Why would a 1994 grand marquis still idle rough and stall occaisionally when in drive at a complete stop even after tune-up?

Answer 1

IDLE CONTROL VALVE GONE BAD Check for vacuum leaks. In particular check for leaks in the PVC valve hoses. Follow the rigid line from the PVC valve (top of right rocker cover)back to a rubber TEE at the right rear corner of the intake manifold then follow the line from the TEE across to a rubber ELBOW on the left corner of the intake manifold. Check both the rubber TEE and ELBOW for ruptures that will allow for a vacuum leak.