I am trying to track down the cause of a fairly severe engine vibration at about 3000rpm. The car is a 1993 940 Turbo with 91K miles (I posted on the 900 section, but got no response.) It starts fine, idles smoothly (but at about 950rpm). Under load or not, it makes a loud vibration at 3000 rpm. The severity of the vibration depends on the octane of gas - 87 horrible and 93 pretty bad. I have replaced the ignition amplifier, spark plugs, distributor cap&rotor. The ignition wires are a few years old and might need to be replaced as well. I did notice a small amount of play in the distributor shaft when I replaced the cap&rotor. Could this cause the problem? The power seems pretty good. The gas mileage is about 15 in the city and 20mpg on the highway.

Any help would be greatly appreciated.

Steve

I had a water pump bearing go bad. It did exactly what you are describing. What a pain to figure out. Good luck.

I am still trying to solve this problem. It has actually gotten worse, and could now be classified as a misfire (I think). Not sure what the definition of a misfire is, but it feels like only 3 cylinders are working under heavy throttle, especially at low speed. This is in addition to the vibration at 3000rpm, which is still there. I have read the FAQ and it seems to suggest that it could be the knock sensor, distributor, or an injector problem. Does that sound about right? How do I distinguish which of these is to blame, or is there something else that could be the cause?

With regard to these three things (knock sensor, distributor, injector problem) this is what I know:

Knock sensor: The physical condition of the knock sensor seems good. Wiring looks OK and connector is in good shape. No codes in ODB.

Distributor: The distributor shaft has a little wobble - I would say about 1mm of play. Could this small amount of wobble be to blame? I have replaced the cap, rotor, wires, and plugs. (everything but the distributor itself). I have also replaced the ignition amplifier, crank sensor, and radio supression relay as preventative measures.

Injectors: I have not done anything with regard to fuel injectors.

I am also thinking about pressure testing the turbo system, and doing a compression test, but both require tools that I don't have.

Cheers,
Steve




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1986 745 w/ M46 200K since new, 1995 965 104K

If the vibration happens in neutral too, consider the accesory bushings to be suspects. These can also cause your belts to fail...which is a symptom that is hard to ignore. If the pullies look to be out of whack, it might be worth a try. Don't buy them at the dealer, they are crazy expensive.

I'll check this out further. Hopefully it is something as simple as these bushings.

Steve
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1986 745 w/ M46 200K since new, 1995 965 104K

Have you tried looking at the rear transmission seal. I'm assuming it's an automatic. Mine started going bad and I got an increasingly bad vibration. It originally only vibrated at certain RPMs but got worse until it was constant - then it blew all its fluid out the back. I used it as an excuse to convert to a 5-speed. When I pulled the tranny, the driveshaft could move about 3/4 inch in all directions.

Thanks for the post. It happens in neutral and park as well, so I would think that it is not the rear transmission seal. Glad to hear you went with a 5-speed. I always have thought that I would like a 5-speed in an 940 turbo.

Steve
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1986 745 w/ M46 200K since new, 1995 965 104K

Have you checked for bad motor or tranny mounts?

Thanks for the response. I replaced the tranny mount - didn't help much. The engine mounts are made of cheap rubber but look OK. The problem actually sounds like there is some big loose part somewhere in the engine. It occurs within a limited rpm range and sounds/feels like a big pully or shaft is wobbling. But none of the pullies wobble that I can see. The sound comes and goes in the rpm range like the sound a loose heat shield on the exhaust makes. However, this sound is consideraby louder, there is more vibration, and it sounds like it is comming right out of the block. I noticed that the distributor shaft is somewhat loose (has some play), but just a bit. I don't know if this could be the cause.

Thank you,
Steve