By itself, not really serious. I guess it depends on your definition of "serious". The problem with bad engine mount is this: the vibration that is not dampened by the mount will now travel to other parts of the engine and will a broken mount will affect other mounts.
Depending on which mount that is bad. If the front or rear, now your engine will "flex" forward and back with every acceleration and deceleration. This create stress to your exhaust system. If it is the dog-bone (front engine - by the belts), the engine will flex side to side, which will transfer stress to transmission.
Wednesday, November 19th, 2008 AT 12:07 AM