Vibration while cresting hill...???

[Birdoprey]

I've got a 90 SHO mtx. I know the front motor mount is bad and i'm awaiting a reinforced mt. While i'm accellerating(normal) and i crest a small hill or large rolling bump i get a vibration when the suspension drops. Could this be caused by the bad mount? The c/v boots are fine and there is no clicking or popping while turning sharply. Also, the pasenger side output seal on the tranny is leaking(tranny rebuilt and seal replaced 20,000mi ago). Is it possible that the motor mount allowing the motor/tranny to rotate caused the seal to go bad? I plan to replace the seal along with the motormount, when it arrives.

 

[sdpatt]

The motor and transmission are securely bolted together. The bad mount lets them both move.

Is it a single bump or more like a tire out of balance (periodic vibration)?

 

[Birdoprey]

sdpatt:
The motor and transmission are securely bolted together. The bad mount lets them both move.

Is it a single bump or more like a tire out of balance (periodic vibration)?

I know that the motor and tranny can't move independantly. The passenger driveshaft has a straight shaft and a mount before the inner c/v joint. Is that mounted to the motor or is it mounted to the frame? If its mounted to the motor, then the mount and shaft would have to move with the motor/tranny. If its mounted to the frame, then the motor/tranny could move and that end of the shaft would remain stationary, thus creating a bind/seal problem. My car is'nt here right now or i would look to see.

It feels simmilar to a tire imbalance(high frequency, but only when the suspension drops(after you've gone over a bump). It does'nt do it over sharp bump or *** holes(at least not noticable). You feel it when going over a wave in the pavement(miniture rolling hill).
Is it possible for the motor to rotate back and down(due to the bad front mount) and the c/v shaft to slightly rub the subframe(when the tire drops after the "bump"? It feels like something that is spinning quickly is rubbing the frame(or something simmilar).

 

[Catfishphillip]

Mark, the right drive shaft is held with a yoke to the subframe, not the engine. Your rear subrame bushings should be checked - have you ever changed them?

 

[220ksho]

The right halfshaft is not mounted to the subframe. It's mounted via a bearing to the engine.

I have had the same vibration that you're talking about. Crest a hill or compress the suspension in a dip under acceleration caused the condition. After replacing the struts, tie rods, bearings and everything, I tied the root cause to the motor mounts. I haven't really figured why the vibration happens, I just know it stopped happening when I put on the new SHOnut mounts.

Let me know how it comes out.

Jeff Grondin