My 87 928 5 speed (stock so far) has a vibration that occurs around 1500-3000 RPM, not at idle however. My buddy thinks it's a spun bearing in the motor, i do not however want to think about something like that yet. The engine runs nice, consistent. It does have a custom chip plugged in, and runs very rich but does not produce smoke. The cats are gutted, and all of the emissions control system is still present. What should i do?

 

Check the motor mounts, if old and collapsed it will give those symptoms.

 

Yeah..
Mine to,have the same symptoms.
Motor Mount$$$$$

 

If you've spun the bearings in the engine, you'd be posting about why your car wont spin over and the battery is good.

Check the motor mounts.

 

Thanks for the replies so far. The first thing that came to mind when i actually noticed it (after i bought it of course... ) is motor mounts. I also have an oil pan gasket to tackle some day, does this not also involve taking the engine support member off and supporting the motor from above? Should i perhaps tackle the motor mounts at the same time? I am 18 and money/time seems hard to come by when everything else is done, so my 928 is kindof a "when i get days off from work/Sundays" project.

My pal is a car guy, but of course he only thinks US domestic is the way to go because they're so simple to repair. I brought him up so he could marvel at all of the stuff on the "exotic" 928 that is all foreign to him. Anyways, he said it might be a spun bearing and that "...if you drive it it'll just eventually blow up...". Glad to know it's probably just the motor mounts.
I am planning to go over the ignition system when i find the time, give it an R&R. I will probably report back and tell if it made a difference.

 

He's right. If you drive it probably will eventually blow up many years from now. You're right about removing the crossmember, supporting the motor from above. Although some clever people have done it other ways, don't even think about it.

Slim possibility it's the torque tube bearings, although they cause vibration (usually with a growling noise) throughout the RPM range. Easy to rule out with a manual tranny. If the vibration continues when reving the motor when in neutral in the driveway after you push the clutch pedal down, it's not the TT. If it goes away, it probably is. The 1500-3000 RPM range vibration is commonly motor mounts, as indicated.

 

Im dying to know what your car guy buddy thinks about a car with 'bad bearings' that only has 'bad bearings' in certain RPM ranges, that manifests itself with chassis vibration.


And..if he's nice, and brings beer, you may only let him WATCH you work on your complicated, yet somehow simple once you grasp it, car.

 

a quick test for spun bearings is to pull out the dipstick drip the oil onto a black piece of cardboard then look for metallic particles in the sun use a magnifying glass,
if you see lots of sparkles then you should check the flex plate drive shaft clamping bolts and crank end play
NOTE this should be done anyway

Note a quick check of the MMs is to look under the car,
the oil pan should be flush with the bottom of the crossmember
If its below it, then the MMs are probably collapsed

 

Something you can do while working up to the big MM job is lift the engine just enough to slip something between the pan and crossmember to soak up some of that vibration. I put a Paul Jaeger "speedmount" in my car before Sean replaced the MMs. Others have said just fold up a section of heavy doormat.

Pretty cost-effective way to get rid of some of that vibration!