How do you confirm the integrity of the rear trailing arm bushings and subframe bushings on an E30? My 1991 318i has a brief speed-related shudder (vibration) when accelerating through about 5 mph. Engine, transmission and differential mounts, central drive shaft bearing have all been checked. Drivetrain is tight. The shudder is not clutch related and not rpm related. Through process of elimination I wonder if its the rear bushings.
Cross member bushings are easy to check. If the cross member appears to be resting on the bottom mounts with the car jacked up, they are bad. You will be able to push up on the cross member and have it move a significant amount. Trailing arm bushings can be tested with properly placed prying, though they rarely go bad. Sounds more like one or both U joints on the driveshaft are going bad than anything else, though.
Could it also be an out of balance drive shaft? Do you get this vibration when coasting and decelerating at this speed? I guess either out of balance or bad U joints has the same solution! Replacement Drive Shaft.
Driveshafts don't usually go out of balance. The joints lose their lube over time or wear nitches into the bearing races that don't allow smooth movement. What happens is when you are putting just the right torque load through the shaft it will try to wind up and if the U joints can't move smoothly, the shaft will momentarily not spin true and cause a vibration. In extreme cases the driveshaft will hit the center support bearing outer ring and make a knocking noise. Your driveshaft is in the very early stages of degradation, and most people don't notice. If you want to fiddle around you can get a grease needle for your grease gun and try to inject some lube into the bearing boots. Can't really hurt anything, anyway.
