DanielD

Ok, I've searched high and low for an answer to this one and still don't have a clue.

1990 S4 Automatic

Car cranks fine but shakes more than it should at idle in park or neutral. There is no bearing noise front or rear, no noises out of the ordinary.
Put the car in drive and move forward; at 1000/1100 rpm the car rumbles; the vibration is through the whole car, I can't tell where its coming from.
As you accelerate normally you again feel a vibration through the car at 1900/2000 rpm. Continue to accelerate to highway speed and car smooths out.

I changed the motor mounts about a month ago. This evening I put the car up on jacks and reinspected the mount install to ensure I didn't botch the job but couldn't find anything there.

The vibration is rpm related, not speed related.

Any ideas are greatly appreciated.

namasgt

the vibration started after the MM were changed ?
was the torque tube taken out of this car before ?

Tom. M

Cam timing? Did mine on my GT a while back, and it cured a buzziness I felt through the shifter around 4500rpm..

WallyP

Check the driveshaft couplings front and rear.
Check bellhousing bolts for torque.
Check torque tube bolts for torque on both ends.
Check transmission mounts.
Check harmonic dampener for slippage.
Check harmonic dampener bolt for torque.
Check all belt-drive accessories for tightness.
Torque converter bearings?

Lots of possibilities - I'm sure that other folks can add more.

Mrmerlin

Follow Wallys post and also consider these.
Did you cut down the stud on the motor mounts where they stick through to the rack??
It might be possible that stud is contacting the rack and transmitting the vibes. ( I try to cut the stud off flush with the new nut)
The oil pan should be flush with the cross member. With good mounts installed
When you check the rear pinch bolt see if you can stick a wire into the TT and see how far it goes before it hits either the damper or the rear bearing, check for the exhaust hitting on the heat shields.
Also look at the transmission mounts , is the trans axle hitting the rear cross member,
When you remove the lower bell housing also take a look at the flex plate for cracks

DanielD

The PO had new bearings installed in the torque converter last fall. There is no bearing noise.
I left the car on jack stands last night so I'll spend some time under there tonight running through as many of your suggestions as I can this evening.
With me luck!
D

the flyin' scotsman

I'd double/triple check all ignition components including the orientation of the spark plug wires, the rotors and condition of the disti caps.

If your unsure as to what wire belongs where the distis have the numbers stamped in the plastic. Remember that #1 cylinder is PS front.

danglerb

While driving do you get the vibration with your foot off the gas slowing down?

If so I would suspect mechanic balance issues instead of fuel or spark.

DanielD

When you let off the gas the vibration goes away. I've rechecked the motor mounts, no problems; everything is tight. I checked the mount studs hitting the steering rack; eveything is clear.
I jacked the car up on stands and ran the car while on stands. The whole driveline shakes at 1000 rpm and again around 1900; front and rear seem to shake the same amount. The transmission mounts look good; they must have benn changed in the last few years but I can't see it in the receipts that came with the car.
Obviously a vibration source will cause the whole driveline to shake as it is bolted together.
I give. I'm going to take it to a shop this week and see what they can do. I'll keep you guys posted.

worf928

Unscrew and pull the carpeted cover that hides the ECUs. Underneath there is a clear plastic cube-like fixture. This is the ignition monitoring relay. With the relay in view from the driver's seat, start the car. After a few minutes of idling do you see a green or red LED light from the relay?

Mrmerlin

follow Dave C advice, also run the car in the dark and see if there are any wires sparking after you have verified the spark plug wire routing

DanielD

worf928, I let it idle about 5 minutes and didn't have any led lit up.

worf928

Was it rough and shaking as in your OP while it was idling with no green or red LED lit? If no, then the test isn't valid until it's rough and shaky. If yes, and the car was warm(-ish) then the ignition monitoring circuit is probably not part of the problem. But, keep the carpet cover off for now and continue to monitor the status of the IMC.

DanielD

I'm not going to admit I had someone work on this problem for me but Smith Motorsports in Panama City Beach, FL does great work.

The solution to the vibration issue: replace the torque CONVERTER. Smith's troubleshot it and got me a torque conveter installed within a few days. The car rides like a dream. I highly recommend those folks.
The 928 community is tremendous.

I drove the car down from Memphis to Destin, FL as the vibrations were gone at cruise speeds. The car drove down just fine but the vibration had gotten noticeably worse at 1000 rpm. It began to worry me enough that I checked with a Rennlist member that lives here and he recommend Smith's Motorsports. He was on the money. Thanks Matt!

My wife even noticed and she's not pissed!

Thanks for all the advice but this one was beyond me and apparently is a rare problem so I don't feel so bad.

D

Mrmerlin

glad your back up running,