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My understanding of the IAC is that the idle air control valve basically supplies air to the cylinders/engine when the throttle body is completely closed/shut. Per wikipedia, "The ECU can control the amount of air that bypasses the throttle when the throttle is fully closed, thereby controlling the engine's idle RPM."

Well, the engine can't run without air and fuel (and spark). And... if the IAC is bad (Say a bad solenoid, or the valve itself is jammed) -- you have symptom's of really rough idle, but your engine still runs.

How is the engine getting air into it with a bad/faulty/clogged IAC?

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When the valve is 'closed', it won't completely block the airflow, otherwise, as you note, the engine would choke and stop. Therefore, even if it's stuck in the closed position, some air will still get through. This should result in a slow idle - but of course if the ECU thinks more air is getting in, it will be putting more fuel in, resulting in rough running...

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In most electronic fuel injected vehicles, there is more air getting through than what the IAC provides. This can happen with either holes directly in the throttle plates or by having them partially open (very small percentage). This is still metered air (air the computer is aware of), but it is air getting into the system.

Newer vehicles with "drive-by-wire" (as opposed to drive-by-cable) don't even need an IAC as the computer can control the throttle plates directly to allow the air in for idle.

Also, there seems to be a slight misconception of how the IAC itself works. It is not a solenoid, but a motor which opens a geared plunger (at least the ones I've seen and worked with are this way). The plunger is regulated (or measured) for how far it moves. The computer reads this and can adjust it according to the needs of the engine at idle. Things which can happen to the IAC are the gear can get stripped or it might get stuck partially open. Carbon or varnish (or whatever debris) might get stuck in between the the plunger seal and base and not allow it to close. It can get stuck closed (motor just stops working), but as I've stated, there are other areas where air does get in.

  • Note that if you have drive-by-wire/electronic-throttle you can still have an IACV. The E39 M5 setup has individual throttle bodies and an IACV. Under light loads the ITB's only open up after around 2000 rpm; requirements below that are satisfied by the intake-plenum-fed IACV. If the throttle actuators or throttle position sensors are not trusted by the DME, the car goes into 'limp home' mode where the throttle bodies stay shut and the car runs purely off the IACV. – Zaid Jan 14 '15 at 13:19
  • @Zaid ... Ayup. Those BMWs are strange beasts! I was just suggesting that some manufacturers have chosen not to include an IAC with the advent of DBW. The IAC's in this setup must be able to flow quite a bit of air! – Pᴀᴜʟsᴛᴇʀ2 Jan 14 '15 at 14:13
  • When running solo the IACV can support the engine upto 3-3.5k RPM. I calculate that to be around 125 l/s on a 5.0L engine. So yeah, a helluva flow capacity – Zaid Jan 14 '15 at 14:49

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