1968 Shelby GT500

Idle Air Bypass (IAB)

First let me say this little thing has many names. But they all talk about the same item under the hood. Here all the names I’ve had the torture of learning throughout the years:

•Idle Air Bypass
•Idle Air Control
•Idle Speed Control
•Throttle Bypass Air
•Idle Bypass
•Inlet Air Controller
•Inlet Air Bypass
•Intake Air Bypass
•Intake Air Control

It seemed that every automobile manufacturer had to invent some name for this thing and all the other brands had to learn it. Going into a local parts store can be a nightmare depending on which name the guy on the other side of the counter understands. Basically if you have any combination of the words; Idle, Inlet, Intake, Air, Control, Bypass, Control, or Controller you have just made a name for this thing.

Moving on now, sorry for the head ache. How’s it work? This is simple to understand, look at the following picture.

It doesn’t get much easier; this mounts on the side of the throttle body. Air is allowed in from in front of the closed throttle body, and “bypasses” the throttle body plate. The air is then released into the open intake track flowing directly to the cylinders. The whole idea behind this is simply a way for the computer to give air to the engine without needing a driver. The IAB is not opened like door half way, its either open or closed completely. The EEC sends many pulses per second to flutter the IAB open and closed. It flutters so fast that you should notice any RPM fluctuation with a functional IAB. This fluttering is called “duty cycle,” when it’s calculated a percentage of on time. So when the IAB is off it is at 0%, when it is fully open it is at 100%, and any RPM in-between.

There are 3 main needs for the IAB and a fourth that is a debate.

•The engine requires an unrestricted amount of air to start, so EEC opens the IAB 100%.
•As soon as the engine RPM self sustains and begins to stabilize we now need air to Idle.

•Idle is more complicated than the word implies.
•Idle needs to be smooth and stable.
•Idle is increased if the engine is cold to help warm up.
•Idle is increased if the engine is overheating.
•This increases the speed of the water pump to flow more coolant to the engine and radiator.
•This speeds up the speed of the radiator fan improving cooling.
•Idle is increased if there is a high load on the alternator.
•Idle is increased if there is a pressure spike in the power steering.

•When you take you foot of the pedal the IAB is at 100% to prevent stalling.
•As the engine returns to idle speeds the IAB lowers the duty cycle to keep the RPM stable.
•When the vehicle comes to a stop the IAB is now controlling the Idle.

Full Throttle:
• The IAB does opens 100% at full throttle. But the debate is if air flow increases.
•Air now has the option to entering the full open throttle body or the IAB.
•Air diverting through the IAB would disrupt the main air flow path through the throttle body.
•I haven’t seen any flow bench test data to suport this.
•You do have “possibility” for higher flow, but it’s more violent and turbulent now.


This is really easy to test. First as with all problems you should gather the trouble-codes from the computer. Follow the codes for testing and repair. If you get a code that points to a problem with the IAB start the vehicle and bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the IAB. If the engine begins to stubble or stalls the IAB is functional and does not need to be repaired. If the engine idle does not change you should remove the IAB for inspection.

The IAB can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it. There are 2 halves to the IAB, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAB needs to be replaced.

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