C3 Rear End Noise/Shift

[Steve B.]

After running the car for a short amount of time I have noticed occassionaly after coming to a complete stop when I release the brake there is a rather loud thump(for lack of a better word) directly behind the seats followed by what feels like a slight lateral shift in the rear end - that's the best I can discribe it. Since I live in an area where there are no real Corvette specialists around (that I know of) I was hoping to get some ideas of what it might be before I sought any professional help. Any thoughts or ideas would be greatly appreciated.

Thank you!

[John H.]

Re: C3 Rear End Noise/Shift

Steve -

Check the rubber bushings and thru-bolt where the diff nose support bracket attaches to the frame crossmember; when the bushings deteriorate, the bolt gets loose, and you'll get quite a "thunk" under accel/decel/braking.

[Joe L.]

Re: C3 Rear End Noise/Shift

John and Steve-----

That's what I'd say, too. The bushings are still available from GM under GM #3788637, upper, and GM #3795333, lower. The problem is that the upper bushing comes in a package of 2. So, you'll need to find a GM dealer that stocks them if you want to buy just one OR you can just order from one of the Corvette parts suppliers like Dr. Rebuild.

[Tom Dooley]

Re: C3 Rear End Noise/Shift

lotta times thats caused by the wrong juice in the rear end. first thing i'd do is put in at least one of those bottles of gm positraction additive. real common problem, especially after the rear is warmed up. run it for several miles to get the stuff mixed in and on the clutches and see if the clunk goes away. pretty common

[Michael H.]

Re: C3 Rear End Noise/Shift

Steve,

Quite possible that John and Joe are correct but it's also possible, and likely, that you may have the exact problem that Tom described. If the problem only starts after a few miles of operation when the differential oil is at operating temp, this would be an indication that your positraction fluid has lost all of it's special additives that are in the formula to eliminate the exact problem that you describe. The clutches in the positraction unit bind and when they release, they make a "pop" sound and this also causes a slight side to side motion of the rear of the car, just as you described. I would definitely try adding at least one bottle of GM positraction additive to see if it eliminates the problem. (most owners add two) It may take 20 or 30 miles of driving to work the fresh additive into the clutch plates. Keep us posted on the results.

[mike cobine]

Re: C3 Rear End Noise/Shift - Another Poss

Another possibility is the trailing arm bushings being bad or the shims missing. Sitting, the bushing can seize the bolt, but after driven a short distance, it is free, so that when you brake and then accelerate, the trailing arm moves out, and then back in.

Start with the differential front mount, then check the trailing arm bushings, and finally the Posi fluid.

The first two can be done with a crowbar while the car is sitting. Motion means the bushings in either place are bad.