My 91 STS recently overheated because the drain plug somehow popped out of the radiator and it lost all its coolant. I shut it off rather quickly though and got the car towed home. I replaced the drain plug filled it back up and it idles now very rough. It won't sit at the same idle speed like it used to about 650RPM instead it surges between 650 and 900 back and forth. I replaced the plugs and wires but still same problem :banghead: . Any ideas?? Otherwise the car runs fine and smooth at speed as soon as the gas pedal is touched it smooths right out. If anyone can gice me some ideas to try to fix this I would appreciate it. Oh by the way the car has 188,000 miles so its no spring chicken. I know it could be a huge number of things but if there is any thing in particular that comes to anyones mind let me know.

Possibly the high heat underhood caused the idle speed control motor to fail... if the closed throttle switch inside the idle speed control motor started to loose contact it would cause an inconsistent idle.


Enter into the onboard diagnostics by simultaneously pressing the OFF and WARMER buttons on the climate control panel with the key on. Watch for any trouble codes to scroll by that might give a clue as to the problem. After any codes display the system should prompt you with ECM? Press the HI button to answer YES. At this point the various icons on the climate control panel will take on new meanings. They will indicate the status of various ECM functions. The LO icon will indicate the status of the closed throttle switch. If the LO is on then the switch is closed....which it should be anytime your foot is not on the throttle. If the LO icon is not on when the throttle is closed or is blinking intermittantly with the throttle closed continously then the closed throttle switch is probably failed and you will need a new ISC motor. That is the can shaped device with the plunger that the throttle lever rests on at the throttle body.


Make sure you changed the oil after the overheat as the oil gets very hot in such a situation and will need changing immediately as it likely got overtemped.

You should also clean the throttle bores and the EGR tubes on that package. Deposits in the throttle body bore can also cause an idle speed control problem as the deposits on the perimter of the bore block air flow around the perimeter of the throttle blades causing the idle speed control gain rates to be off. Hold the throttle blades wide open and spray some carb cleaner around the bore below the blades and scrub with an old toothbrush.

See the two tubes sticking up at you from the floor of the intake with the throttle blades wide open..??? Those are the EGR tubes and they are not supposed to be full of carbon. Rod them out all the way thru with a piece of stiff wire. If the tubes are blocked it will screw up the air/fuel control as the EGR compensation will be off with the EGR flow blocked.

This is some excellent info thanx alot I will try it tommorow and tell you how it goes. I appreciate it. I am a pretty good mechanic working on motorcycles and my 5.0 mustang but this high tech stuff puzzles me sometime. Sounds like you are pretty experienced with it.

i think its your Idle Speed Control motor.. (ISC from here on out)
What happens is when the throttle is in a relaxed posisition, The ISC switch tells the PCM to control the idle to a certain RPM.. which is what a little motor with a plunger does.. moves it in and out

When you touch the throttle, it lifts off the ISC and opens the switch, telling the PCM to stop controlling the idle, because you'd be in motion (or revving up) .. sound like the cause?

Mines acting up also, soon to be replaced.
Today i idled down to 350 RPM, man did that improve gas mileage !
Sometimes it will stick at 2k, sometimes 700.. usally varies, like you're experiencing. The part is about $120 from the dealership.. easy DIY thing.

I wouldnt go rushing out buying things because i said something..
will check back later on this situation. (damn do i sound smart.. lol)

edit: Oh yeah, forgot about OBDI
Bobbonski is pretty authoritative around here..