98XJ321

So I have been getting a really bad clunk when I let off the gas or hit the gas. I have a brand new trans mount. And year old autozone cheap motor mounts. (I have brown dog brackets, had the mounts but went back to stock because of bad vibes at idle.

But I also changed my diff fluid to all synthetic FRONT- 75-90 REAR-75-140.

I also live in central florida and I noticed it is worse after I get off work from it sitting in the heat for about 4-6 hours. Could it be to thin of Diff fluid in the rear from the synthetic plus the FL heat?

I do have a towing package on mine, but I never tow anything at all. I have been thinking of maybe trying like 80-140 synthetic in the Rear or would that be too thick.

I don't think that it is my motor mounts, but could be? Any way to test?

drhoward1988

Check your u joints in the rear driveshaft

98XJ321

Replaced the u-joints because I thought it was that. Its more than just a u-joint clunk I can feel the truck move.

SteveMongr

Did this start just after the tranny mount was installed?

I had Anchor brand motor mounts, one only lasted six months before the rubber cracked and sleeve was loose.

You say you feel truck moving/shifting in conjunction with this noise. Any issues with leaf springs, like a broken leaf hiding in the pack?

Larry's XJ

Loose Ubolts, shot bushing in spring or shackle, loose/broke shock, sagged springs allowing shackle to hit.

SteveMongr

^Excellent ideas. Time for some 'noggin pokin'. Climb under jeep with a shop light and give it a thorough visual inspection.

98XJ321

No I replaced the trans mount after because I thought that was the culprit.

Also my leaf springs have been shot since I have had the jeep. Don't think anything is broke and I am pretty sure it started after I changed the diff fluid. I really need to know if I should just go back to non-synthetic or try the higher # synthetic like 80-90 or 80-140?

SteveMongr

Regarding the gear oil, it should not cause issues.
Jeep recommends synthetic in the rear diff (handles heat better). 140w if you tow or offroad, 90w for normal driving. A thinner oil will transmit more sound out of differential.

Can you describe more by what you mean by 'feeling the truck move'?
A clunk, accompanied by chassis movement should be suspension related.

Fred/N0AZZ

Sounds like U-Joints or the differential is where I would start looking. My XJ is mostly off road and I run a std. 85-140 gear lube and change it often because of the very wet conditions I trail ride in, lots of mud/snow/water crossings.


Do not overlook those worn out leaf springs either, you just need to get under it and start checking things.

98XJ321

Its like when I let off the gas it a feels like a huge release of torque and Thud forward. Then press the gas and it Thuds the opposite way. I also have a giant thud when shifting from P to D or R to D. When going to D the whole truck moves forward under torque

Basshark7

So, kind of like accidentally starting a truck with granny gear engaged?

This sounds like improper gear mesh or something else within the driveline. If the problem is inside the differential, I doubt the oil is the problem. Is your description based on physical sensation or sound?