[s-cars] bad vibrations (long)

Wed Jul 18 22:22:14 EDT 2007

The problem you're describing sounds to me like a compression difference
between cylinders. Injectors, spark plugs and coils can also create that
kind of vibration but injectors would be more noticeable at high RPM, and
spark plugs as well as coils would be more noticeable at WOT.

Always looking for the cheapest, easiest diagnostic first, I would check the
compression first, dry and wet, to know if it's related to the head or the
pistons. I just hope I'm wrong as it's the easiest to diagnose but the most
expensive to fix.

Vincent.

-----Message d'origine-----
De : s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] De la part de Aaron Ryba
Envoyé : 18 juillet 2007 13:41
À : s-car-list
Objet : [s-cars] bad vibrations (long)

Listers,

I have been trying to diagnose a vibration issue in my 95 S6 for quite a
while. What I originally thought was worn engine and/or tranny mounts turns
out not to be the case as I recently replaced them and the vibration is
still there. Although the two engine mounts were shot, the new ones did not
get rid of the problem.

Basically, there is a significant vibration under load in low gear. For
example driving in 5th gear and cruising at 70 mph. The vibration gets worse
as the load increases, i.e., slower speed in the same gear or going up a
hill. No vibration while shifting, only as I apply the gas. Vibration does
not get worse when cornering or braking, only under engine load. Happens in
every gear, at low RPM when under load and is worse when load is greater.

I had originally thought it was a drivetrain issue but after replacing the
mounts, front bearings and CV joints in addition to properly balanced tires
the problem persists. I have inspected the drivetrain and can't find any
obvious play in the driveshaft or rear end components. This is inspection is
however by eye & hand inspection only.

Problem areas I have thought about include proper driveshaft alignment,
subframe mounts and rear-end subframe mounts/bushings which I have yet to
replace. The issue is that I recently noticed that the car vibrates in a
similar way when at idle, sometimes more than others. The idle is steady but
depending on ancillary power draws such as steering pump, alternator and AC
the idle will drop bit and vibration is noticeable. I can get the vibration
to get worse if I start from a stop in second gear. As soon as the RPM picks
up and the load is lower the vibration goes away.

This has led me to believe that the vibration is parallel with the RPM and
engine load and not the speed and therefore is not a drivetrain issue but a
combustion/fuel supply/emissions control issue. This is especially true
because of the vibration at idle while not moving.

I did a bit of research in Bentley and noticed two possible causes. One is
the proper alignment of the torque support or snub mount at the front of the
engine. Now that I have new secure engine mounts in I will try this. The
second possible cause could be a developing exhaust leak somewhere near the
catalytic converters/downpipe.  Bentley states that a leak upstream from the
Oxygen sensor can throw the engine out of whack.  It sounds like the leak is
after the sensor possibly at the downpipe/cat connector but I haven't yet
had the chance to source the leak. This vibration became all the more
evident that it is possibly an engine operation issue than a drivetrain
issue as the exhaust leak got worse and I can hear the leak and feel the
vibration in syncopation with one another.

Does anyone have experience with similar problems with an exhaust leak
causing such a big problem like the engine vibration under load I am
experiencing? It is really an issue at highway cruising speed. How about
some other leads as to why the engine may be acting this way? 

In the mid to upper RMP range and under low load the engine runs smooth. It
is hard to determine where the vibration is coming from, it seems like the
center of the car however there is not all that much transfered through the
shifter.

As of last 5k miles I have done : the timing belt & tensioner, cleaned the
throttle position sensor, confirmed operation of ISV, WFV, injectors, &
others in diagnostic test cycle, replaced MAF.
There are no autocheck diagnostic lights coming up and I run 93 octane gas.
The car did this when I got it a year (& 5k miles) ago before all the work
mentioned above.

One thing I have not replaced are spark plugs, but previous owner claimed
that work.

I'd like to test the Oxygen sensor readings as Bentley prescribes to
diagnose an exhaust leak near the sensor or in the wastegate however I don't
yet own the VAGCOM tool/software.

For some reason turbo pressure shock comes to mind right now as well......

Any thoughts/speculations/condemnations are appreciated.

Aaron Ryba
95 S6 140k 

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