I have an early 85 944 with a bad vibration at low RPM (around 1000), so I assume it a bad motor mount. I have a couple of questions. How do I know which mount is bad, where/how do I jack the engine.

 

If the vibration goes away at about 1200 rpms, then its the motor mounts. Go ahead and do both mounts at the same time, you'll be glad you did. Also, be careful which mounts you get, you want the OEM German made ones, not the aftermarket oriental ones.

Clarks-Garage.com has a good write up for doing the mounts. I usually grab a couple of 4" x 4" lumber that I cut to fit across the shock towers to support the engine and then use a ratcheting tie down strap to hold it up while I drop the crossmember. A bottle jack and a piece of wood can be used on the oil pan to raise it if you need to, but I wouldn't use that as the only support.

Regards,