OK guys, here is the scoop. Shes a 91 coupe n/a.
The problem: As I was driving from work today, I noticed a wierd vibration from the back of the car. The vibration seems speed dependent, ie i cant feel/hear it in first gear. but around 30-50 mph its very noticable. I can describe the vibration most closely to a flat tire. Its kinda like a rotating, thumping sound (very quiet, but vibrates the whole car). Also I noticed, that when I drive straight the sound is there, If i turn left the sound is gone during the turn, if I turn right the sound and vibration increases even more than when its going straight. When I apply the brakes - no change. In neutral - no change.

What the car has: new rear brakes, pads - got done one month ago.
I drag raced at E-town one week ago.
25mm wheel spacers w/3rd gen wheels.

Please help, this is my daily driver. I don't know what can be wrong, and how to diagnose it.

 

Ummmm, please help me!

 

So the vibration is speed dependant. How about load dependent? Does it increase in volume under WOT, and decrease in volume on deceleration?

 

I dont think so, it seems fairly constant at speed (not first gear - no sound/vibration). The only increase in volume/vibration I noticed is during right hand turns.

 

From your description, it is something on the LR of the car. Since it's speed dependent, the tranny is not suspect. And, since the noise goes away when you turn left and take weight off the left-rear of the car, that puts the problem in the LR of the car. You may have lost a wheel weight, wheel bearing could be bad, half shaft could be bad, or the side bearing in the differential could be bad.

Do you have LSD?

 

I think that I do have LSD. Its a 91 n/a, I think all those had it. Wheel weights are all there.

How do I test these things that you described?

 

To check for LSD, jack the car up on stands and look at the diff housing. The LSD equipped diff will have a tag on the top bolt. If you can't see it, then start the car and put it in 5th gear. Get out and see if both wheels are turning(not the best way to tell). If the car does not have LSD, one wheel should not be spinning.

Check that first.

 

Then, while it's on jackstands, turn the wheels fwd & rev and feel & listen to where the backlash is comming from. Good luck..

 

It wouldn't be the LSD on that car unless the wrong weight gear lube was used.

The few series 5 cars that come with LSD have the fluid type LSD, which do not wear out.

 

jack up the car and yank on the wheel to test the wheel bearings. my TII had a blown passenger rear bearing when i got it. SOUNDED HORRIBLE!

 

I don't think it's the diff, I just want to know if he has LSD because it's easier to chase a rear end noise when both wheels have power.

 

Thanks for the advise guys...I will jack the car up on my lunch brake and will report back with my findings.
Oh ****...its raining here...the sacrifices we make for our cars!

 

BTW on a 91 he won't have a LSD unless he has the 4 piston brakes in the front.

 

I do have 4 piston fronts, and vented rear rotors. But I will check for LSD anyways, just to be sure.

 

OK, I went home for lunch and jacked up the rear end.
I do have LSD, but thats not the problem. The problems seems to be a loose passenger side caliper. I could just jiggle the caliper around with one finger, also the pads are very loose in the mounting support. I will tighten that down tonight. Looks like the shop that did the brakes a month ago did a poor job, and the drag racing must have aggrevated it.

What do you guys think?

 

you have LSD ? interesting. your car isn't supposed to ! that's very good

 

What do you mean? I though my car has all the goodies short of a turbo, ABS, and automatic suspension.

I also got the 4 pot calipers.

 

he means that in 91 only a very few NA models had LSD. It is a very rare option that year.

 

Yea your right most dont have it. I didnt ever hear of anyone even having them in there 91's until now. Wish I did

 

I think you guys are right actually...I missunderstood the "LSD test".

When i turn one wheel, the other turns opposite. That = No LSD.

I was just a confused little boy. I go from thinking I have it to being jealous that i dont, just like Flyby116.

 

yeah my 89 GTU had an open diff. nick you still want my LSD 4.33 GTUs that's in my garage

 

ROFLMAO @ sunshine. its ok i was gonna be very surprised if you did have LSD. if either of you want one i have a 89 GTUs 4.33 rear flyby116 has dibs though

 

My guess is that you got a lump on one of your rear tires. At low speed is not fast enough to feel until you speed up an it trows your tire out of balance. As when you turn all the weigth transfer makes it worse or simlpy not so noticeable. LSDs dont make your car shake they just ultimatly wear and slip. Jack the car up spin the tires look across the threat and feel it. Hopefully that take care of it. Ev7

 

Thats not it for sure...the tires are brand spankin new...less than 1k on em. But thanks for the post. I also dont think its the diff....I'm not sure how this thread got on that tangent.

I'll try tightening that caliper when I get home, see if that takes care of it!

 

Yes! lol Thanks I forgot to ask you