I'm still trying to figure out what this thump or thud sound is. It sounds like its hitting the floor behind the back seats when I accellerate from a stop. I noticed that one of the shocks was leaking so I changed them but that didn't fix it. When I inspect the suspension there's nothing obvious. It almost sound like it's coming from between the floorboard and the gas tank but I'm not sure. Any input on what I might look for?

 

Same thing on my 99.
Does it also make the thump when you stop sometimes?
I've been thinking its my badly worn rear springs. They sag a lot and I think its allowing for slight rotation of the axle. When stopping rotating forward and then back then taking off.
I'm going to hold out till the lift arrives and hope that's it.
I have no play in the drive line front or rear and just this past weekend serviced both diffs and tcase and all looked good.

 

I have the "thump" at various times on 3 different XJ's, an 87, 90 and 92 and the only thing I can figure is it is the stock leafs do not go in a complete circle around the front mount in the rear so when it unloads it bounces sometimes. Does it in one every time I put it in reverse and you can see it move there. I have kind of gotten used to it but I think part of the problem is worn bushings in the springs. Once I get the new rear springs in I'll find out if that fixes it as they completely surround the front mount and the bushing is actually inside the spring and not seperate.

 

slip yoke is worn possibly

 

Could be worn or broken motor or exhaust mounts.

Have you crawled under at all? Looked for any spot where metal has rubbed?

 

X2, I bet the engine is torquing over and the transfercase is hitting the floor.

 

I noticed that there is a brake cable with a wear mark on it like it was rubbing against metal. When I changed the shocks I made sure that it was clear of anything that would cause that. I also checked all the mounting for the exhaust and it looks OK. I will look at the motor mounts and that slip yoke as soon as I figure out what that is.

 

Look at the entire underbody. Look for any spot that has been rubbed at all.

 

It's much further back than the tranny or transfer case.

 

It may sound like it does, but until the problem is found and fixed, that's just a guess.

 

No guessing here, the thump can be felt it occurrs and someones sitting in the back seat they feel it right at the floor where their feet are, and right next to where that front mount is for the rear springs. And it deffinately sounds like thats where it is coming from too.

 

Then you should know exactly what the problem is. You've pinpointed the exact source.

 

Check the front spring eye bushings for excessive wear.

 

How does one do that ? How would you define "excessive wear" as apposed to normal wear ?

Thanks !

 

If its loose and moving in the bushing its excessive wear.

Have someone jump on the rear bumper a few times and watch the fronts of the leafs. Id they move separate from the bolt and bushing the bolt runs through its probably bad.