Tracking mystery driveline vibration

[E30godz]

Update:

So i've been trying to find the cause of the driveline vibration that occurs at 50+ MPH.

Replaced CSB, Guibo is fine. I also disconnected the rear half shafts, jacked up the rear of the car and 'drove' it up to the speeds I feel the vibration at....Still there.

So it's not anything wheel/halfshaft related. Is there anything OTHER than the driveshaft that could be causing this? Do the output shaft bearings in the transmissions go bad and cause shit like this?

Interestingly enough, the vibration seems worse at some times than at others.

[Teaguer]

Unlikely that its the trans output bearings .

Was the CSB preloaded correctly ?
yOu can also run it up to speed and look at the CSB or u-jounts on the end and see if you see any unwanted movements .

[DCColegrove]

PreLoad:
Install the Center support bolts just snug then knock it forward (evenly) about 5mm - 8mm then tighten.

And are the U-joints binding at all? Or even just a bump as you "work" them?

[E30godz]

I did preload the CSB....I didnt notice any binding in the u joints but it is possible that i just didnt check.

[Teaguer]

Recently i checked a members car for a similar problem .

Got it up on the rack and had him put it in gear and take it up to speed while I stood underneath and checked the CSB , front and rear u-joints for movement .
Turned out his d-shaft was shit as it had a nice hop in front part of the rear section and had a nice hop in the CSB because of this .

[jlevie]

If the CSB required replacement because of a failure of the rubber mount, it is almost certain that the cause was binding u-joints in the drive shaft. That being the case, the new CSB will have a short life and the binding can cause vibration from the drive train.

[frantic]

How would you fix a binding driveshaft?

[jlevie]

How would you fix a binding driveshaft?

Replace it with a new or reman unit.

[T.Dot_E30]

un-balanced flywheel is a possible but unlikely cause.

[E30godz]

that would tell me its engine speed dependent, which it is not.

[Stevie30]

that would tell me its engine speed dependent, which it is not.

Try getting to vibration speed and disengage the clutch, allowing the car to coast. I did this and my vibration stopped, even with the engine running at speed. I have narrowed mine down to the transmission; most likely the input shaft, clutch disc or pilot bearing. Good luck!

[jlevie]

Try getting to vibration speed and disengage the clutch, allowing the car to coast. I did this and my vibration stopped, even with the engine running at speed.

While that might implicate the transmission, in my experience it points to binding u-joints in the drive shaft. When binding u-joints are present they cause axial mis-alignment when the drive shaft is subjected to torque loads and the more load the greater the affect. When you de-clutch at speed you unload the drive shaft and that can lessen the mis-alignment and thus the vibration.