As you can feel the vibration during stand still revving of the engine, it points to a rotational assembly misalignment or out of balance. Sorry do not have any theories about that.
However, if there is a secondary issue, I was thinking it could be related to air pressure / pulsing. While driving at speed if you are getting any kind of air pulsing that could give you a naseaus effect. The most common is the sides of the windshield. On the 90+ Porsche started putting foam rubber on the sides of the windshield to reduce noise.

 

As I understand it, if the damper on the front starts to fail you get more vibration at higher revs. As the engine is good below 2600 and above 3100 I actually don't think the engine is the problem.
Indeed it could be the damper in the Torque tube, the bearings hadn't moved when I had the tube off but couldn't check the damper without removing the bearings. I've rebuilt a tube before and if I do it again will use super bearings from Black Seas and do away with the damper.

Make me laugh when they say it failed due to abuse, It's not like it was taken rallying and they are built strong but not strong enough to withstand a component failure elsewhere. I know running on 4 cylinders can cause damage though.

With the new tyres rated at 68db and I retrofitted the rubber inserts you speak of it is actually fairly good on the noise levels up to 70mph. It's not the same as a modern car of course but it's better than it was.
I'm sure once I find and fix the rotational assembly issue it will cure the other.

 

If this came into my shop with this complaint, I'd put it in my chassis dyno and have the owner drive it to see if the problem was still there.

That would eliminate a whole bunch of things...one way or another.

One other thing that seems obvious, to me.

If the Volvo mounts helped, wouldn't it make more sense to install the proper mounts that were designed to absorb the harmonics of the 928? After all, the incorrect part for the incorrect application can't really be expected to do the correct job.

 

Hi Greg,

Thanks for your input. I asked a local company about putting my car on their rollers but they said because the vibration was not diagnosed they would not be happy putting it on the rollers in case it damaged them.

I've obviously got a problem with vibration and new mounts have masked that a little. It's more than the inherent vibration you get from any car and to an extent bumps and ripples in the road when travelling may have confused the issue. I really don't think that the premium you have to pay for the Porsche mounts is equal to the percentage of difference they might make over the Volvo version. Nor do I think any mount would de-couple the amount of vibration away from the seat of my pants.

I understand if you haven't read all of the thread, I get vibration at a certain rev band while in N and not going anywhere. I'm going to separate the engine from the drive train and see what happens as this will tell me which end of the car to look at.

 

I have read the thread and understand what you are saying.

All of these cars vibrate to a certain extent. I've never seen one that made anyone sick....so this is a severe issue (and interesting.)

Hurt the metal rollers? Were they serious? Never mind, doesn't matter, that is their choice.

Your plan is a great place to start, given that you can't run it on a dyno.

I'll be really interested in reading what the results are.

For the harmonics to actually cause you physical issues, they have to be emanating from a fairly massive source. The engine would certainly be my first consideration, as a source.

I've been seeing a tremendous amount of destructive wear on main bearings, increasing over the past 10 years or so. I'm certain that the harmonic balancer (it really doesn't "balance" anything.....it's actually a "harmonic absorber"), on the front of the engines, all now have "rock hard" rubber between the two pieces of iron, and quit "absorbing" harmonics years ago. Combine this with the fact that Porsche made this "harmonic absorber" a slip fit onto the crankshaft (which negates a huge percentage of the ability for the harmonics to be transferred to the absorber) and you've got a huge potential for some really crazy harmonics to come from that source.

This is such a huge issue that I'm now actually having "proper" hubs CNC machined to use ATI dampers on 928 crankshafts to replace the "rock hard" original dampers.

I'd also guess that the torque tube shaft has the potential to create some powerful harmonics. Keep in mind that the "later" torque tube shafts in the automatic cars were very prone to failure [because the larger shaft diameter had to be reduced right at the very end of the shaft, which transferred virtually all of the twisting (and harmonics) right to this area.] This could create some crazy vibrations, just before failure, I'd guess.

Good luck. Keep us informed. There's things to be learned here!

 

Greg,

Yet more fascinating insights from the oracle! Now I can make more sense out of the "harmonic balancer" for whatever it does [or does not] do.

When the OP posted his problem it immediately reminded me of my problems 16 years ago- whether or not there is a connection remains to be seen. I felt this strange vibration at exactly 3050 rpm [that number sticks in my head] that to my mind had to be some kind of harmonic. The drive shaft sheared at the splines and with a new torque tube the vibration reoccurred when the clamp slipped and Porsche had no answer to this.

Similarly I have been puzzled why Porsche saw fit to only put two bearings on the A/T torque tube [OK it is a bit shorter than the manual] yet they still had to fit that monstrosity between the torque tube bearings presumably to dampen harmonic noises in the cabin. Given what I have seen it does not take too much imagination to understand that if the torque tube rubber bearing cushions have gone solid, whereas they may still hold the bearing in place, they may have degraded to the point of causing strange vibrations given the shaft speed and that shaft taper can only make things worse I suspect.

I have never seen anyone report the specific vibration I experienced and cannot believe my situation was unique however it is what it is and has never reoccurred since Loctiting the splines and suppressing the clamp migration.

At the moment my money is on the problem emanating from the torque tube bearing area but nothing would surprise me. The engine and drive line are a "total system" so the interactions can be rather complex.

On my Cayenne Turbo S Porsche seemingly could not design a drive [Carden] shaft with intermediate rubber supports that last more than about 6 years so the 928 is not doing too bad by comparison.

The OP has an interesting issue here and I sure hope we can help him resolve this matter given all he has done to date. Doing things by process of elimination is far from ideal but sometimes it is all we have left to turn to.

The only other suggestion I have at the moment when the system is decoupled is to clock the drive shaft and see if the clock reading is concentric and if changes to the readings can be detected with applied lateral [sensible] loads. Just thinking out of the box here a little and wondering if such test might give a clue?

Rgds

Fred

 

Decoupled engine runs fine without any excess vibration through the 2600-3100 range, it's normal right through to 3500. I didn't take it any higher as the engine is cold and it was fine over 3100 before.

So I am looking at a Torque tube or Torque converter issue.

I've removed a TT from a manual car without dropping the gearbox. I remember having to push the gearbox back a bit. I've only ever dropped the TT and box together on an auto.

Is it possible to decouple the TT at the rear clamp so I can spin the TT at engine speed without including the TC?

 

With the engine decoupled it was smooth right through to 3500rpm...

With the TT reconnected as it should be I feel the bad vibration between 2600-3100 rpm but I can feel some vibration right through the range now! It's only slight either side of that band but definitely there and I wouldn't have noticed the difference if I hadn't disconnected the engine from the TT.

I don't think it's a coincidence that there was a bad bearing in the TC housing when I bought the car and my money is on a balance issue with the torque converter.

Unfortunately that is a whole heap of work but on the other hand I have a week off work from the 18th May so I guess I will be dropping the gearbox and finding a place that can test and balance my torque converter. The TT has to come down with that anyway and if the TC is good I will look at that.

I will update when I find out what the problem is.

 

So I've sent the torque converter to get it balance tested and will get a report on Monday.

I span the drive shaft with a wheel on my drill in a friction manner. I'm not sure how fast I got it spinning and it stops quite quickly once the wheel is taken off.
The damper is about 2-3" from the rear of the tube and the front bearing is about 11" down the tube which is too far isn't it?
The splines look fine and the bearings are quiet and tight still but if the front has moved I am into a rebuild anyway. That's OK, I've done one before and still have the threaded bar etc.
Just wish Constantine wasn't closed until June as I could do with getting it all done before the end of May ready to take it on a road trip. It looks like it wont be done and I will have to take my BMW M135i instead (you don't get the 1 in the US)

 

I removed the drive shaft and the two bearings are 9" and 27" from the engine end of the tube which seems about correct to me? It was tough getting the shaft out, it was IN there.

 

Thats the dimensions provided by Constantine for his superbearings and where I set mine:

 

I have not read this whole thread in detail, so I don't know if I miss anything. But this sounds exactly like the vibration I had in my 90GT. I bought the car with 74,000 miles on the odo. At 80 mph there was always a fairly strong vibration in the rear. It felt like the whole rear suspension and/or transmission were resonating. And it was strictly road speed related. It would disappear around 120 mph. It annoyed the hell out of me for about 5 years.
After balancing the wheels several times, and changing tires, running the car with and without the spacers (which didn't help either), I decided to find the problem by running the car on stands in my garage. Same vibration, with or without tires.
To cut this story short, my brake rotors were the problem. After statically balancing the rotors, the the car now runs smooth at any speed.

Peter deJong
CHarleston/Houston

'90 GT_TT

 

Cool, glad you found the source of yours. Mine vibrates with only the torque tube and torque converter spinning and no wheels turning so it's not my wheels or brakes but I should have an idea what it is in the next few days.

 

This diagram is for the 5 -speed TT. It's a bit different for the auto TT.

Cheers,

 

The torque converter rebuild company failed to stick to my instruction of testing the balance, they just opened it up and reported back that it's all Ok. So I don't know if that was the cause of my vibration because they will balance it after the rebuild, just replacing o-rings etc as springs and clutch all good.

It's frustrating as I needed to rule that out and now I don't know!

I shall go ahead with the most likely culprit and rebuild the tube with Constantine bearings once they open for business again in a few weeks which will delay me putting it all back together ready for a road trip I had planned and I will have to take my daily driver instead.