Hello everyone,

I'm having some trouble with an 89 corvette with the ZF 6 speed and I'm hoping that some of you might be able to help me. I just recently changed the clutch, master and slave cylinder on the car. It seemed to run great. However, after a few days, there seemed to be a sound coming from below like rocks in a can or a diesel motor on a truck. Since this corvette is low mileage (~35,000 miles) and the flywheel looked alright visually when the clutch was out, I figured that either 1) the clutch was wrong for the car, or 2) the clutch was not installed properly. So, since I'm a glutton for punishment, I decided to take the whole thing apart and try a different clutch brand. I was absolutely meticulous in my re-installation. Frustratingly enough, the problem is still there with this new clutch. This fact would lead me to believe that the "rocks in a can" sound is possibly coming from the two discs chattering on the dual mass flywheel.

I did indeed read the FAQ on www.ZFdoc.com. One of the questions seemed relevant to my problem--except for two differences between my problem and the one diagnosed on zfdoc: 1) my "rocks-in-a-can" sound only occurs when the vehicle is hot and has been driven for a little distance (~ 20 miles each time) and, more importantly, 2) the sound does not go away when the clutch pedal is pushed to the floor.

Has anyone had a similar problem? Is it possible that the new clutch put more of a stress on the flywheel and that the rubber between the two discs on the dual mass was thus compromised? If so, does anyone know where I can find a new dual mass flywheel? (I wouldn't necessarily mind converting to the single mass solid steel flywheel, but I'm worried about the potential for some weird transmission noises, which was the very thing I was trying to avoid in replacing the clutch/throwout bearing/pilot bearing.)

Any help you guys could give would be greatly appreciated! Thanks!

Joshua

 

push the clutch in while idling, what happens?

 

When the clutch is pushed in at idle the sound does not go away. Obviously, the whine from the throwout bearing goes away when the clutch is depressed, but the "rocks in a can" sound remains. Additionally, when the motor is under a slight load (e.g., when the air conditioning is running, or the motor is revved up) the sound seems to lessen or--sometimes--go away completely.

 

Suspect clutch hydraulics (defective or need bleeding), clutch fork (bent or wrong one), or clutch-fork pivot ball (incorrect or not installed all the way with locking 'nut'), or clutch disk/pressure plate (wrong ones) as it sounds like the clutch is NOT disengaging the tranny.

When the clutch pedal is depressed at idle while stopped (assuming clutch hydraulics are working correctly), it should dis-engage the tranny from the engine (i.e. the flywheel and pressure-plate should continue to rotate --but-- the clutch disk and tranny input shaft should stop spinning).

This would take the load off of the dual-mass flywheel and effectively eliminate the need for its 'damping' -- so in my mind, IF the clutch is disengaging the tranny, this would eliminate the dual-mass flywheel as causing the noise at idle with the pedal depressed.

Also while stopped at idle with clutch pedal depressed, the 'release bearing' should be 'working' (i.e. ID on tranny shaft IS NOT spinning, OD on pressure plate springs IS spinning). So if any noise when the pedal depressed, might be 'release bearing'. Also the 'pilot bearing' is 'working' too, so same applies.

When the clutch pedal IS NOT depressed, the clutch hydraulics are supose to keep a small bit of pressure on the clutch-fork (via an internal spring or retained pressure) and thereby the release bearing -- this actually helps the bearing's bearings spin, and keeps it quiet -- if your getting a bit of 'whine', it might imply the slave cylinder is defective or needs bleeding.

BTW, there's still a chance the Dual-mass flywheel is defective, as at idle in neutral, with the clutch pedal NOT depressed -- is when the DM 'damping' is supose to happen (engine roughness is dampened so it's not transmitted to the tranny).

Your noticing noise 'going away' under slight load makes sense, as it would smooth-out the engine roughness, eliminating the need for the DM dampening.

There is a DM test, if you have an FSM, it's in there -- if no FSM, post back and we can dig it out for you..

 

Maybe the pivot stud is loose in the BH allowing the fork to rattle.

But if it still rattles with the pedal pushed in, this might not be it.

Also, consider the thin sheet metal inspection cover could be loose and rattling.

Just a thought