Mitsubishi Repair: 1992 plymouth colt vista 2.4 mitsubishi engine, plymouth colt, stepper motor

Question
hope u can help my mitsubishi 2.4 engine goes up to high revs 3000 rpms and then down to 100 it does this when in park and in drive. (when u turn on the air it goes down to normal idle),this is anoying constant surging up and down. when it is cool day the car works fine.I have just changed the ox sensor, air filter,engine oil prior to the emission test it worked fine,but on the way home it was hot and humid and it started to do the same thing, i am LOST,PLEASE HELP Bill (what is odd it passed the emission test today and idled fine)

Answer
Bill,
This problem is commonly referred to as 'idle surge'.  It's typically caused when your Idle Air Control (IAC) motor is malfucntioning.  It's also referred to as the Idle Speed Controller (ISC).  This is simply a little stepper motor that regulates the amount of air that bypasses your throttle body when the throttle plate is closed (e.g. when you're sitting idle at a stop light).  

The IAC is not an emissions components and will not alter the emissions of your car whether it's functioning properly or not.  It really only does it's job when your car is idling.  Also, on cold starts, the IAC is not even used by the engine until the engine reaches normal operating temperatures.  It may just be starting to go bad since it sounds like it's an intermittent problem at this point.

I would recommend having your IAC tested to make sure it's working properly, and get it replaced if it's not.  It won't hurt anything to have a malfunctioning IAC in your car, but it will causes stalling more often over time.
Good luck!