The IAC valve is an electric "step motor" that operates a plunger type valve.
It is mounted on an air passage that by-passes the throttle plate, so it is in essence a controlled vacuum leak.
A Step motor is a "digital motor", it will have at least 100 "steps" the computer can call up
Each "step" will cause motor to turn 5deg, so computer can send a voltage pulse to IAC Valve, like a morse code, and motor will turn to a specific "step".
Each time motor turns the plunger will move and the amount air bypassing the throttle plate will change.
This is how the computer sets the idle, since fuel injection doesn't have "jets" like a carb, an idle screw will not work, well it wouldn't work well, lol.
When you turn on the key computer will open IAC valve all the way, so when you start the engine idle should be high, and then it will drop to pre-set idle range for cold engine or warm engine when computer starts closing IAC valve.
The vent on the side is for the back side of the plunger/valve, this allows it to move in and out without air being trapped behind it.
There should be no active air flow coming out or going in via that vent, the cap doesn't seal it, it is so it doesn't get clogged up
Last edited by RonD; 08-03-2015 at 12:14 PM.