After a recent track event my car started making a thud when transitioning on and off of gas.

Background...

http://www.classiczcars.com/forums/showthread.php?t=41028

The track mechanic put the car up on the lift and diagnosed the problem as a bad differential. However, after searching the forums I can't find a rear end thud that has been tied to a bad diff. From what I read when the diff goes bad it generally gets loud or fails catastrophically. My car does not exhibit and new gear noise it just thuds on and off gas. It also thuds rocking the car back and forth with the e-brake engaged.

I checked the usual suspects with the track mechanic:

prop shaft: A little play on the diff end but not too bad.

diff mounts: front and rear seem fine

mustache bar mounts: seems fine.

suspension: no obvious slop on either front or rear suspension

motor and trans mounts: A little loose but nothing that looks like it would thud

exhaust system: Seems solid

Any advice?

Did you check the u-joints and bolts?

Did you check the universals in the drive shaft and the spline on the drive shaft as it goes into the transmission?

Thanks for the responses guys. I took another look at the universals etc and while I was there I found that the track mechanic mis-diagnosed the rear mount for the diff. He was checking all the mounts with a pry bar but the rear mount is solid to the mustache bar. If you pry on it there is no play but it turns out that the bolts were loose enough to cause the diff to rock side to side.

It's raining very hard here so I can't really test drive it right now but after tightening the bolts it no longer thuds when you rock the car back and forth in gear. Hopefully this is a $0 fix. My favorite kind!

I had problems with loose bolts in the past. It can make for one heck of a racket.

That's good news for sure.

Thats exactly what I was about to post....check the two nuts for the diff behind the mustache bar. Usual suspects.

Edit: I added locktite to mine.

Do you have an R/T mount for the front of the diff?

I have a new Powder coated (black) R/T with a new Urethane bump stop if you'd like to buy it. PM me if you're interested.

If you're gonna be doing more ON/OFF throttle, then this would be ideal for your Z. It'll lock your diff down so it can't lift up and Thud against the diff strap.

Use your breaker bar between the diff and the front crossmember to see if the rubber of the diff mount seperates...

Dave

Dave,

I have one of your R/T mounts. It worked flawlessly The issue was on the mustache bar end of the diff.

Ya, I saw that after I posted. My bad.

I use a breaker bar on my diff to mustache bar nuts, just don't break the studs or diff. Like previously stated, some threadlock sounds good.

Glad to hear it was a simple 0 $ fix.

Torqued them to 60 ft#s as per fsm and went for a test drive. All seems good. Thanks again guys!