What is an IAC?

82 GT · 05-08-2001, 09:21 PM

What does the IAC do? I heard this mentioned before and always wondered what it does.
I know it's on the T/B but what does it do?

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'82 GT 351W C-4,BBK headers,Carter 625 carb,Comp.Cam,Flowmaster exhaust.
1988 GT...T-5,bone stock

jimberg · 05-08-2001, 09:59 PM

It's the idle air control. It allows air to bypass the throttle plate to keep the car idling. It also opens up at WOT to allow more air into the intake.

If it's malfunctioning, it can lead to an unstable idle.

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351W 89 Mustang GT Convertible

82 GT · 05-08-2001, 10:27 PM

Would it also cause the car to stall after slowing to a stop from highway speeds?

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'82 GT 351W C-4,BBK headers,Carter 625 carb,Comp.Cam,Flowmaster exhaust.
1988 GT...T-5,bone stock

smokedchevy · 05-09-2001, 12:43 AM

Check your float levels. Mine used to do that when it was set too low.

jimberg · 05-09-2001, 10:17 AM

Possibly, but that is also a symptom of a bad EGR valve, an inoperative thermactor, or a bad speed sensor. Does your cruise control work if you have it? If not, it may be the speed sensor.

When you're slowing to a stop, your computer tries to keep the engine at around 1200 rpms and then when you stop it should drop to normal idle of about 700 rpms. Without the speed sensor, your computer doesn't really know if you are still moving or not. The IAC is what the computer uses to control the RPMs.

You should definitely get a code reader and do the quick test followed by an engine running test. These are really helpful at finding out what is wrong.

One thing that is happening when you're slowing down is that vacuum jumps. A problem while stopping could mean that a vacuum control solenoid is stuck open. There are three of them on the backside of the passenger side strut tower. EGR is on top, and thermactor ones are the two below that. If you've experienced some backfiring, it may be the thermactor. Oh, that's the air pump system, btw.

Another possibility is that you have a spark plug wire shorting on a header or something. When you're coming to a stop, it may be flipping forward and contacting the header.

Just some ideas.

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351W 89 Mustang GT Convertible

82 GT · 05-09-2001, 02:59 PM

Thanks jimberg for the info. It only does it at highway speeds. Driving around town is fine for some reason.

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'82 GT 351W C-4,BBK headers,Carter 625 carb,Comp.Cam,Flowmaster exhaust.
1988 GT...T-5,bone stock

05-08-2001, 09:21 PM	#1
82 GT Import Slayer Join Date: May 1999 Posts: 2,241	What is an IAC? What does the IAC do? I heard this mentioned before and always wondered what it does. I know it's on the T/B but what does it do? ------------------ '82 GT 351W C-4,BBK headers,Carter 625 carb,Comp.Cam,Flowmaster exhaust. 1988 GT...T-5,bone stock

05-08-2001, 09:59 PM	#2
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	It's the idle air control. It allows air to bypass the throttle plate to keep the car idling. It also opens up at WOT to allow more air into the intake. If it's malfunctioning, it can lead to an unstable idle. ------------------ 351W 89 Mustang GT Convertible

05-08-2001, 10:27 PM	#3
82 GT Import Slayer Join Date: May 1999 Posts: 2,241	Would it also cause the car to stall after slowing to a stop from highway speeds? ------------------ '82 GT 351W C-4,BBK headers,Carter 625 carb,Comp.Cam,Flowmaster exhaust. 1988 GT...T-5,bone stock

05-09-2001, 12:43 AM	#4
smokedchevy Registered Member Join Date: Jun 2000 Location: ventura,CA Posts: 438	Check your float levels. Mine used to do that when it was set too low.

05-09-2001, 10:17 AM	#5
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Possibly, but that is also a symptom of a bad EGR valve, an inoperative thermactor, or a bad speed sensor. Does your cruise control work if you have it? If not, it may be the speed sensor. When you're slowing to a stop, your computer tries to keep the engine at around 1200 rpms and then when you stop it should drop to normal idle of about 700 rpms. Without the speed sensor, your computer doesn't really know if you are still moving or not. The IAC is what the computer uses to control the RPMs. You should definitely get a code reader and do the quick test followed by an engine running test. These are really helpful at finding out what is wrong. One thing that is happening when you're slowing down is that vacuum jumps. A problem while stopping could mean that a vacuum control solenoid is stuck open. There are three of them on the backside of the passenger side strut tower. EGR is on top, and thermactor ones are the two below that. If you've experienced some backfiring, it may be the thermactor. Oh, that's the air pump system, btw. Another possibility is that you have a spark plug wire shorting on a header or something. When you're coming to a stop, it may be flipping forward and contacting the header. Just some ideas. ------------------ 351W 89 Mustang GT Convertible

05-09-2001, 02:59 PM	#6
82 GT Import Slayer Join Date: May 1999 Posts: 2,241	Thanks jimberg for the info. It only does it at highway speeds. Driving around town is fine for some reason. ------------------ '82 GT 351W C-4,BBK headers,Carter 625 carb,Comp.Cam,Flowmaster exhaust. 1988 GT...T-5,bone stock