Dana Anderson

I have a 1990 C2 that I purchased about a year ago. Great car, tons of fun. One thing (for now) that I would like to figure out, it has always been pretty noisy when the back wheels hit any small to moderate sized bumps (highway segments, lane dummy bumps). Noise sounds like a thump. On some roads it is like I'm inside a base drum. The car has been lowerd about an inch but shocks seem to be in good shape. Bounce test shows things are working okay. No strange noises in the garage.

Any ideas on how to diagnose the cause without purchasing new equipment? Is it likely to be suspension or do failed engine mounts cause this. Any suggestions would be much appreciated.

Adrian

Could be the engine mounts but my guess is that the shocks are standard with lowering springs and they are bottoming out. If this is the case one day you will come out to the car and see a puddle of oil under the rear wheels as the casing has cracked and the shock oil has dumped itself upon the ground.

The rear engine mounts are easy to check and they also leak fluid as well. The transmission mount which is made from rubber is harder to check. You need to get underneath. I have seen these mounts cracked but you will usually feel a problem when changing gears as well.
Ciao,
Adrian.

nine64

also try check the rear shocks upper mount, there is a rubber part(not sure what it call), that what cause the thump sound from my car, it was crack.
hope this help

Dana Anderson

Thank you to you both...I'll gather more data on the spings (they're green, that much I recall) and the shocks to see if there is a miss-match.

This next weekend I'll see if I can't get the rear ones out and determine if the mounts are the problem. I'll check back in later this week. Thanks again for the support.

Adrian

Dear Dana,
Green springs normally means H&R lowering for the 964 Carrera and Carrera 4. They are more often than not fitted to standard Boge struts and shocks which is really not a good idea especially on rough roads.
Something that is often done by backyard mechanics when fitting these springs is to incorrectly pre-tension the rear springs and not to bother to torque the nut down to the correct value. I have seen too less and too much.
The other issue which often comes up albeit usually with the front struts is that the assembly is incorrectly put back together. Things like spacers in the wrong place. This causes all sorts of knocking and strange sounds as well as serious handling problems when driving hard.
Ciao,
Adrian.