'88 951 - Vibration problem

[arnebret]

Hi guys,

I just took '88 951 stock car for a spin on a local industrial area. I did
this to test a few rubbers i replaced. The result was pretty good, the car
took corners as it was designed to: extremly stable at extreme high speeds
without even doubting of oversteer. The suspension, steering etc is all
pretty good now. The only thing that still irritates me is an immense
vibration in the stick. When i pull the car through the revs, there is a
tremendous vibration in the stick at exactly 3000rpm. I still haven't got a
clue what it could be. Of course i tested a few things, i noticed:
- The vibration is exaclty at 3000rpm and is gone below 2800 and above 3200.
In ALL gears, but less felt in the higher gears.
- The engine runs smooth at 3000rpm
- The vibration is exactly the same when the clutch is engaged. For example:
i'm doing 4000rpm in 2nd gear, engage the clutch and let the car run out,
exaclty where the engine should have done 3000rpm the vibration is very
heavy, while the engine is actually already at a steady 900rpm idle. The
vibration disappears after slowing down even more.
- When shifting back or up to a gear in wich the engine should do about
3000rpm the vibration is there, with or without the clutch engaged.

Concluding from this, i think it has nothing to do with the engine, but
there is something along the drivetrain that is very unstable at 3000rpm and
lets my stick vibrate heavily. It has to be something that is not speed
dependable, but revs dependable.

WHAT COULD IT BE??

Thanks for any help or input,

Arne

[Pilot_951S]

I don't think I understand what you are saying.

All 951s have a vibration in the gear lever at around 3000 rpm.

Are you saying that you also have a vibration that is not dependent on engine speed in the gear lever?

Clarify this

The vibration is exactly the same when the clutch is engaged. For example:
i'm doing 4000rpm in 2nd gear, engage the clutch and let the car run out,
exaclty where the engine should have done 3000rpm the vibration is very
heavy, while the engine is actually already at a steady 900rpm idle. The
vibration disappears after slowing down even more.
- When shifting back or up to a gear in wich the engine should do about
3000rpm the vibration is there, with or without the clutch engaged.

-Drew

[arnebret]

Yep, even when the engine is at a steady idle the vibration still comes into the stick when the engine should do 3000rpm in that gear. Even in neutral i can really feel the unstability, not in the stick but in the car. Trust me, if there is a vibration in the stick at 3000rpm, okay... but this is way too much. The stick is really bumping up and down, especially in the lower gears and it really doesn't matter if the engine is engaged to the rest of the drivetrain or not. So, there are 5 speeds (in mph or kmh) where my stick will vibrate, being the 5 speeds where the engine would give 3000rpm to realise that speed. It must be some sort of bearing that needs replacement.

[Pilot_951S]

So let me get this straight, what you are saying is that it is dependent on the RPM of the clutch assembly, the torque tube shaft and the transaxle input shaft. Which also makes it RPM dependent on teh engine but the vibration is there regardless of whether the engine is engaged to the rest of the drivetrain, and regardless of engine RPM once the clutch has been disengaged.

Well I think you have a problem I never heard of. I would take it to a specialist and see what they can make of it.

[arnebret]

I'm taking her to a specialist in a few days. He already had a few ideas based on my story. The problem propably lies somewhere in or from the clutch on to or in the transmission. As these are the only parts that rotate at 3000rpm when the clutch is either engaged or disengaged.

[PorscheG96]

It's the torque tube, mine does the same thing since the clutch was replaced. Either the shaft got bent or one of the bearings is becoming worn. -Trevor

[arnebret]

Thanks for all the feedback.

But i'm getting a little confused here. Some people say it HAS to be the balanceshaftbelt, some people say it HAS to be something in the clutch (flywheel or alignment), some people say it HAS to be the torque tube. First of all, let's first make clear i have a basic knowledge of cars and the 951 is pretty new to me. How exactly is the drivetrain in this car:

Enigne -> Clutch -> Driveshaft -> Transmission -> Driveaxles (CV) -> Wheels ? Or i'm i missing something here? By the way, i'm not from the USA, but from Holland, so i'm not too fimiliar with terms as the 'torque tube'. Could anyone help me out here and tell me a bit more about the drivetrain and things like 'torque tube' on the 951?

THANKS!!

[slevy951]

Arne-

I have the same vibration as you describe only at 3200 RPM.

Torque tube=driveshaft....The driveshaft spins inside a tube on bearings and each end of the tube is bolted to a bell housing. The front bellhousing holds the clutch and is bolted to the engine. The rear bell housing is bolted to the gearbox.

I don't think it's the torque tube as the vibration is there with the clutch pedal in (pushed to the floor) so the torque tube would not be spinning.

My mechanic thinks it's something in the balance shaft drivetrain causing a nasty hamonic at that RPM. It started after he replaced the balance shaft belt tension pulley on my car about 6 months ago. We noticed that the old pulley was completly made of steel (or whatever metal it is), but the new pulley is a combo of steel and plastic. The plastic part being where the belt teeth ride (black in color). We're wondering if the lighter pulley is the culprit here. Haven't had a chance to change back yet as I have other problems right now (turbo, ouch!).

If you get a chance to check that out, let me know.

[arnebret]

Originally posted by slevy951
[B]Arne-

I have the same vibration as you describe only at 3200 RPM.

Torque tube=driveshaft....The driveshaft spins inside a tube on bearings and each end of the tube is bolted to a bell housing. The front bellhousing holds the clutch and is bolted to the engine. The rear bell housing is bolted to the gearbox.

I don't think it's the torque tube as the vibration is there with the clutch pedal in (pushed to the floor) so the torque tube would not be spinning.

Thanks for explaining the tube to me. I was already thinking it had to be something like that. But is it true, that when you use the clutch, the driveshaft stops spinning as well? You only disconnect the engine from the transmission...
Well, as i said i'm new to the 951 and rear-wheel drive. So can you explain me what happens when engaging the clutch?

[slevy951]

The physical link between the motor and gearbox is the clutch disc. The disc is on a splined shaft (driveshaft). When engaged (pedal out), the pressure plate holds the disc against the flywheel, turning the driveshaft. When you push in the clutch pedal, the fork and release bearing pull back on the pressure plate releasing pressure from the clutch disc and allowing it to free spin with no assistance from the motor. The disc and driveshaft will eventually spin down. The pressure plate will spin with the flywheel as they are bolted together. The driveshaft (torque tube) will not spin with the pressure plate if the clutch is pedal is disengaged (pedal on the floor).

[arnebret]

Ok, so let me get this straight. When i drive, let's say 4000rpm in 2nd gear and release the accelerator and put the clutch pedal to the floor, the car runs out with an idling engine. What parts will still spin with 4000rpm and will gradually decrease as the car slows down? Propably, one of these parts needs looking to.

Thanks for the reply and maybe another answer

[slevy951]

"the car runs out with an idling engine. "

I'm sure I understand what you mean by "runs out". If you're at 4k, in gear, and push in the clutch, the driveline is now disengaged and the clutch disc and driveshaft will spin at whatever speed your going (being driven from the rear by the rear wheels). The engine will idle down and will spin at idle (pressure plate/flywheel/etc.). If you blip the throttle, the motor will spin up, but the clutch/driveshaft will not.

Now, if you're sitting still, in neutral, pedal out, motor/clutch/driveshaft/gearbox input shaft (remember we're in neutral) will spin at engine speed. Push in the clutch pedal, clutch disc, driveshaft, input shaft will stop spinning. Clear as mud?

[arnebret]

Yep, completly. But then the problem has to be somewhere from the clutchdisc on to the transmission, and the only thing that i can imagine that vibrates at a certain rpm (not specificly engine rpm) is a worn/bad bearing... what else could cause such heavy resonance/vibration that can be felt in the stick when in gear?

[slevy951]

Well, I think it's something in the motor/clutch forward. I can duplicate my vibration with car sitting still, in neutral, clutch pedal on floor, at 3200 RPM vibration is there. That means from the clutch back isn't spinning. So it's something from the clutch forward. Remember, shifter is bolted to torque tube which is bolted to front bell housing which is bolted to motor so any motor vibration will go from bell housing to torque tube to shifter.

[slevy951]

Another interesting note.....

My mechanics '86 951 has the same vibration. He never noticed until I pointed out mine was doing it. Now we've found the same vibration in a number of his customer's cars. We're mostly stumped on this one.....

[arnebret]

Well, i can't recreate the vibration when the car stands still. When i put gear in neutral and the car parked, all rpm's are pretty fine. No vibrations whatsoever. To add a bit of other peoples experience: all 951 owners i spoken to have a vibration at 3000rpm (or around). I also asked my Porsche Dealer, he said that the engine in the 944 is a pretty unstable block and around 3000rpm the engine is very unstable. Porsche tried to correct this with the balanceshaft and it worked out pretty nice, but the system has to be maintainted (don't we all know ) So, i'm still pretty confused of what could ever be the cause of this. I really think it has to be a bearing in the tube, or a problem with my driveshaft... i can't imagine a problem with the engine, as she is working fine at 3000rpm (also under workload).
Haynes Manual, says:

Vibration with manual gearbox
- Rough wheel bearing
- Damaged driveaxle
- Out of round tires
- Tire out of balance

Still, all these problems would only give vibration at one speed, but our problem is in ALL gears...
Anybody... any ideas?

[slevy951]

Hmmm....ok, maybe I misunderstood the nature of your problem. It sounded very similar to mine. But, if you don't have it sitting still, that may be something entirely different. Please note, I don't think I said this earlier, my vibration under the conditions above, is more subtle that when driving. It is magnified under load. You may want to check one more time to be absolutely sure.

Anyway, if it's only while driving at 3000, not sitting still, could be torque tube bearing or something in the gearbox. Any strange noises from the back? Try sitting still, in neutral, with clutch pedal out (engaged) and rev motor to 3000. Drive line is connected that way and driveshaft would be spinning at motor RPM. If it's in the torque tube, you should feel it then. If it's only in gear and while moving, I'm leaning toward gearbox.

[arnebret]

Of course! Didn't think of that earlier, i'll give her a try tomorrow (it's already 22:00 here in Holland), and i don't want to wake my neighbours with the dark rumble of an Ansa Muffler

[arnebret]

Damn! I'm really getting confused here! I just tried 3000rpm etc in all gears with the car standing still, of course with clutch disengaged. I could feel the vibration in the stick, but only a very small bit... i think it gets about 1000x worse when under load. When i put the gear in neutral and go 3000rpm with clutch engaged, i can't feel anything in the stick, and can't feel anything noticeable from the car... It can't be the box, as the box is disconnected when in gear standing still... might it be the flywheel or anything related to that? I think not, as the car does the same thing when driving with a disengaged clutch... BUGGER! I'm getting lost now, i will do a few test-runs soon and post the results... Anyhow, the car will see a specialist the 7th of jan... i hope he can point some things out...

[slevy951]

Now it sounds like my problem. Gotta be something up front. Let us know what your specialist thinks. I'm stumped.