Bill Ball

I suppose the rear axles. Since this is road speed related, I've not focused on anything from the torque converter forward. The torque converted and primary pump were replaced with no change. I suppose it could be anything from there rearward. since the vibration has been somewhat erratic, not present sometimes, harsher at others, I suspected a worn suspension bushing that would hold its position sometimes, then oscillate at other times, but they all look good. I did replace the rear upper control arm bushings anyway, since those often get compressed and make it hard to get rear camber within the desired range. No change. Even though I get no vibration through the steering, I did replace the front upper control arms (rebuilt) and lower control arms (good used). The steering rack, tie rods and ball joints are good.

I've just lived with it , but I'm tentatively pleased that it appears to have receded recently.

Sander_Baas

Bump on this more than two year old topic,

Bill Ball, did you ever find the source of the vibration? I’m experiencing the same vibration and can’t find where it comes from

Thanks in advance
Sander

928wolf

Just discovered this thread, I have the same problem. Approaching 80 mph on the highway in 5th gear the car starts to shake. feels like it's coming through the torque tube, starting to get a rattle in my dash because of it. Kind of ruins a good run down the highway/.

From browsing through this thread it looks like a pretty common problem. Anybody come to any conclusions? Seen several recommendations that an alignment job has fixed the problem for some, any consensus on that?

Supposedly the local Porsche dealer (Charlotte) has a guy who really knows 928's, but I'm sure it will cost me. Don't want to drop the cash if it won't do anything.

Pagnobito

Mine happened about the same speed, I thought was wheel balance but after ruling that out I noticed it was between a particular range of engine speed and I could replicate it in N and just increase the revs. Disconnecting the drive at the engine end and the vibration was gone so that pointed at TT or TC. I couldn't see anything wrong with the torque tube, I had the TC rebuilt and the engineer said it was fine. I replaced the TT bearings (I used Constantine Super Bearings). I think the bearings have masked the issue as I still feel it briefly sometimes when I come off the throttle. I've possibly got a very slightly bent drive shaft, would like a new Greg Brown one but getting one to the UK is cost prohibitive.

Have you dipped the clutch at that speed? If the vibration stops it could be several items in the driveline, if it doesn't you are looking at rotational mass such as wheel balance or tyres. Checking wheels is easier than all the other parts that can cause this.

OTR18WHEELER

There are so many variables for vibrations, it's hard to narrow it down to just a single item.
When I first got my 928, the old tires were a given, the cracked flex plate definitely was a cause, a new bearing in the TT helped, after all that, the front wheel bearings became evident of need of replacement, while having the front tires balanced, although the rim was true, the "newish" tire was out of round.
I still have slight vibrations at certain speeds. At 55, it rides smooth, then again at 78 it gets smoother, 120 , not so smooth.
But I've not ballanced the rears yet, I thought it may have been the tread, but it's all gone now.
A bad strut could come alive at certain speeds also.
Too many variables.

928wolf

I'll play around with it some more this weekend, but it doesn't seem to be engine speed related. It doesn't seem to be an issue at various engine speeds in the lower gears.

pdejong

My '90GT used to have it. It was strictly road speed related. I got totally rid of the vibration after balancing my rear brake rotors.

Peter deJong
Charleston SC

'90GT twin turbo
'83 S Auto

M. Requin

Very interesting- what led you to that as the source of vibration?

928wolf

Took the car out for some subjective data gathering. A couple things are going on:

The high speed vibration around 80 mph is present on-throttle, off-throttle and coasting with the clutch disengaged. I am more and more convinced that there is definitely something coming from the rear. I can feel the vibration through the seat more than anything. The large number of people who have experienced a similar problem tells me the rear suspension in general may be somewhat sensitive to imperfections in the this speed range.

However, there is also a distinct vibration in the 3000 and above rpm range, certainly more pronounced under load than with the clutch in. I suspect this is a totally different cause, engine mounts, flex plate, I don't know?

It's not a coincidence that when these 2 modes add together starting around 70 mph (~3000 rpm) they add to quite a noticeable shake.

GlennD

928Wolf,

For a suggestion, look where your front torque tube bearing is sitting in relation to specs. I had a similar problem and after lots of troubleshooting (replacing clutch, throw out bearing, pilot bearing, etc.) I narrowed it to the front TT bearing had migrated back. This results in the actual TT shaft not having sufficient support and beginning to wobble like a wet noodle at higher RPMs.

In my case, the "vibration" was noticeable from the seat, from the stick shift, and hit around 2800-3000 rpm. Did not matter which gear but yes, more pronounced under load.

If this is the case for you (migration of the front TT bearing towards the rear), you may wish to check your pilot bearing as well. I found that the vibration caused the pilot bearing to start working its way out - it was only 1/2 in when I stuck my hands into the clutch.

Good luck
Glenn

Nicole

928wolf: Given the age of your car, it's possible that you have the common speed-related vibration AND another one from worn motor mounts, issues with the torque tube, or else.

worf928

928Wolf: see https://rennlist.com/forums/928-foru...l#post12502425

Adk46

I'm wondering now if CV joints might be the guilty party in my car. Both had torn boots as-received, one for many years, the other even longer (guessing). No pitting or other signs of distress, so I put 'em back in.

It's the sort of vibration that begins at about 80 mph and slowly gets worse with speed up to [redacted], where it and other anxieties combine to form a limit. Sure feels like it's from the rear, just like a tire imbalance, except it does not come on so suddenly and vaguely feels more mechanical than rubbery, if that makes any sense. I guess the procedure must be: get a very good tire balance and have the brake rotors balanced (NAPA?). If no change, change out the half-shafts. If no change, stay under [redacted]. Don't worry, be happy.

Someone ought to sell a vibration diagnostic kit, consisting of sensors you clamp onto various places in a car that transmit signals to a central unit. One on each corner might be sufficient. This would be equivalent of what we've all thought of doing - loading up the car with passengers at each corner, selected for matching butt sensitivity.

Protocol is to drive the car and hit "record" (a short sample ought to be sufficient). Do it when there is no vibration, when there is, at different speeds, in different gears, in neutral. Swap wheels left to right and repeat. Look at the traces (better: Fourier transform to frequency domain) - that should narrow down the possibilities. For example, vibration at left rear both before and after tire swap would indicate left CV joint, bearing, rotor, or shock. Careful examination might say which - e.g., wheel hop from a bad shock will be at a constant frequency, 4/rev suggests CV joint. Vibration generally everywhere, independent of gear indicates something in final driveline.

mkriete

I'll add my experience, FWIW. I have noticed 3 distinct vibrations. One at idle (in gear, felt in the steering wheel), one at 45 mph (the whole car shock) and another at 75 mph (felt in the seat).

I suspect the idle shake is due to shot engine mount dampers.

I heard a constant clicking coming from the rear and I noticed grease was leaking around the CV joint boots, so I pulled them. One joint was loose and had notable wear. I ordered a used half shaft from Mark, repacked the joints and replaced the boots. Replaced rear bearings at the same time. The vibration is gone at 45 mph.

The 75 mph vibration was not noticeable until I replaced/repaired the CV Joints. I need an alignment, so I hope that is the issue.

928wolf

Update: both outboard CV joint boots are shot. I haven't done a detailed look at it yet but the grease (I could get a whole finger in there) was at least viscous and not gelled or dried out. Hoping and praying the boots are the only problem.

The car is parked until I can replace the boots. Any other suggestions while I'm in there? Not sure what a new set of wheel bearings and CV joints costs, is it worth it as a preventative? The car only has about 37,000 miles on it, but 30 years.

I still need to do the motor mounts, but I'm dreading that job. I don't have a lift or anything other than ramps and jack stands, neither give very much room under the car, and jack stands make me nervous.

Thanks all for the help, I'll report back on the results.