The car is a 1993 940 Turbo sedan with 220,000 miles. I have the last 10 years of service records if I need to look into certain things as this thread unfolds.

Ok so here is the situation:
Recently, the car has started producing a strong, fast vibration while in the 3000 RPM range. This all began after I had the A/C compressor replaced, however I couldnt tell you if these two things are related. The vibration is present with or without the AC running. Below 3000 RPM it is smooth as silk, and above 3500 or 4000 it trails off and becomes less noticeable. This vibration can be felt through the entirety of the vehicle, but especially through the accelerator pedal. within the last 50-75000 miles the Left engine mount, and both transmission mounts have been replaced. I put the car in neutral and revved it to about 3000 RPM to see if perhaps it was the transmission, not sure if the fact that the vibration was still present eliminates the tranny from the equation. The Right engine mount checks out ok, as well as the other mounts which were recently replaced. I am concerned that this vibration, while it does not seem to effect the performance of the vehicle, might actually cause more issues with other components of the car. The car has been to my local mechanic 3 times to trouble shoot this issue, with nothing being conclusively blamed for the vibration.

Please let me know if you have had a similar issue and if so, what you did about it, and if no one else has had such an issue, I am open to hearing what anyone thinks the issue might be.

Thanks!

 

I assume this vibration is while the car is not in motion, otherwise you'd be mentioning speed?
I'd REALLY look at parts of the exhaust touching the body somewhere...

 

Ahh, good point. It makes the vibration regardless of the speed of the vehicle. It makes the vibration around 3000 RPM, whether the car is moving or not, and it doesn't matter what gear the transmission is in, or if it is in neutral or park.

 

New things I have noticed about the vibration. I had the engine mounts replaced, It clearly feels dampened, but the vibration is still there.

Its not a constant vibration, and it happens more often with heavy acceleration, and in fact, if the boost needle does not go into the white, the noise/vibration is drastically reduced if not eliminated.

I also recently replaced the o2 sensor between the cat and turbo and I replaced the ignition coil. This helped lessen the vibration a lot, but its still prevalent especially under heavy loads and under hard acceleration. Basically, the car runs much better to the extent that I don't need the turbo as much as I used to, and I think that the vibration is mostly when the turbo kicks in.

I also noticed that the vibration and noise both sound like they are coming from the transmission tunnel, and that it usually happens between 2500 and 3500 RPM, below 2500 RPM the vibration is not there, and above 3500 the vibration is not there.

There were two instances of the noise/vibration not happening. The first was after I had disconnected the battery for a few hours while I was doing an oil change and replacing the idle control valve, the car drove noise free for the rest of the evening. I'm assuming that it was due to the fact that the computer reset.This led me to believe that it was electrical in its origin. I drive it for about 15 minutes and for 10 miles.

The second time it didn't make the noise/vibration, was after it had sat at my mechanic's for about 2 weeks, not being driven as it waited for me to take it home. I have no idea what they did, they were looking for the vibration, and never found anything. All I know is that it sat without being driven for an extended period of time. This time, it drove noise free for half of the drive home which was about 20 miles and 20 minutes.


Please! This is bugging the s#!t out of me! please help me with this. I don't know what to try next.

 

Have you looked into the possibility of engine misfire? If it went away after some event then may a high speed misfire... and BTW, did you make really sure that exhaust is not touching the body somewhere?

 

I was under the car for an hour with a bright light looking for signs of rubbing and didnt find anything, but that doesnt mean that its not happening. If you think that there is a common fail point in the system where this tends to occur, and that I might have overlooked it, I would be happy to check again.

As far as a high speed misfire, what would cause a high RPM misfire?