My problem is hard to explain and gets complicated, so here goes. Right now when I get up to around 70mph the car starts to shake a bit, mainly the drivetrain. It almost feels like one of those cheap back massagers that vibrates on your back. It is a little less severe in fourth gear though. Then when you let off of the pedal and shift into nuetral it starts to shake the whole drivetrain even more violently. Then shift back into gear and all is fine.

For the background: I have a 92 RS with a 9 bolt, 3.45 gears and t-5. Back when I rebuilt my t-5 I put new U-joints on the drivshaft and had a company spin balance it. This was over a year ago. In the summer I put in a 355 and had it spin balanced. same transmission, new Hurst shifter. Before the swap when the bottom end and rer end were stock every thing funtioned fine. I had an old 2.73 ten bolt by the way then.

I then ploped in the 355 but kept the 2.73 ten bolt. That is when I started to notice the problem. Though it was not quite as bad. I then pulled the ten bolt in favor of a nine bolt and the problem only worsened. I know it is not engine related because it can spin to 6,000 RPM with no problem. I have heard aftermarket shifters can cause some additional whine to be heard as well as gear swaps.

Could the problem be that the driveshaft is bowed or has flexed? I do not suspect the transmission because it still has a good rear tailshaft seal there. I use Synthetic oil and have even tried adding the posi additive, oil level is good. All new bearings in rear end. The carrier was shimed and the original axel tube washers were used. Driveshaft is bolted nice and tight with the rear. Whe I removed the 10 bolt it was far from healthy with the clearences being way off.

 

mine used to do that to. one day i crawled under the car and looked at the trans and where it bolts to the bellhousing all the bolts where loose and one was missing. it used to vibrate like hell till i tightened the bolts. this may not be your problem but it might be worth looking at.

 

Sounds like a good idea to re torque all of the bolts and such. I have also heard that worn u joints can cause bad vibration on decel. Any truth to this. U joints are cheap enough, I think I will replace them again.

 

from everything i have heard that is true about the joints. the only reason i replaced on of my ujoints is because i broke it launching to hard.