Jim Grant's Tech Tips
Mercury Grand Marquis, High Idle Problem
Q: For the second winter now my Ď91 Mercury
Grand Marquis has had an idle problem. At temperatures below 5 degrees
(F) 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
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 motorís electrical connector. If the idle drops then you have a command
problem. If it doesnít you may have a sticking IAC motor. The fix for
a sticking IAC motor is a cleaning or replacement. Most of the time Iíve
found that a problem like yours lays with the IAC motor.