[s-cars] bad vibrations (long)

[Aaron Ryba]

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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