TrevorNelson

I have a 97' xj that makes a rattle when you start it and when you turn it off. it sounds to me like someone is hitting something with a hammer inside the oil pan a couple of times.

TrevorNelson

here is a video for you guys if that helps.
(its not that o2 sensor wire you see vibrating in the beginning)

sam24th

Check your flex plate bolts. Its possible one of those bolts might have came loose and its pretty damn close to the oil pan also.

JeepingDan

I almost wanna say it's a rod but it's just when you start it and shut it off.. Thats it? There is no noise if you power brake it?
Could also be a cracked flex plate if its an Automatic.

TrevorNelson

Alright, dumb question... What the heck is a flex plate???

F1Addict

It's the equivalent of a flywheel, only on an automatic.

TrevorNelson

Can anyone tell me how to get to this so called "flex plate"?? Or how to check it? Tighten the bolts possibly?

atteburyc

Search flexplate knocking there's a nice write up on the forum

JeepingDan

Here's one but it's only good for loose converter bolts.
https://www.cherokeeforum.com/f51/ho...ocking-125547/

Sometimes they crack around the crank shaft. It's happened to me twice on my DD.
If that's the case then you have to pull the tranny to replace it.

Radi

I'd be inclined to pull the pan and take a look.

CCKen

If you pull your pan and find a chunk that has part number 4798329 on it, it's a size B piston skirt.

Willys55

It sounds like a cracked flex plate, you would have to pull the trans to replace it. Keep this in mind, most time this is caused by an overheated torque converter, the gear housing expands between the plates inside the torque converter when it becomes over heated causing it to push the flex plate forward, since the end of the crank is not going to give any the "flex" plate over flexes and cracks. I have seen two in the last year that were cracked because the rear main bearing flange was worn, but that is easy to spot just by watching the front crank shaft pulley and having someone rap the throttle a couple times, if it moves in an out, time for crank bearings.

Turbo X_J

If your watching the front crank pulley can't you just "rap" the throttle yourself?

Willys55

You can, but it's easier to pay attention to what your doing , no injuries that way mate.

JeepingDan

First time I had a bad flex plate I thought the bottom end was bad.
But after searching this forum I got an idea. I took the bell housing cover off and watched the flex plate as I moved the crank shaft back and forth with a breaker bar.
Sure enough it showed "unusual" play and made a thunk sound each time I moved it.
Thats an easy way to diagnose it.