The Car Care Clinic with the Dr John

91 Mercury Grand Marquis - High Idle Problem

Andrew in North Bay

Question

Now that the temperatures are dropping my ‘91 Mercury Grand Marquis is having an idle problem. At temperatures below 5 degrees (C) the engine will race when I try to slow down. The engine will start and run normal until sometime after it's been driven at least 10 minutes. It will then start racing at a very high RPM and will not go down. The engine must be shut down for a few minutes for it to return to normal, only to repeat the problem. All the usual culprits have been replaced including the throttle position sensor, air sensor, and temp sensor. Any ideas?...."ll be listening to your show on the pc tonight.

Answer

Andrew, What has to be determined is if the idle problem is a command or mechanical problem. A command problem is the computer deciding the desired idle speed. If it is a command problem then it can be isolated rather quickly. Have your technician show you where the Idle Air Control (IAC) motor is located and how to electrically unplug it. When the condition occurs, unplug the IAC motor’s electrical connector. If the idle drops then you have a command problem. If it doesn’t you may have a sticking IAC motor. The fix for a sticking IAC motor is a cleaning or replacement. Most of the time I’ve found that a problem like yours lays with the IAC motor needing a darn good clean.

Good Luck!
Dr. John