my '90 RS has a weird vibration @ about 60-75 mph. if i ease out of the throttle, or down-shift into 4th, it slows down. does anybody have any ideas? Thanks

 

From front , or rear?
Does it come through the steering wheel, or does it feel like its shaking the body?

 

It feels like a shacking in the body, i don't feel it in the steering wheel, it feels like it is coming from the rear of the car mostly

 

Could be an out-of-balance driveshaft.

Try two things here:

·Maintain the same speed, but in different gears (1-2-Drive-OD)
·Maintain the same RPM, but in different gears

See if there's any correlation to a certain RPM, speed, etc or it's just always outright bad at one particular speed.

 

Yep, could be a few things, check first tires, for being out of shape, driveshaft, it could be the u\j thats going on the shaft, or it could be a bearing in the axle.

 

Does it come back when you get back on the throttle? You may have a bad trans mount letting the trans lift too much & throwing off the drive line angles, which will cause a vibration.

 

had this problam myself and i would bet my left leg that its your drive shaft! i replaced mine with an ally drive shaft and now i can do 130+mph with NO vibration

 

thaks for the info guys. i will look at all of those possibilities. you guys have given me an entire day of stuff to do away from my wife THANKS AGAIN!!!!!!!!

 

i had the same problem.. it was 2 things.. #1.. the wheels needed to be balanced better.. and #2.. the torque arm was a little loose.

 

Could also be a bent rim. I've struck a pot hole, and have a bent rear rim - try switching wheels from back to front if you can, and see if vibration moves to front. Or, switch from side to side, see if vibe moves left to right.

 

You've done the obvious like checked your tranny fluid right (while parked and running)? I had a bad shake about the same speed range and it was a bit low on ATF...filled her back up and she runs much smoother.

 

I would go with wheels and tires first.

If it happened all of a sudden, a wheel weight could have come off.

 

Well it's a lot easier to check the ATF than find out if a tire is out of balance.

The key here is that he says it slows down when he down shifts...so it changes when the tranny changes. He doesn't say he slows down...if he (the car not the sound) slowed down (tires going slower) and the sound slowed I could see it being an issue with something "wheel" related.

I'd really have to lean tranny/drivetrain here....

 

I changed the tranny mount last night, and it seemed to reduce the vibration but it is still there. I also checked the u-joints and they seem to be OK. Also I have a 5 spd., how do I check the fluid level?

 

Only manual I have is in my Ford Ranger...
Not much help from me on that one.

On my Ranger there are a few plugs that can be remove and I can use something to slide in to check.

 

DOH!...i musta been smokin' somethin...I even said you down-shifted...obviously you have a manual. My bad. I'd still lean tranny-related though.