I'm trying to figure out a small issue with my '98 XJ.

For the past year or so I've noticed that the rear end makes a dull "thump" sound when I go over sharp bumps. It's not a big problem but it's really annoying. I figured it must be the rear shocks, so I replaced them but it made no difference.

I removed the rear sway bar years ago so it's not that. The Jeep is on 180k miles and the rear springs have done around 145k miles. The sound definitely isn't coming from the interior or rear hatch. That's all tight and it's quiet as a mouse inside! I'm also pretty sure the exhaust isn't hitting, as it's on there tightly and doesn't come close to anything.

I'm thinking it must be to do with the rear leaf springs. Would worn out shackles/bushings make this noise, or should I look at something else?
I'd also thought about greasing the shackles, but can't see how that would stop a thump noise.

Also, are there any good ways to thoroughly check the bushings?

 

Remove, clean and grease the slip joint at the front of the driveshaft where is slides into the t-case. All my post '97 XJs had the "bowling ball in the trunk" feeling until I greased the splines. Its easy to do and I'll bet that's the issue.

 

Thanks Diskman, I should look at doing that.
That said, it doesn't make any clunk noise when starting off or coming to a stop, and it doesn't do it over smoother bumps either. Wouldn't the slip joint make a noise when I set off?

 

No - starting doesn't require the shaft to move in and out very much.

The slip joint issue is when the rear end rises up or drops down enough, the slip joint needs to slide on the output shaft and it's not sliding.. it's sort of sticking.. because eventually it's forced to move so it's just the slightest bit "jerky"... but it's an "in and out" jerk.

It's NOT a spinning jerk (like play in the gears, or a bad u-joint) where you would feel it on any abrupt change in driveshaft speed.

Those are highly technical terms of course..

 

You'd be surprised. When braking to a stop, the whole rear axle and differential unit rotates forward, which pushes the pinion down, lengthening the driveshaft at the slip yoke. When taking off from a stop, the opposite happens.

 

No doubt there's some movement. This theory is presuming the slip yoke is the source - it could very well be weak leaf springs or something like that.

The movement, in order to be the slip yoke AND show up only on "big bumps", means the movement of small bumps, or start and stops means those are too small, or too slow to be perceptible.
On a really big bump, the movement is substantial AND quick... so it is possible that the high speed and substantial travel allows perception of the insufficiently lubed slip yoke, where the smaller, and or slower movement are not.

That was the question -- "how can that happen for big bumps but NOT start and stop"... that would be the way...

Certainly there's a lot of forces at play and personally, I tend to favor the weak springs as the likely source. Lubing the slip yoke is a heck of a lot easier than replacing leaf springs, however, so it's worth doing that first since others have experienced the same symptoms.

 

Let me retract - I misread the original post - the springs (with only 35k-ish miles) should NOT be the likely cause.

Grease up the slip yoke and let us know what results!

 

Thanks everyone, I'll get the slip-yoke greased and see how I do. Now that I'm paying attention to it, I have noticed it that it does thump a little as I come to a stop or set off.

I won't rule the leaf springs out yet either though. I realize the way I wrote it in my first post wasn't clear - to clarify, these springs have been on the car for 145k miles and 17 or so years. For some reason they were replaced when the XJ was only a few years old.

 

In my experience, moly grease works well for the slip yoke.

 

A little late, but I've got a follow up to this.
Last week I had all the front control arms replaced, and got the rear driveshaft slip-yoke greased whilst it was there. The "bowling ball" feeling when pulling away which I'd eventually started to notice has now gone, but it's made no difference to the rear suspension noise.

I've been really trying to observe when the thump is happening. I've found that when I hit a sharp speed bump (like the yellow and black rubber ones which bolt to the road), the rear suspension doesn't hump when the wheels hit, instead it's after the wheels drop on the other side of the bump.

I think now it's pretty much got to be something to do with the springs, but I don't want to throw money at it until I've got it figured for sure!

 

Check the tail pipe throughout, but especially where it crosses over the transmission support bracket. Check motor and transmission mounts.

 

Thanks, I'll take a look at all of that.
I'll be surprised if it's the exhaust, because this noise hasn't change with two different cat-back systems on there. I won't rule it out until I've checked though!

The noise sounds like it's coming from under the cargo area floor.

Is there any good way to check leaf spring bushings?

 

Grab your rear shocks and shake them back and forth a few times as hard as you can. Do they move at all or clunk around?

 

They don't! They're 6 months old, and the noise was there with the old shocks too. The old shocks were in terrible shape so I assumed that new ones would fix the problem, but it hasn't.

 

I have exactly the same kind of sound coming from under the cargo area
on a 87 XJ.

Will check the slip yoke