In a fuel injected computer-controlled situation idle speed is being regulated by controlling the amount of air that is allowed to enter the engine with the throttle plates closed. Some engineering types would call this a calculated vacuum leak. If you're idle speed problem is a stalling condition when coming to a stop, start by looking for the obvious problems like vacuum leaks from deteriorated gaskets or broken vacuum hoses and lines. On vehicles from the 80s and 90s many were equipped from the factory with plastic vacuum line that joined a small rubber hose where it was then mounted onto a vacuum fitting of varying types.
Adjusting base idle speed
On vehicles from the 80s and 90s you will find instructions listed on the emissions decal that explains the necessary conditions that must be met prior to attempting an idle adjustment. On older GM vehicles for example, the vehicle is placed in a diagnostic mode that extends the IAC motor to its outward limit. The IAC motor is then disconnected and the diagnostic mode exited in that order. Now when the vehicle is started it is on the base idle setting which at this point can be adjusted.
If you find an adjustment screw on the throttle plates turning it without following the procedure will not accomplish anything. This is because the computer is designed to adjust idle speed and keep it in a specific range. If you're not in the setup mode the adjustment will not hold.
If you just want to set you’re idle and you are looking for professional help with that task visit the help with car repair page. There you will be able to connect with technicians that are familiar with your specific year make and model. For more free auto repair articles this next link will take you back to the homepage from this page about idle speed problems.