What could be wrong when a 91 Ford Tempo motor shakes when idling?
Sounds like you ave a misfire. Could be as simple as needing a tune-up.
A car can shake because of a problem in the transmission although this usually is not the problem. I will assume it shakes while it is idling or just starting out. It can also shake due to broken motor or transmission mount(s), bad u joints, a loose harmonic balancer, an unbalanced driveshaft or problems in the rear differential.