sparks259

I have had the x-74 on my 2000 c2 for 30k+ mi. With no prob. Whatso ever.

Silk

I made a mistake above.

The transmission mount was not replaced as stated in my opening post. The motor mounts were instead. Should have done the transmission mount too at the same time as it seems to be the culprit.

CV joints, boots and drive shafts are in tiptop condition after detailed inspection. Differential ok. Wheel bearings in good condition.

The transmission mount was shot. My mechanic says he is sure now it is the mount who caused the vibration and knocking. As he could somehow simulate it (at lower frequency).

So it should be a cheap and easy fix. A coincidence that it started together somehow with the newly installed suspension setup which made the diagnostic procedure more complex than necessary.

After all, the posts I referred to were indeed giving a good lead.

alpine003

Still trying to comprehend how you can have a "grinding" noise as you described it from a worn/failed transmission mount.

Also trying to figure out where the "grease" came from if your CV boots are in tip top condition. Only the engine mounts have fluid in them.

Do you have a pic or did the mechanic give back your old tranny mount by chance?

Silk

Yes I understand your doubts.

Maybe the noise does not qualify as grinding. I made some recordings with my phone pointed to the back. One video is a roundabout: short right turn followed by a round left turn. The other video is a left turn at a traffic light, followed by a long 180 degree turn up left. The noise is in reality much louder.

click for video here

I spoke to my mechanic on the phone yesterday. He first dismounted both shafts left and right and all the CV joints were okay, flexing and moving smooth. He said he could simulate the knocking noise by manually shaking the transmission when he jacked it up slightly, and that there was a lot of clearance that should not be there. Also the fact that both motor mounts were crap and changed some months ago he blames himself a little bit not to have tested the transmission mount to such an extent as now.

The grease I found leaves me also in doubt. Could it be from when the new shocks were mounted they used grease for something in the suspension? Maybe it doesn't come from the car at all. All I know it was greasy and very sticky. I maybe wrongly suspected it to be joint grease. So I am not considering my diagnosis in the first post as bullet proof.

The parts were ordered this yesterday, Installation was today or tomorrow. I will pick the car up tomorrow Thursday of on Friday.

Maybe any further questions I should ask him?

Your comments are much appreciated!

alpine003

I've had grease leak out of CV joints with the boots seeming to be perfectly intact. Sometimes they do escape out of the boots occasionally. So this could still be a possibility.

From the feedback from your mechanic, it sounds like it could be torn to me. "knocking" though rather than "grinding" would be more consistent of a failed mount.

Do you by chance have a Tiptronic in your 996.2? Those can fail but I have yet to see the 996.1 or 996.2 manual mounts actually fail. I've seen "worn" mounts where they sag but not any that have torn personally. Once again, I'm talking about 996.1 and 996.2 NA manual trannies.

I'd request the worn tranmission mount back from your mechanic just so you can see for yourself.

Silk

Yes I have a 2002 996 3.6 tiptronic.
I will post pics of the mount as soon as i get there.

Did you check out the two videos via the link to have an idea of the noise?

The occasional release of grease could be related to the vibration going through the drive-train? Is it something to worry about?

Could it be that the shot mount allowed the whole drive-train to flex to much outwards causing stress on the joints. I mean, as such the transmission shifting to the right a bit causing the clearance that the right shaft normally has to allow for the joint rotation with the suspension. Hmmm I hope you get what I mean.

alpine003

Ah I knew it had to be a tip tranny mount. I think I know what you mean and I could see it as a possibility especially with swift transitions and high g loads.

It would be interesting to see if both sides were torn or only one side.

laphan

Make sure your mechanic replace it with 997 tiptronic mount. Porsche changed the design to solve the mount issue with 997 part number.

Mine was toast at 60K miles. 996.2 tiptronic mount is a known issue. It should take 1-2 hours to get it replaced.

Silk

check out the videos I made last week here

See it simplified as follows (I will number the passages to make it easy to comment)

A) the suspension makes the wheel center connection point translate up an down, an absolute vertical movement (yes very simplified).

B) The outer end of the outer CV-joint can only move vertically up and down bue to rotation around the joint. Hence the clearance available in the shaft.

C) To be able the outer joint end to slide horizontally back and forth when rotating around the joint under progressive loading of the suspension, i.e. in a curve under accelerating.

Now here comes my wild theory

1. Taking in to account the lowering (less clearance available compared to original setup). The car sits lowr in its suspension.

2. The bad transmission mount causes:
a) Considering turning left the engine tends to move right. Is it the same way the engine will move/rotate in the engine mounts when accelerating (loading the crank)?
b) Then the bad mounts cause the whole drive-train under acceleration to shift too far to the left i.e. using up all horizontal clearance in the CV joint shaft.

3. Considering that the shaft doesn't have enough space to slide further horizontally upon further rotation around the CV joint (higher loads on the suspension). It will cause high loads on the shaft and makes it to bounce back at a max. threshold, after that forces load again, bounce back, loading, bounce, etc Hence causing a rhythmic knocking noise, also a little grinding audible.

4. This would mean
a) excessive longitudinal loading on the CV joints. Which could maybe explain grease pressed out.
and/or
b) following this longitudinal forces the ***** in the joint start 'grinding' 'knocking' due to the high pressure and temperature (friction). The grease could seep through under pressure.


I think my mechanic did not open up the boots to see if there are any traces of wear or frictional heating (yellowish/blueish/purple/black coloration of the steel). He just checked the boots and flexed/rotated the joints and found them to be in good shape. Wheel bearings were also fine.

Maybe I am speculating to much. But the grease leaves me puzzled.


That is however not the situation here. It is not a a swift transition that causes the noise to appear. The knocking appears after long persistent one way loading. In my case long sharp turning left with good acceleration, i.e. from standing still. It seems as a certain threshold of loading needs te be reached. It also stays there as long as the same acceleration is used. When lowering the load it seems to disappear only under a lighter load then it started, see it as a kind of hysteresis phenomenon if you would plot noise vs loading in a graph. It will appear under load A and get louder then lower again and dissappear under load B with B lower then A

Silk

As you can see in the photos the transmission mount was totally kaputttttt.

Photos 2 and 3 are seen from the right; 4 and 5 from the left.
On the right inner flange you could see a spot where metal has been rubbing over metal, scratched and burred. That was were the knocking noise came from.

So no CV joint failure as in Title.
Mount is replaced. Car drives smooth again!

alpine003

Glad you got it sorted. The torn mounts is what I had pictured in my head. Hope you got the solid 997 replacements put in there.

Silk

Your tip to replace it with the 997 mount came too late. I need the car tomorrow for a DE. And there was no time to order the part for him.

But the coincidence is that I ordered a couple of days before the mount started to knock already a set of sport bushings for the tiptronic mount from FVD. They are in back order. So we agreed the old mount will get the new bushings. And the new mount will be replaced.