I have a 1991 XJ 4.0L manual 5 speed tranny. 4X4. this past week I have noticed a vibration when I accelerate. as I go thru the gears, and when under power the entire car vibrates. I took it to the Jeep Stealer, and the mechanic there told me my Harmonic balancer was going bad and I also had a bad U-Joint on the front drive shaft. made since to me, so they replaced both the crank pulley and the U-Joint. when I picked it up, same thing, still vibrated when under power. The mechanic said that the joint at the end of my front prop shaft has a rubber something inside of it sort of like a CV Joint and if that is bad it will also do this. well I eliminated all he said because I removed the front drive shaft and it still does this. Has any of you had this or heard of this problem ? I have had the Dana 30 in the rear rebuilt with new ring and pinion and bearings, the main drive shaft has new U-Joints and all wheel bearings seem good. as long as I drive and accelerate slowly and easily it only vibrates a little, but normal driving is shakes a lot and I am afraid it might break something. any ideas ? Thanks in advance.

 

Check the transmission and engine mounts. Engine mounts can harden and crack. Transmission mounts can turn spongy. Aside from that, my only guess would be u-joints or something with the suspension.

 

good idea, I will look at them. I replaced all mounts about 1 year ago, but that doesn't mean one of them didn't fail. I did hear a squeaking sound one day last week, I removed the main drive shaft and greased the U-Joints, put it back and the squeaking stopped but the vibration when accelerating is still there...maybe it is a U-Joint...I don't know. I will look at the mounts like you said. thanks for the reply.

 

I would also have the clutch inspected as well. The fly wheel could be slightly warped giving you a vibration. I can see that you are in Abu Dhabi so it is pretty hot there and the heat from the clutch on top of the ambient temp could have caused it to warp quicker.

 

It does sound like it could be a clutch problem. Do you know how old the clutch is?
The disc could be worn (or the pressure plate springs are weak) so it slips when it is loaded.

If you do replace the clutch set, the factory manual says to not have the flywheel
machined, as some shops are prone to do, as that will damage the surface shape.
Rather, just smooth it with emery cloth.

 

Could be clutch flywheel or disc warped

Have you checked the driveshafts for ANY play (basically just grab ahold of it and shake it up and down, should not have any significant movement at the u-joints).

Have you checked to see if all the wheels are balanced properly? A wheel weight could have fallen off.

Check the engine and trans mounts. Use a pry bar and examine for cracks or if it is super squishy or anything.
Cheap replacement engine mounts are garbage, most of them fail pretty quickly for whatever reason.

Trans mount is easy to change and probably should be done every 10 years or so, as they are rubber and definately do not last forever.

 

I would venture to say a mount failed, especially if it was a cheaper replacement

 

well all of these are fantastic ideas and I thank you all. BUT I did get it fixed. a few days ago I crawled under the jeep and checked everything, I couldn't move the U-Joints on the rear drive shaft, so I thought they were on. well I learned that I am not as strong as the motor, it turned out to be a bad U-Joint at the tranny yoke. I changed both of them just to make sure and all the vibration went away and it is like new again. I never had a bad joint that I couldn't detect bad by my hand, but this time that's what it turned out to be. all fixed, and again thank you for your replies and input.