[s-cars] got a new sound

Sun Mar 16 14:07:09 PDT 2008

I had a "knocking" sensation in our A4that only occurred when accelerating hard from a standing start. Turned out to be the center bearing was worn, but not torn. It was evident that it was allowing sufficient lateral movement that the driveshaft was rubbing the metal part of the center support. The solution was to replace the shaft as the bearing isn't easily replaced on that car. On my ür-S, the vibration is speed-relative, @ 45 and 65 mph, and is not a knock, its certainly a high-frequency vibration. Could be in the driveshaft-end joints (CV/U?) the rear diff, I don't know. 

I'd love to make it go away, but it isn't bad enough yet for me to have started tearing into it yet. 

Wylie Bean
90 cq
91 90q20v
92 UrS4
01 allroad

-----Original Message-----
From: Theodore Chen <tedebearp at yahoo.com>

Date: Sun, 16 Mar 2008 12:27:00 
To:Jerry Scott <jerryscott at wispertel.net>, Mike Fitton <rfitton at vt.edu>
Cc:s-car-list <s-car-list at audifans.com>
Subject: Re: [s-cars] got a new sound

i have a vibration that isn't very consistent, but occurs mostly when accelerating fairly hard at relatively low speeds.  it seems to be driveline shudder, and some listers have suggested that the U-joint in the prop shaft may be going bad.  i wanted to take a look at the prop shaft, but it seems to be covered by metal heat shields.  does the exhaust have to come out in order to remove these metal shields?  bentley seems to say that the exhaust has to come out first.

according to the bentley, the driveshaft is not serviceable and only removal, installation, and adjustment are possible.  but i know other people have replaced the u-joint, including one lister who was kind enough to send me a short write-up.  is this joint a universal joint or constant velocity joint?  bentley says it's a u-joint and the picture looks like a u-joint, but other people say it's a CV joint.  i've been given a part number for the replacement:
porsche 923 332 03200
anybody know what year and model porsche this is for?

it's been suggested that i might be able to fix the vibration by simply re-greasing the joint, so i may try that if it has a good chance of working.  the vibration isn't consistent, and most of the time isn't noticeable, so i think the joint may be seizing up occasionally.

-teddy

----- Original Message ----
From: Jerry Scott <jerryscott at wispertel.net>
To: Mike Fitton <rfitton at vt.edu>
Cc: s-car-list <s-car-list at audifans.com>
Sent: Sunday, March 16, 2008 12:02:54 PM
Subject: Re: [s-cars] got a new sound

Get a stethoscope from NAPA and probe around the drive tunnel while someone 
drives the car. You might hear a center bearing noise.
Jerry
----- Original Message ----- 
From: "Mike Fitton" <rfitton at vt.edu>
To: "s-car-list" <s-car-list at audifans.com>
Sent: Sunday, March 16, 2008 11:33 AM
Subject: [s-cars] got a new sound

> And I'm at a total loss.  It's a very clean vibration.  Don't feel it,
> only hear it.  Frequency rises with ground speed.  Amplitude varies.  I
> don't hear it below about 20mph and not above about 65mph.  It's loudest
> around 50-55mph.
>
> I've ruled out everything engine by turning the engine off at 55mph in
> neutral.  Stick and clutch position don't change anything.  Steering
> wheel doesn't change anything.  Wheel bearings all pass the
> autocrosser's tech inspection test.  All CV boots are tear-free as far
> as I can tell, and none of the usual grease sling that comes with torn
> boots.  No perceptible alignment changes.  No corollary bugs at all that
> I've noticed.
>
> About two months ago, I changed out my transmission oil (finally).
> Amsoil's synthetic 75W90 GL4.  All the plugs are still tight and the
> stick still feels great.  The sound started very suddenly about a week
> ago, so I'm guessing it's not that.
>
> Two weeks ago (one week before the sound started) I switched back to my
> summer tires.  Drove on those just fine for the first week.  Pressures
> are consistent.  Verified lug torque at 90 lb-ft for all 20.  Rotated
> them anyway with no change.  Tires and wheels all look fine, and no
> feeling of imbalance in the drivetrain.
>
> All I can think of is the prop shaft center bearing.  I've never
> experienced that before, so I don't know how it should sound.  I played
> with my prop shaft (no Krausky, not like sexually; or at least not right
> then) and found a little front to back play which seems like it would be
> normal with a CV joint on either side.  The two halves seem to move
> equally during this exercise.  There's also a little play perpendicular
> to the shaft at the center - that is, I can bend it up/down/left/right a
> little at the center U joint.  I never did that before, so I don't know
> what's normal there.  But it seems to me like it shouldn't do that, so
> that's my best guess so far.
>
> But before I give up on diagnosis and resort to just throwing parts at
> it, I want some second opinions.  Anyone?  Bueller?
>
> -Cheers!
> Mike
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