[s-cars] bad vibrations (long)

Wed Jul 18 14:59:42 EDT 2007

I do have a problem at high boost which could be a misfire.It is sporadic. I thought it may have been a boost leak or something else and this might be the something else.

I understand that I can jump to the empty 6th channel if one coil channel is dead.
I'll give it a looking into.

Thanks,
Aaron

----- Original Message ----
From: Robert Myers <Bob at chips-ur-s.com>
To: Aaron Ryba <aaronryba at yahoo.com>
Sent: Wednesday, July 18, 2007 2:23:01 PM
Subject: Re: [s-cars] bad vibrations (long)

If it is a miss it sounds like a failing coil.  Easily
replaced.  Also easily identified.  To determine which coil
unplug injector #1 and go for a drive hard enough to cause the
misfire.  By unplugging injector #1 you are causing cylinder #1 to
misfire.  If the engine runs even worse under load then #1 cylinder
is OK.  Go on to #2, etc., until you find one which does not get
worse under load.  That is the bad coil.

At 10:40 AM 7/18/2007 -0700, you wrote:

*This message was transferred
with a trial version of CommuniGate(tm) Pro*

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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