Hey everyone, so I have a minor issue. When driving at or above 75mph and precisely at 78 mph, I get a noticeable vibration throughout the car's body. It is most felt when you put your hand on the dash and through your butt from the seat. The trick is that it sometimes comes and goes for no particular reason. (example: driving 78mph on cruise control and the vibration comes, then seemingly like magic it will go away....and then comeback) Now, before you ask, the tires and wheels are new (it did this before the new tires and wheels); I have switched the wheels front to back with no change, and to top it off there is absolutely no vibration coming from the steering wheel. As mentioned above, the vibration is only felt through the seat and dash. I think the center support bearing, u-joints, or something else in the driveline is to blame but I'm curious what others might think.

I have had the car up and took a peak at the driveshaft and nothing seems to be amiss besides what seems to be a "ring" of oil and and dirt near the back of the center support bearing which might be from the U-joints. Also the car has 74k miles

Any help would be appreciated

 

Likely road surface: seeing the same. But be sure a grease the zerk fittings both sides of the differential output shafts.

 

Unfortunately tires can be unround from the factory. Tires can be too soft too and this will cause vibration that you are feeling. Does the vibration come on and go away if there is loong constant curve in the highway where you have to keep steering in one direction for some time?

 

The speed at which it appears and the constancy at a given speed seems to point to a tires/wheel issue.
I had something similar with a BMW and it turned out to be a wheel out of round.

 

check all the ball joints , control arm bushings, engine mounts..

 

I'm more inclined to think the oil ring near the support bearing is an indicator of the issue. You may have some wear / gauling in a U joint, but it hasn't completely chewed it up yet. the extra slop could be the caused of the oil ring as it would overheat from the increased friction and also push the seal slightly out of place, causing said oil ring. Or possibly the support bearing could be having a similar wear issue. But I'm still thinking the U joint. Just a couple of thoughts. Hope it helps.

Jack

 

Let’s face it, every rubber component on these old cars needs replaced. I removed a fuel tank out of a 96’ XJS yesterday and the rubber gaiter between the fuel filler neck and the fuel tank turned to mush the moment I touched it.
#oldcarlife

 

I had tje same problem it ended up being g a boot/bubble in the left rear tire. It would start vibrating bad at 55-75 mph. I hope this helps!

 

These vibrations can be caused by a plethora of cirumstances, sometimes a combination of them occur:

- Tires/rims out of balance
- Differential output shafts (greasing mine did help)
- Flex couplings (hardy disc) x 2
- Prop shaft center bearing
- Wheel bearings
- Prop shaft out of balance
- Hub bearings rear axle (2 per side) on the hub carrier (when these are rusted, they push the wheel out of alignment)
- Anything loose in the suspension. Be it worn out or just loose

 

I agree with this. Check the U joints, carrier bearing, and jurid couplers.