Editing Idle air control valve overview

The idle air control valve http://www.partsamerica.com/product_images//img/nie/21757.jpg (IAC) is a stepper motor http://en.wikipedia.org/wiki/Stepper_motor http://upload.wikimedia.org/wikipedia/en/thumb/f/f8/Stepper_motor_1.png/241px-Stepper_motor_1.png which regulates the amount of air entering the engine during idle and 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 the idle based on the throttle blades alone with the IAC passage closed. Running (or commanded) idle is programmed in the chip.

A GM engine's IAC is measured in steps. For example, 0 steps is all the way closed, and 160 steps is all the way open. So, for instance, if your commanded idle is 600 rpm, and your actual idle is 500 rpm, the IAC will 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, etc. This helps to keep the engine from stalling. The IAC also has different step settings for Park/Neutral, as the engine naturally requires less air when it is not under load. Think of the IAC as a smart air choke. 

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 throttle valve. It temporarily opens to a certain step (also programmable in the 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 receiving minimal engine power right after they let off the throttle). 

The IAC also has a "Keep Alive" function. If it detects a drop in RPM, it will open 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 running.


















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