Editing Idle air control valve overview

The idle air control valve (IAC) basically regulates the amount of air coming into the engine during idle, and various other times during an engine's run period (such as deceleration). There are actually two idle settings on most fuel injection engines: minimum idle and running idle. Minumum idle is your idle based on the throttle blades alone, with the IAC passage closed. Running (or commanded) idle is what is programmed in the chip.

A GM engine's IAC is measured in STEPS. For instance, 0 steps is all the way closed, and 160 steps all the way open. So for instance, if your commanded idle is 600, and your minimum idle is 500, the IAC has to be open a certain number of steps to obtain 600 RPM.

Now, remembering that the IAC (at least GM's) works on a range of 0 (fully closed) to 160 (fully open) you want the IAC to be about 40 steps at warm idle (it is also set on a curve based on coolant temperature, with cooler temperatures allowing more steps to help warm up the engine). This is so that it has room to move both back and forth, to account for various loads, occurances, etc. to keep the engine alive. The IAC also has different step settings for Park/Neutral, as the engine naturally requires less air when it is not under a load. Think of the IAC as a smart air choke. And, since fuel is measured by incoming air (either directly via a mass airflow system, or calculated based on manifold air pressure sensor), it works to choke off fuel as well.

The IAC also provides another function called "throttle following". This function helps keep the engine alive when you floor it, and then suddenly let off and close off the butterflies. It temporarily opens itself to a certain step (also programmable in a chip) to curve the drop-off in incoming air. Some people disable this function, as they feel it hurts their braking power (as they are still recieving minimal engine power right after they let off the throttle). 

The IAC also has a "Keep Alive" function, where if it detects a drop in RPM it will open itself up to help "keep alive" the engine. This is why you want your warm idle setting to be at or below its half open range. For instance, if at idle the IAC was at 140 steps (almost fully open) it wouldn't have much room to open up to keep the engine alive.