I searched the archive for info on a severe vibration above 5000 rpm and found most advice aimed at the driveline. My '91 GT, with 110K, has severe vibration about 5500 rpm. It showed up abruptly and was bad enough to almost cause it to jump out of gear.

I am planning to install the twin screw later this year (Hi DR!) and wanted to make sure the foundation was stout. Everything from the engine back has been rebuilt and replaced. I even changed out the MMs since they looked like they were shot even though they were only about two years old. (I think the vibration caused them to fail)

With everything disconnected from the engine, the vibration seems to be located in the motor. The timing belt, tension and harmonic balancer look fine.

We are about to drop the pan to inspect the bottom end for bearing issues.

Any thoughts on where else to look? My mechanic is stumped and has checked with various folks with no solid leads.

Thanks!

 

One of the "retaining straps" on your pressure plate has possibly broken throwing your pressure plate out of balance? Wally and I have both experianced this first hand on our 90 GTs and the symptoms were exactly as you described. Time for a GTS clutch kit ????

 

Where do you feel the vibration from? Strg wheel..........the front end. Through the seat then perhaps the rear end. When you say everything from the engine back is rebuilt and replaced does that include the transmission, diff, drive shafts?

Have you listened to the engine with a scope?

 

I do not know much about 91, But my 85 had a very very bad vibration at 5500 also.

It was the front torque tube bearing moved back about 30 inches.

when you have the bottom bell off.. also check this bearing. it should be about 10 inches from the end of the tube. It was verified by someone before I bought the car, But I also varified it after buying the car by sliding a pc of wire coat hanger back the tube till it stopped,, sure enough it went back 30-some inches....

 

We changed out the whole clutch assembly and put in a lightened flywheel. I'll double check on the pressure plate question. The vibration was felt in the driveline primarily and was quite violent, IMO. I've had the car for 10 years and always marveled at the smoothness.

All of the parts back of the motor have been R&R'd. The TT was pulled and rebuilt and the runout was less than 0.010. The driveshafts were checked and all bearings replaced.

The frustration was that after it was all back together the vibration changed up a few 100 rpm and was improved but still obvious.

Thanks for the thoughts. I'll keep you posted.

 

If sitting in the driveway revving the engine with the clutch pedal depressed but the shifter in gear there is vibration, that rules out the TT, as it would not be spinning under these conditions. So, that would point to the motor or pressure plate/clutch assembly.