GlennD

I am looking for some advice on my vibration issue. I have tried to solve myself and before I take the next step, I thought I would check in for some guidance - in case I am missing something.

I have a vibration on my 90 GT. When I rev the engine up to approx 3000 rpm, the vibration appears and continues to increase as the rpms increase. This occurs sitting or while driving. When the clutch is pushed in, the vibration is less pronounced. And to confuse me even more, sometimes the vibration decreases enough to notice and other times, it does not start till about 3200 rpm.

So far, I have pulled the front end off the car and checked the timing, the belt, the gears (cam, oil, crank) and all is great. Timing is dead on. I removed and re-installed all the equipment driven by the accessory belts and all is good. I thought that perhaps my timing had slipped from the replacement job I did last year. All was as expected.

I looked at the fuel injectors thinking that perhaps they were plugging or getting flaky and having trouble with the duty cycle at higher RPMs (perhaps one or two were not dispensing correctly). So, I replaced the fuel injectors (they were original) with new ones (thanks Roger). Fuel lines are replaced, new fuel pressure regulator and fuel dampeners. All is good. Spark plugs are good, ignition wires good, distributor caps and rotors are good.

Only thing left for me is the torque tube. The previous owner had the torque tube replaced by a 928 Intl supplied rebuild. While I trust the tube, I do not have the same trust for the outfit that installed it. Perhaps it was damaged during the installation (bearings moved).

Before I attempt removing the torque tube, I would like to check if there is something I am missing - something I should be checking before I take on this challenging task. Any advice would be greatly appreciated.

Thanks
Glenn

MainePorsche

Motor mount status ?

ammonman

When you had the front of the engine apart did you carefully inspect the harmonic balancer for separation of the rubber connecting the inner and outer sections?

Mike

GlennD

Thanks for the ideas.
1) Motor mounts are good. Nice rocking action of the motor when blipping the accelerator.

2) good idea on the harmonic balancer. I will check it. What exactly should I be looking for and where on the balancer? I plan to use a boroscope to look (save ripping the under belly covers off). At least I can confirm it is on correctly (not reversed) - looked when I pulled all the accessories off.

Glenn

GlennD

So I investigated the vibration dampener. Pulled the front end apart again and took off the dampener. All looks good.

Running out of ideas. Does anyone have any other suggestions before I tackle the torque tube?

Are the symptoms I am describing associated with a typical torque tube bearing migration or pending failure?

Appreciate the help guys. Thanks

SeanR

Been trying to chase this down on a '90 GT that is semi-local and each time I think I've got it figured out, I fail.

I've done the torque tube, motor mounts and a few other things and now we are looking at doing the entire clutch assembly.

The issue wasn't a huge deal to the owner until I was right seating it and I felt his shift *** (stop, that's dirty, don't go there) and commented about the amount of vibration. He thought it was normal.

Suck.......

hacker-pschorr

Oddball thought, when the green wire shorted out on my 79, I had similar vibrations and the motor rocked more when revved in neutral.

GlennD

Sean. Thanks. My vibration gets quite bad at 4000 rpm to the point I am afraid something may get damaged. Idle is very smooth. 2000 rpm is smooth. Starts to show up at 2800 and continue from there. I am puzzled by this - never had this in a car before.

Glenn

ammonman

Is the vibration more a mechanical imbalance or is it due to mis-fire? Worked on a car with a CPS that was going bad. It would idle fine and engine would run smooth up to about 1500 rpm, then start missing a smidge and get worse as rpm's increased. The CPS was sending a clean enough signal when the frequency was low but as the revs and CPS signal frequency increased the noise in the line started washing out the signal and the car would miss. Just a WAG.

Mike

JET951

How old are the engine mounts ? , the reason I ask is because the engine will rock just like you mentioned when blipping the throttle with completely stuffed mounts that( when collapsed ) are not isolating the engine from the subframe / car itself & the vibration they cause is dreadful

MainePorsche

On an S4 a quick look for engine mount status can be by comparing the intake height relative to the crossbeam. I'm not sure on your MY/intake, but someone will chime in.

GlennD

Thanks for the suggestions - keep them coming - thanks

The motor mounts look good - both by my assessment (oil pan not touching the crossmember, mounts not looking collapsed) and by another 928 member (Malcolm - who also checked in on me when I did my first timing belt, water pump, gears job).

The vibration really does not seem to fit motor mounts - I am talking the drivers seat turning into a vibration chair, feel it first in the gear shift, it gets to the dash, I can even feel it in my legs from the pedals. I have had bad motor mounts before (albeit not on a Porsche) and this is a completely different vibration.

This definitely feels like something is not in balance as the vibration increases rapidly and in amplitude as RPMs increase - perhaps the clutch assembly (as Sean suggested)?

I may substitute ignition wires as well - just to rule misfire out (the wires that came with the car look and feel new)

Turning into a fun investigation. I sure feel comfortable taking the front end of the engine off now ... lots of practice.

I really appreciate the assistance - thanks

Glenn

hacker-pschorr

Sometimes you can diagnose bad / faulty wires by running the car in a pitch black garage and misting water over them. If there is any arcing going on you will see sparks.

You can also check the plug ends with a multimeter, I'm not sure what the resistance should be, but I doubt even a bad plug end would cause this kind of a miss unless it was so bad that one cylinder wasn't firing at all.

The miss I'm describing with my 79 would be the equivalent of the ignition amplifiers shorting out, the two mounted down in the nose of the car. If anyone local has a good running car, it would be easy to swap those and see if anything changes. Maybe even the EZF too.

WallyP

Based upon two factors, my first suspicions would center in the flywheel/clutch area.
- The vibration changes when the clutch pedal is depressed, but does not go away. If the torque tube were the problem, the vibration should normally go away when the clutch is depressed. Pressing the clutch changes the pressures on the pressure plate, clutch disks and flywheel. (And the crank, but that kind of vibration on a crank often turns disastrous relatively quickly.)
- The vibration seems to change over time, with no known cause.

Some possible causes might include:
- Loose pressure plate bolts. Not too likely, as I would expect rattling at idle.
- Loose flywheel bolts. Ditto.
- Loose pressure plate ring. This might be the most likely cause. When the clutch is pushed, the ring is free to move off center. If it does, vibration will grow with RPM. If the ring settles nearer center, the vibration will be less. If it settles on center, no vibration.

My next step would be to pull the clutch for a very close inspection.

davek9

OP question, when you pushed in the clutch did you put the trans in gear?
That would insure the Driveshaft (in the TT) would stop spinning as the car is not moving, in Neutral the driveshaft could still spin due to friction and or clutch plate drag.

Dave K