'91 Grand Marquis shuts off when idling when I turn on the heat or ac. What might this be ?

Question

Most times there is just engine surges during idle but sometimes it shuts off. I've talked to a mecvhanic over the phone and he say's it might be the egr valve. Could it be anything else?

Okay now it just completely turns off in idle.

Answer 1

The most common is a slow reacting IAC (idle air control valve). It is located near the throttlebody. This controls fast idle and keeps the idle speed up when engine loads change.
The only other thing could be a locked up compressor, but that probably isn't the case.