My car has a vibration that is RPM dependent. It seems to come and go through the range being the worst around 2500-3000RPM. I have replaced the vibration damper and it has a new FW/clutch/PP that were balanced together.
Could the vibration just be the harmonics of the engine made worse through poy mounts and a semi solid transaxle mount? or, can it be the fuel trim in the tune? If anyone has other ideas I would be glad to hear them.
Thanks.

 

If you're using poly engine mounts, you should be using a poly trans mount as well. Short of that, I'd also have the driveline checked for balance.

 

Crank harmonics/oscillations are not felt within the car as vibration.

Does it happen in that rpm range when free revving (something is out-of-round/balance) or when under load (misfire)...?

 

But bottom-end related vibration/oscillations can be.....think of cracked flexplates or flywheels, and all the hell they can play with diagnosis....not to mention the transmissions behind them.

 

Does the engine vibrate in park/neutral also, or only moving?

 

Thanks, for the replies. The vibration comes in and out at the same RPM's under load or not, going up the band and when coming back down. The Flywheel/pressure plate/clutch assembly was sent in to be checked for balance, it was balanced. I put on a new vibration damper with no change. I do have poly transaxle and motor mounts. It happens when still and the clutch is in so it is not the drive shaft or transaxle.

 

Check your driveshaft angle, different mounts might have changed it.

Good luck

 

Never thought of that, thanks.

 

Hi bm,

he has a transaxle, his driveshaft is located in a rigid tube running between engine and transaxle (on his 944);

and earlier he said this:

 

Can the fuel trim at the RPM when the vibration happens cause a vibration? There is some roughness to the idle, or it may be from having semisolid motor mounts. I had a mail order tune done, but not street or dyno tune.

 

Misfire can sometimes show up as a vibration.

 

Thanks, that is what I am trying to figure out. How about O2 sensors? That comes up when I am searching.

 

I pulled the plugs and they look good, the insulator was just a little tan colored. New NGK plugs with new MSD plug wires. I pulled on fuel injector connector at a time, each one was essentially the same. Then I pulled the serpentine belt, no change.
Can a street tune be useful?

 

I still have the vibration when the motor is under load around 3K RPM and when the RPM's come back down through 3K off throttle. Does that indicate an ignition/tuning problem? If it is then I will take it to a tuner. If not I am going to take the preload plates off the energy suspension motor mounts. Any help is appreciated, I don't want to remove the motor mounts.
The clutch/FW/PP have been off to check for balance and the vibration damper has been replaced with a new unit, no change.

 

It was the clutch. Swapped in a LS7 kit and all is well.

 

It could be one of your coil packs going bad.I had this problem.I never had any codes.I found it by taking a temp gun and checking each header and found one temp lower than the rest.Coil 8 was bad.The coil worked fine at full throttle but not part throttle or idle.It took me awhile to figure it out.