Kevin in Atlanta

Locally owned. Less than 80k on the clock, sat in a storage unit for ten years. Museum piece in most respects. Local is going through it and asked me to drive it.

About 5000 RPMs crazy driveline vibration. Clutch in or out. Vibration is present at lower RPMs but nothing close to above 5000.

Twin disc clutch. Knowing little of them, can they be out of balance? I've felt TT related vibrations in the past in the 1500 to 2000 RPM range.

Another thing I found interesting. Under normal driving the shift from 1st to 2nd is uneventful.

Under hard acceleration shifter wants to go from 1st to 4th. I've never experienced anything like that. I had to repeat it few times to confirm it.

Could that be related to driveline vibration?

gazfish

What state are the engine mounts and harmonic balancer in?

Kevin in Atlanta

New mounts. Original harmonic balancer with 76k miles.

rexpontius

is it running on all 8 cylinders? if not that creates huge vibrations in driveline

Constantine

Probable front TT bearing migration backward (very typical failure for 5-speeds) which leaves a long unsupported drive shaft length at front.

Kevin in Atlanta

I think we're going to start with a compression test.

jcorenman

Speedtoys

So it's probly cracked...but this wont cause that much vibration.

Id go with TT bearings moving first too.

Kevin in Atlanta

Car is not firing on all 8 cylinders. Next to check injectors and spark.

All the the spark plugs look like this:

ROG100

Harmonic balancer looked OK when we did the TB. Rebuilt Injectors - one could be sticking.

Rob Edwards

[Pedantic] Rebuilt injectors? Or ultrasonically cleaned injectors? [/Pedantic]
Those plugs look fine from here.

How many dowels are installed in the pressure plate and tapped into their holes in the flywheel? Just because you have all 6 pressure plate bolts installed doesn't mean the clutch assembly is centered radially, and it can vibrate like crazy. Ask me how I know.

ROG100

Let's go with "refurbished" - new filter, O rings, spacers and pintle cap, cleaned and flow matched.

Rob Edwards

That's a pretty good OED definition of 'refurbished', Roger, I'd go with that.

I had a very similar driveline vibration in the '89 GT I redid last year until I installed the dowel pins back in the pressure plate. I know the dual disc is a much smaller PP but it's still dowel-centered in '84 through '86, if I'm understanding PET correctly.

Kevin in Atlanta

Owner brought the car to a knowledgeable 928 mechanic to swap the TT and remarked upon listening to the engine that the car was not running on all cylinders - that was unprompted.

Not saying either one of us is right, but I would still like to rule out the engine as the source of the vibration.

Owner plans on doing a compression test this weekend.

Constantine

Talked with the owner a few nights ago and we discussed multiple failure scenarios for these vibrations.

I too have some reservation it being only the TT, but thought to throw it out as a suggestion. Hopefully we'll get the report shortly with a complete diagnosis.

Owner did say how much he appreciated Kevin's and Roger's help with the 928 and what a great 928 owner community we have as 928ers.