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ChevyTech · 03-21-2007, 08:19 PM

Question:
When the engine is started, does the engine have a fast idle that slows down as it runs? The IAC (idle air control) is what gives the fast idle speed when started cold.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on. If you have A/C and turn it on while listening carefully you should be able to here the IAC controlling the idle as the A/C clutch engages and disengages.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

03-21-2007, 08:19 PM	#3
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Re: 1993 suburban very low idle in gear Question: When the engine is started, does the engine have a fast idle that slows down as it runs? The IAC (idle air control) is what gives the fast idle speed when started cold. The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage. The IAC (and computer) control the idle speed. The IAC (and computer) is what gives the engine a “fast idle” speed when started cold. The IAC also supplies more air to compensate for the extra load when the A/C is on. If you have A/C and turn it on while listening carefully you should be able to here the IAC controlling the idle as the A/C clutch engages and disengages. IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.