jeff spahn

I have encountered a new problem with my 90 S4.

After the engine pull and re-installation this last weekend I have encountered a very intermittent vibration.

It happens at highway speeds and only comes on about 10 minutes into the drive. It feels like the whole car is shaking in the middle.

If I shift into neutral at highway speeds, it does not affect the vibration.

If I pull over to the side of the highway and stop, then accelerate back up to speed it doesn't come back for the rest of the 10 minutes I have to drive home.

Almost feels like a bearing somewhere. Not coming through the steering wheel like a wheel bearing would feel up front. Gets faster with speed up, slower with slowing down until you stop, then gone. Put car in neutral at a stop and rev, it is gone. Feels speed dependent, not engine rev dependent.

What did I do differently this time when I pulled the engine? Only different thing was to remove the rear transmission bolts when I slid the transmission back instead of just loosening them. I also unhooked the parking brake cable from the rear crossmember to allow me to get to the rear transmounts easier.

Front wheel bearings feel fine. Wouldn't a wheel bearing be something there all of the time, not just once during a drive? What about torque tube, but wouldn't that be engine speed related? Tires out of balance would be there all the time wouldn't they unless I am getting some strange harmonic feedback or something with one tire out of balance.

Everything up front that I took town when doing the engine install (crossmember, rack etc ) has been torqued to spec. Super clamp torqued to spec, but then wouldn't that be a constant vibration if it were something on the flywheel or something .

Could the trans have shifted around or something or did I miss something critical when I put the two trans mount bolts back in?

Any suggestions?
It is enough that it shakes the whole car like being on rumble strips on the highway. Again, when I pull over, stop and then get back up to speed, it is gone. Happens maybe around 60-65 mph.

Speedtoys

Look for something not connected well in the central driveline..not a tube bearing, but..yokes...?

NC928S4

An out of balance tire usually only is felt at a certain speed(resonant frequency). A worn ball joint or tie-rod could amplify the effect too. Maybe a loose mount or missing bushing in the rear would cause a problem.

Since the vibration is also felt when in neutral at I assume is idle speed I would rule out Torque Tube bearings and dampener since they spin at engine speed.

I agree it sounds like a bearing issue since it is temperature dependent.

Good Luck!

Bill Ball

I can only sympathize. I have a similar vibration (road speed related, shakes whole car rather than steering wheel, not always there, not wheel balance or axles), although mine is at 80+ MPH. Almost always these vibrations are rear wheel balance or roundness issues, but I have tried 3 different sets of wheels and tires, repeatedly re-balanced. Not torque tube because that would be engine speed related.

My vibration seemed to start soon after replacing the tranny mounts. The tranny mounts are pretty straightforward, so I can't see how I might have goofed that up. There are shims for height matching side to side AND there is a >1mm gap that needs to be present on both side between the mount and the trans case. Somehow I think the tranny mount is an issue, but I can't locate the specific problem. Since you slid the tranny, perhaps that is something to look at (see WSM below).

I assume you have ruled out rear wheel balance, out of round/broken belt tire, axles and rear wheel bearings.

jeff spahn

Bill. I was not aware of this. Perhaps the trans moves over, gets in contact with the subframe or something and vibrates. That could also explain the parking brake handle being super tight now. Never considered this. I'll look at it this weekend.

SteveG

If I shift into neutral at highway speeds, it does not affect the vibration.

I read that as -- it continues to vibrate. If so, my vote is TT bearing.

dr bob

Symptoms point away from TT since it does not change with engine speed but does with road speed.

I'm with Bill. Start with the correct trans install/align procedure, especially since that's what has changed in your procedure.

jeff spahn

What are these shims they talk about made of? I don't recall any falling out.

Bill Ball

As to the parking brake, when I slide the tranny back so I can remove the bellhousing and work on the rear main engine seal or TT, I remove the parking brake cable brackets from the crossmember and pull the cable down and out of the way so I can get a bit more room for the tranny to go back. Getting the cable and its brackets back up on the crossmember and mounted correctly can be a chore. If you did this, then take a look at the cable mounting there. The cable housing may not be seated properly.

As to the TT as a possible source of vibration, I would expect the vibration to be engine speed related in an automatic. With manual trannies you can isolate this source by putting in the clutch. With automatics the TT always spins at engine speed, so you should feel it as you go through the gears at roughly the same RPM range. I only feel my vibration above a specific road speed, same as you. Still, I removed the TT and replaced the TC bearings (although they were fine) and carefully examined the TT. It was fine, so I put it back in. With bad TT bearings in other cars, I could feel the vibration at an engine speed range whether the car was moving or just sitting in the driveway reving in park or neutral, and the vibration is generally always there, increasing with engine speed. That's not what you or I have. Strangely enough, Nicole has a very similar vibration. She had Greg Brown take a very close look at it, and he rebalanced the wheels, but it was not eliminated. We had already very carefully looked at her brake rotors, wheels and tires for run-out with a dial gauge and there was nothing significant. Also, we recently replaced her rear wheel bearings and that did not change the vibration. You should focus on rear tires, wheels, brakes, axles - they are the most likely to cause a road speed related vibration. That's why I am stumped. I have had similar vibrations a couple of times in the past and always found it to be bad tire balance. Not this time.

Bill Ball

The shims are squarish metal pieces with a bolt hole in the middle, like a thick but square washer, and sit on the crossmember tab at the tranny mount - they would probably just stay in place if all you did was slide the tranny back a bit.

tmpusfugit

how about a bad CV joint? I have not had that problem in a 928 but I have had it in another car with CV joints. It took a fair amount of time for them to get hot and when they did it was nasty with a lot of vibration. They were shot and were worn and rusty too and required complete replacement. The boots were intact and in good condition...it was suggested the car had been driven in saltwater....

Mrmerlin

its also possible that the shims have fallen out and got caught between the crossmember.
your other clue is that the E brake is now hard to operate.
I would remove the E brake housing from where you touched it and examine how it was reinstalled

the flyin' scotsman

How does the transmission lever shfit Jeff?

I ask as this is very similar to the issue with the 87 S4 Im working on. Didnt move the tranny but did do the pinch bolt/flex plate release. the shift lever is quite still compared to my car.

Look forward to following this one.

jeff spahn

Are the shims on the bolt or are those square washers something different? Does the orientation of the square washers on the bolts make a difference. I mean, do they have to be perpendicular to the trans or can they be at an angle (yes I know perpendicular is an angle but lets not split hairs here)

the flyin' scotsman

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