supercedar

We have a 3,000 rpm vibration which we thought collasped MM's were causing. The vibe is still there so we tried the following.
We disconnected the front flex plate from the flywheel. We ran engine up to 3,000 rpm and never felt smoother. Now What? Has anyone tried this? I know there are others that have this problem and also know many things can cause it. We now know it isn't caused from the engine. Would it be wise to try this on the rear flex plate? then we could isolate the TT maybe as the problem. I have been told I might want to rotate the TC to flex plate 120 degrees.
Any advice?
Thomas

Mrmerlin

It sounds like the damper in the TT .
OR rather a lack of the damper in the TT,
has the TT been rebuilt?
If so was the damper refitted?

Bill Ball

If it's always RPM related and not present when you disconnect the flexplate from the flywheel, then you have isolated this essentially (but see note below) to the flexplates or the torque converter. Loose or migrated TT bearings tend to vibrate throughout the whole RPM range and are worse at higher RPM. The front flexplate and clamp are a balanced unit. If you or someone unbolted the clamp from the plate and reinstalled it in a different orientation on the plate, this could affect balance. It's not really terribly likely since the clamp is so central, but it's a theoretical possibility. Of course, verify that the REAR TT clamp bolt is very tight (~60 ft lbs) and that the rear flexplate is intact. It's possible the TC cover bearings are worn too. Hard to tell unless they make noise or your remove the TT to check them.

IMPORTANT NOTE:
There was a recent report of a vibration at this RPM range caused by bad spark plug wires, I believe. It might be the case that running the motor with no load does not reveal the vibration in this case and is not really pointing to an issue with the TC or flexplates.

supercedar

Thanks guys!

To clear this up a bit. My car which I just did MM's, trans mounts, Constantine bearings and clamp does have a slight vibe in the 3,000 rpm. It is wayyyy better than before these repairs.

On Bill's (Hb4), we just changed the MMs and are still chasing the vibration. The TT was changed by a PO about 40K ago. The transmission mounts are collapsed, but there's still a half inch between the trans and the crossmember. The vibration was stronger when he had it on the Dyno, and is pretty noticable on the ground in Neutral or Park; about the same strength as mine before the TT and TC bearings and trans mouint change. We did look at the car in the dark and no fireflies. The torque convertor was changed abt 30K ago as part of a bunch of transmsission work by a PO (the 2500 RPM stall convertor was removed and a -rebuilt?- one installed. That seems like the next thing to check.

Black Sea RD

This sounds like a problem another Rennlister had even after rebuilding his TT.

Turned out to be his trans mounts.

Strange though about your continued vibes, even though they are better. Another lister recently found vibration problems caused by his fuel injectors, IIRC.

Never ending with chasing down vibration causes with these 928s.

Good luck,

jeff spahn

It's really easy to eliminate the vibrations. Just ignore them.

Bill Ball

Let me correct my post - there was a member (malcom or somebody else in Canada??) that had a vibration at 3000 RPM under load and it was fixed by replacing the injectors. Trying to find that thread.

Ah, here it is:
https://rennlist.com/forums/928-foru...vibes-man.html

Bill Ball

Here's one that was tranny mounts.

https://rennlist.com/forums/928-foru...ine-fixed.html

Bill Ball

And here's one that was ignition parts.
https://rennlist.com/forums/928-foru...parts-wow.html

supercedar

Thanks again guy's. I am now vibe free. I was told that Porsche advised to alway's align the TC to flex plate the same as when removed. After talking with mechanic who replaced my rear flex he told me he did not have this opportunity with the new flex. So he told me to rotate the rear flex once and see if it helped and do it again if needed to the final pattern. We then re installed the exhaust and noticed the cats were touching the heat shield. We pulled down on the exhaust then tightened the clamps and mounts.
Now I believe someone could pay $70,000 back in 1988 for this car. I knew this car should be smooth.

Thanks to all for your help and a special thank you to Bill (HB4)
Now I hope we can do the same for his car.

Thomas

Bill Ball

Interesting. I know this has been the experience of at least a few people, although in theory it should not be as the flexplates are balanced units, not balanced on the TC. Glad to hear it worked.

Black Sea RD

Good show Bill about finding all those threads!

Thomas, glad to hear you are now vibration free.

I must say however that I never had these vibration problems with the flex plates after doing all my TT R&Rs during the R&D work for our products. I don't remember these flex plate cautions in the WSMs either.

Happy Motoring,

supercedar

I am told that wear particales of trans and TC sludge settle in the TC. This extra weight lives in a area which will balance imperfections in drive train. This is why they recommend keeping it in the same location when removed.
This is not to say that my cats did not help also by removing them from rubbing.
Bill, Thank you for those threads, I appreciate your help.
Thomas