HELP! '99 C4 engine/driveline problem

[wbgordon]

My 1999 C4, a 6-speed, has 53,000 miles. The motor was replaced by Porsche at 37,000 miles (two weeks after I bought it) due to a sudden crack in the block. I had a new clutch put in at the same time but did not replace the dual mass flywheel. The issue now is a subtle yet annoying body vibration at 2800-3000 RPM. It feels like it's underneath the seats and almost sounds like a hum.

1) It seems RPM related rather than speed related. No vibration leading up to 2800 RPM and no vibration after 3000 RPM. I cannot feel a vibration when driving in 1st or 6th gears. Most prominent in 4th.

2) Clutch performs normally with no abnormal noises. Ditto for the transmission. There is, however, a noticeable buzz in the clutch pedal when you shift. I have driven other 996's, including a new C4S, and they all seem to exhibit some degree of buzz in the pedal, although not as much as mine. Could this be related to the vibration I am feeling?

3) Private mechanic looked at motor mounts and said they look normal. Does that necessarily rule out a motor mount? Or a tranny mount?

4) Mechanic also performed test to determine if dual mass flywheel center was pivoting normally (supposedly it is supposed to able to pivot about 15 degrees, which aids in defusing vibration). We have not removed the flywheel, however, so we can't be sure if it is out of balance. That seems like a possible cause.

5) Car has been aligned and wheels balanced recently. Car has brand new set of Michelin Pilots and ride quality is superb.

6) The dealer, Reeves Import Motorcars in Tampa, thus far says this is normal, particularly for a car with 50k plus miles.

Thank goodness for discussion groups like this! I welcome your input!!!

Bill Gordon

[Loren]

2) Clutch performs normally with no abnormal noises. Ditto for the transmission. There is, however, a noticeable buzz in the clutch pedal when you shift. I have driven other 996's, including a new C4S, and they all seem to exhibit some degree of buzz in the pedal, although not as much as mine. Could this be related to the vibration I am feeling?

IMHO... Yes, I think there is a relation. I think what you are describing is drivetrain related.

3) Private mechanic looked at motor mounts and said they look normal. Does that necessarily rule out a motor mount? Or a tranny mount?

Sometimes a small crack is very hard to see in both motor or transmission mounts.

4) Mechanic also performed test to determine if dual mass flywheel center was pivoting normally (supposedly it is supposed to able to pivot about 15 degrees, which aids in defusing vibration). We have not removed the flywheel, however, so we can't be sure if it is out of balance. That seems like a possible cause.

The flywheel test must be carried out as follows to rule it out:

1. The twist test on the DMF described below must be carried out by hand and without any tools. The test can be carried out in installed or removed status. (In removed status, a second person is required to counter hold).

2. Can the DMF be twisted by approx. 15 mm to both the left and the right, and does it then return to approximately its initial position? -- Yes, flywheel is okay.

3. Can the DMF be twisted beyond the dimension stated above (excessive turn) without a noticeable increase in the spring force? -- Yes, then the flywheel is faulty.

Other potential causes could be an out of balance clutch plate (since you said this was replaced) or a loose or bad throw out bearing. The release arm (assembly) of the throw out bearing was changed in MY2000 so you might want to make sure the newer parts were used.

In the end is sounds like someone familiar with these changes should inspect the car. Again, just IMHO

[wbgordon]

Hi Loren,

Thanks for your prompt reply. Re: DMF, does the flywheel have to be removed from the car to accurately test its status. My private mechanic (Frank Eibell, Largo, FL) did perform some tests on it without removing the flywheel and came away convinced the flywheel is performing OK, although, again, he cannot be sure it is not out of balance. Second, of all the things you mentioned above, which to you seems the most likely culprit?

Thanks.

Bill

[Loren]

The flywheel does not have to be removed for the test.

It's really hard to say with an RPM only type of vibration. If the clutch feels funny (buzzing) or you've heard any "other" noises from the clutch/bell housing then I would go after the throwout bearing and actuator as those are known to be a problem on the early cars.