Geza

1987 S4 Automatic - 130K miles. Clamps tight, preload relieved on the flex plate, Crank axial play well within specification.

I have an engine vibration at ~3000 rpm, which I suspect is related to the TT, perhaps the TT damper. Does it make sense, or is it even possible to disconnect the drive shaft from the flex plate, such that the engine can be run on its own without the driveshaft attached? My though is to loosen the front clamp from the driveshaft, unbolt the clamp from the flex plate and move it rearward as far as possible, thus separating the engine for independent testing. Is this a feasible approach?

Assuming the TT has to be removed for a rebuild, does it make sense to do this while doing an intake refresh/rebuild? With the intake manifold and associated hardware out of the way, perhaps bell housing bolt access would be easier? Thoughts?

Thanks,
Geza

928NOOBIE

You need to replace the engine harmonic balancer if it hasn't been already as part of the work to remove vibrations...I had one right about the rpm you describe; maybe around 3300 or so and it went away when I got Greg's Super Damper.

Geza

Thanks for the input - you've got me thinking, and I have a plan.

Constantine

Hi Geza,

Besides the engine dampener check which is a good idea, you didn't mention the health of your engine mounts which could also contribute to the type of vibrations you have.

The age of your 928 and mileage would mean your vibe dampener is also suspect to no longer operating as it was designed due to the crushing deformation of the rubber holders.

As far as disconnecting your flex plate from the flywheel, just remove the flex plate clamp bolt, remove the six bolts holding the flex plate to the flywheel and slide the flex plate away from the flywheel. After double checking the clearance between the flywheel and flex plate, you can then start the engine and the drive shaft/bearings will be disconnected from the crankshaft. However, as you well know, the TT body will still be bolted to the engine for possible transference of engine vibrations.

Good luck with the investigation.

Geza

Thanks Constantine. That's what I was looking for - there is enough assembly space available to pull the clamp away from the flex-plate. I also replaced the engine mounts recently using the non-OEM (Volvo, I believe) - made a big difference.

dr bob

Geza -- Be Sure to mark the position of the drive plate relative to the flex plate on the engine. You'll want it to go back together in the same position relative to the crankshaft.

Motor mounts: Put a straightedge on top of the intake, forward over the crossbar. If there's more than -maybe- 10mm of daylight between the front of the manifold and the straightedge, plan on new motor mounts before you spend time disconnecting the drive line.

Look also at the mounts on the gearbox, specifically at the space between the bottom of the diff housing over the crossmember casting. If there's less than about 3/4" measured gap there, inspect the mounts themselves to make sure they haven't collapsed.

Picture shows the intake height with new Porsche mounts. Look for gap at the letters on top of the intake casting, under the 7" mark on my straightedge.

GregBBRD

The quick test is to put a floor jack under the oil pan and jack up gently.

If the vibration is gone, you need mounts.

Geza

Thanks all. I can give the jack under the sump a try, but I did replace the engine mounts ~9 months ago with the Volvo mounts- made a big difference - With the exception of the severe roughness I get @ 3000 +/- 300 rpm, the drivetrain is pretty smooth, and isn't making any funky noises. My plan is to the put it up on the lift and have my wife run the engine up to ~3K while I fiddle around underneath around the TT, and move on from there. I'll also check the trans mounts. I'll check back after I've had a chance to work it.

Geza

Update: While in Park, I slowly swept the engine speed from idle to 6000rpm. I noticed engine roughness/vibration in the range between ~1700 and 3400, with the most pronounced roughness being at or about 1700 and 3400. Below 1700 and above 3400, the engine is as smooth as one would expect it to be throughout the entire rev range.

I had my wife slowly sweep between idle and 3400, while I was underneath trying the sense vibration on the TT through contact with a long screwdriver at both the front (by the O2 sensor) and rear of the TT. I did not notice any unexpected vibration in the TT until the engine was turning towards to high end of test range (~3200-3300). Nothing abnormal at the lower speeds near 1700.

I then disconnected the driveshaft clamp from the flex plate. The driveshaft turned freely with no noticeable noise, etc. I was quite impressed, actually, how well it rotated. Then I ran the engine through the full range from idle to 6000 (driveshaft disconnected). The engine was somewhat smoother in the 1700-3400 range, but still not as smooth as I would expect in this range. I reconnected the driveshaft and the vibrations increased to the previous levels.

Things I’m pondering:

I saw somewhere here in a Porsche document (can’t find it now) that the TT vibration damper is tuned to dampen vibrations @3300 for the automatic. Well, that is certainly around one frequency (3300rpm = 55Hz) I’m having an issue with.

Is an engine vibration issue (with the harmonic balancer, for example) causing an exacerbation with the TT, or is the TT, which is still connected to the engine (with the driveshaft decoupled) causing the engine vibration issue? I wonder if decoupling the TT from the engine (by loosening the TT bolts, which would significantly reduce the connection between the engine and TT from a vibration transmission perspective) would yield a different result.

When I changed the TB/WP, I didn’t notice any problem with the harmonic balancer, but, then again, I wasn’t really looking closely at it. Does it make sense to just buy a new/used one and swap it?

Since I have new Volvo engine mounts (and because I forgot), I didn’t try to raise the engine with a floor jack yet – I’ll have to try that, and take a look at the trans mounts.

Observation: the engine is definitely designed to run with the additional inertia of the driveshaft and torque converter in place. Without these attached, the engine is not happy, and nearly stalls when the throttle is let off – reduced inertia – not enough “flywheel” without the driveshaft and TC.

I also noticed there is an alignment mark (with a marker or paint) on the flywheel and flex plate. Is this a factory mark, or had someone in the past disassembled the flexplate, etc.? I have photocopies of the service records since practically new, and I don’t recall seeing this work, but the older records are difficult to decipher.

Thanks for reading and for any thoughts/contributions.

Geza

I seem to have finally solved my engine vibration issue this weekend.

I change the crankshaft Torsion Vibration Damper to one of my own design. At rest in neutral, the engine seemed to rev more freely, but the vibration roughness I was experiencing from ~1700 to 3400rpm still persisted, which was disheartening. When driving, the same crudeness I've been experiencing in that RPM range was still there, but felt somewhat different. It no longer felt that it was coming from the engine, but was coming from somewhere back by the transaxle.
Next day, I replaced the transaxle mounts - transaxle was sitting on the cross member. Problem solved. No more crude vibration roughness, engine revs freely. Big improvement. Looking forward to a nice run down to my first Frenzy in a few weeks!