DougyFresh

So our 91 4.0 aw4 231 XJ has recently developed a problem that i cannot pinpoint. As the title says, it has a thump that i can feel in the floorboards while idleing or driving slow. It goes away after about 50 mph. please help!

DougyFresh

No love?

I was originally thinking it was the front driveshaft u-joints, but then it started making the sound at idle too, so I ruled that out. I did crawl up under there and check them though just to completely rule that out and they all feel fine, both front and rear shafts. Would worn out motor/trans mounts cause this? It has roughly 220k on it and the mounts are most likely original. I did notice that while I was checking the front shaft joints, I was twisting the t-case up almost an inch.

I should also say that this is fairly intermittent, as it only does it roughly every other time you start it. When it is not doing it, I can make it happen by nailing the gas and letting off.

DFlintstone

Sounds like something- engine-drive-train is radioing vibs to the frame, then to you!

For mounts prying with a bar can show if it's shot. (you can see that LEFT-hand engine mount a whole lot better with the air box off). Then the exhaust too, it it's hitting you will hear it.

DougyFresh

went out there with a prybar and did some checking... engine mounts feel ok-ish but the trans mount is shot

DFlintstone

Not on either of my Jeeps, but in at least a few other rigs the Tanny mount transferred plenty of vibration. Usually accelerating. Might be "back in the day", leaking oil would end up there and eat the rubber. ??? Anyway I doubt it's actual rubber now. Anyway....sound REALLY "Radios", if any metal is touching metal. Sounds like you found it!

I eddited my earlier post. LEFT (drivers side). (that one pulls UP on acceleration)

DougyFresh

Well, this is driving me nuts. I ordered a transmission mount yesterday, should be in tomorrow. the motor does not move AT ALL even with it in gear, holding the brakes and giving it gas. the exhaust is pushed back and out of the hanger (possibly from catching the muffler on something offroad) and is sitting on the crossmember, but i don't THINK that is the source of this sound/thump.

DougyFresh

That was the problem. Some creativity with a floor jack and a comealong put the exhaust back in it's hanger at the trans mount and now all is well.

Still haven't replaced the trans mount but at least the problem is gone

DFlintstone

Cool! Glad you found it. I should have mentioned that in my first post....anything vibrating touching the frame will do that.