Unknown whump in the XJ

[bradleyheathhays]

Stock 96 XJ here and the old girl's had this problem ever since I bought her 11 years ago. There's a whump sound that matches exactly the rate of tire rotation and I have no idea what's causing it.

Here's the facts ma'am...

- It's been happening since before I got this recent set of tires so I know it's not an unbalanced tire.

- It seems to vary somewhat but is always present

- At speeds under 10-15 you can sometimes feel it in the seat with the truck body bobbing slightly up and down in rhythm with the sound

- Does not translate to the steering wheel

- I just got all the wheels off the ground w/ jack stands, first just the rear then all 4, had it in both 2 wheel and 4, gassed it to get the tires spinning and didn't hear or feel any whumping at all.

My buddy that I bought her from and a couple other yahoos loaded her down and went tearing through a field one fourth of July, you could imagine, and ended up bottoming it out on a pile of cut logs. Best I can recall, and by that I mean not being able to recall anything at all, I think those logs we had to yank out of there were jacked up under the drive shaft. I thought that might have something to do with it, but talking with him recently he says he didn't notice any problems with it the next day so now I really don't know.

Any ideas what this could be or maybe what I can do to start testing/examining parts for the problem?

 

[JPNinPA]

Check all u joints in drive shafts and in the axels. Check the wheel bearings all the way around.
Check that the drive shafts are not bent. You could just line a string from front to back on the driveshaft and check the gap. If the gap is not even its bent.

We're the old tires cupped, chopped or worn unevenly at all?


Regards,
JPNinPA

Sent using TapTalk

 

[bradleyheathhays]

Ok for sure I'll start checking on all those things.

My thinking on the drive shafts is that since they turn at a different rate of the thumps it probably isn't them, but I honestly don't know. Although regardless i'll still check the driveshaft with a string like you suggest.

How would I go about checking the u joints in the axles? I've taken them out before, and replaced the ones in the front axles. But what should I be looking for when checking them say in the rear. Just loose play?

As for the old tires I really don't remember about their wear.

 

[JPNinPA]

Yes
you should be able to twist the axel a bit , just like you would twist a driveshaft. Look for play in the u-Joint.

There are no rear axel u-joints. However the axel itself can get bent and be hitting inside the tube. Though I would expect the seals to fail and leak if it were bent.


Do you know if it is front or back?

How do you know it thumps at the same rate the tires spin?


JPNinPA
Using Jeepz.com app

 

[Jeepernaut]

Also, Check the drive shaft, rear diff setup and rims. I had a 98 Ram and a 94 XJ both had bad differential pin yoke;s and got thump/whumps randomly when driving, and always when going up a hill and shifting ( sometimes felt like the rear was bouncing in essence). On the XJ I also had a bent rim that caused issues and the Ram had a bad drive shaft. Good luck!