What control the cold idle speed on a 1992 mercury grand marquis?

The cold idle speed on a 1992 Mercury Grand Marquis is controlled by the idle air control (IAC) valve. The IAC valve is a solenoid-operated valve that allows air to bypass the throttle plate when the engine is idling. This allows the engine to maintain a consistent idle speed, even when the load on the engine changes.

The IAC valve is controlled by the engine control unit (ECU). The ECU uses a variety of sensors to determine the engine speed and load, and then adjusts the IAC valve accordingly to maintain the desired idle speed.

If the IAC valve is not working properly, the engine may idle too high or too low, or may stall when the load on the engine changes. If you are experiencing any problems with your engine's idle speed, you should have the IAC valve checked.