Hi josephjrm. Welcome to the forum. I'm not sure what Ford makes their engine mounts out of, but $800.00 will get you five or six installed on most cars. What are the symptoms? Mounts have been made for many years in such a way as to not allow them to come apart, but depending on which one it is, it will let one side of the engine droop a little or the engine can rock back and forth more than normal. Either way, the added stress on the other mounts is not going to be significant.
Very often people don't even know a mount is bad unless there is a different problem that becomes noticeable when the drivetrain shifts position a little. The most common thing is a worn inner cv joint housing. As long as the rollers inside it stay within the worn area, you won't notice a problem. When the drivetrain shifts, it is possible for those rollers to move to a new position just enough to bind on the worn spots and cause a shimmy in the steering wheel, particularly when accelerating. This is just one example of what could happen. If the mount that keeps the engine from rocking forward and backward is worn, it could put extra stress on the flexible exhaust pipe coupler and it will twist wiring harnesses and hoses more than normal. Those things are normally designed to allow some engine movement which can't be avoided.
Monday, June 21st, 2010 AT 4:57 PM