto it's $126. If you need a rod bearing and oil pan gasket it's really cheap. Drop the pan and find out what you need and fix that junk! The motor will let you know. The 4.0 is almost indestructible and can be torn down and rebuilt in a day inside the engine bay.
Good luck- Big Dave

Bobby Fitzkee

You can replace the piston if you have piston slap and be ok. I just did my jeep xj 4.0 6 months ago and it's fine. And I got my piston out of a junkyard jeep that was a 99 and mine is a 93. But you will have to hone it out to get the glaze off the piston wall but don't over do it or piston won't fit snugly.
you would know if you have a piston slap trust me on that. But if you don't don't what it is you'll could think it's a lifter or something. But it don't go away even after warmed up.

Bobby Fitzkee

It probably is a rod bearing. Mine needed done on #5 piston rattled on start up and driving at certain rpm then rattle again. I didn't have the money at the time for all the rod bearings so I went the the junkyard again and got one like my piston. Lubes it up stuck it in so far no noise or problems. But that's me. I got it out of the same year jeep my piston came from. Took a chance but didn't have $100 bucks at that time for the bearing kit. If can buy a new kit and lube them good if not you will have the oil from your finger on it and it does something to it but as long as you use your finger to put motor oil on it you're good I used royal purple to lube it. That's me though. Good luck.