Idle air control valve (IAC)

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By Reinier (Contact Me)
Last Updated 2021-07-23
Automobile Repair Shop Owner

What Does the Idle air control valve (IAC) Do?

On vehicles that are equipped with an Idle Air Control Valve, the IAC valve is typically incorporated into the throttle body, and it regulates and controls the volume of intake air the engine needs to run at idling speed when the throttle plate is in the closed position.

Why is the Idle air control valve (IAC) Needed?

SPECIAL NOTE:  Vehicles with electronic throttle control systems, also known as “drive-by-wire” throttle control systems, do not have Idle Air Control valves. In these systems, the PCM can exercise sufficient control over the position of the throttle plate to allow for small volumes of air to pass directly through the valve body to allow the engine to run at idling speeds. END OF SPECIAL NOTE.

On vehicles with conventional throttle control via a control cable or mechanical linkages, the PCM has no direct control over the movement or position of the throttle plate. The PCM can only infer the position of the throttle plate by comparing input data from two or more position sensors.

As a practical matter, the PCM has no way of controlling the amount of intake air an engine needs to run at idling speeds, and especially because mechanical wear of control cables, linkage pivots, and even the throttle plate spindle bushes can eventually reach the point where even the correlation between position sensors becomes impossible to verify.

Therefore, to provide for an accurate method of managing the idling speed, conventional throttle control systems incorporate Idle Air Control valves that allow a small volume of air to bypass the throttle plate when it is in the closed position. As a practical matter, the PCM monitors the engine speed directly after starting and uses this information to open the Idle Air Control valve just enough to allow sufficient air to bypass the throttle plate, which is what allows the engine to idle.

On a fully functional system, the PCM will use input data from the engine speed sensor, throttle pedal position sensor; throttle plate position sensor, and others, including the intake air temperature sensor, to maintain the position of the IAC valve spindle as a means to maintain a stable idling speed.

Moreover, the PCM will close the IAC valve to shut off the airflow bypassing the throttle plate when the engine speed exceeds idling speed, and re-open the IAC valve when the engine speed decreases, to establish and maintain stable idling when pressure is removed from the throttle pedal.

Thus, from an engine management perspective, a fully functional IAC valve is a critical component since, without this valve, an engine with a conventional throttle control system cannot run at idling speed.

How Does the Idle air control valve (IAC) Work?

This image shows the inner workings of a typical Idle Air Control valve. In this example, the red arrow indicates the valve pintle, the yellow arrow indicated the threads that mount this particular example on the throttle body, and the green arrow indicates the electrical connector.

In these applications, the electric motor is known as a “stepper motor”, because the PCM can activate them in such a way that they rotate in very small increments, or steps, which allows for very precise control over the position of the valve pintle. In practice, rotating the motor in one direction extends the valve pintle, while rotating the motor in the opposite direction retracts the valve pintle.

Nonetheless, the valve pintle is the most important part to consider here, since its distance from a seat in the throttle body determines the volume of air that is allowed to bypass the throttle plate. This is the air the engine uses to run at idle, so when an issue such as excessive carbon build-up blocks the passage of air, the engine is starved of the air it requires to run at idling speeds.

Moreover, the PCM can adjust the position of the pintle relative to its seat in exceedingly small increments to allow for more air to pass when electrical consumers such as the A/C place an additional load on the engine. By making continuous small adjustments to the position of the valve pintle to increase or decrease the volume of air that passes through the idling circuit in the throttle body, the PCM can maintain a stable idling speed regardless of the loads placed on the engine during idling.

Where is the Idle air control valve (IAC) Located on the Engine?

This image shows the location (circled) of the IAC valve on a Hyundai application. Note though that while the IAC valve attaches to the throttle body in all cases, the actual appearance and location of the IAC on the throttle body varies somewhat between different vehicle makes and models.

Thus, to avoid mistakes, possible misdiagnoses, and the unnecessary replacement of parts and components, we recommend that you research the exact location and appearance of the IAC valve on your particular vehicle before you begin to dismantle or disassemble a throttle body on any application.

What Does the Idle air control valve (IAC) Look Like?

This image shows an example of a typical Idle Air Control valve, but note that the appearance and design of IAC valves vary greatly between different vehicle makes and models. Note that since the design of IAC valves does not follow a general pattern, we recommend that you research the appearance and location of the IAC on your vehicle before you purchase a replacement valve.

While it may be possible to exchange an incorrect or unsuitable IAC valve bought from a brick-and-mortar store, many online retailers will not exchange electrical parts and components such as Idle Air Control valves.

What are the Symptoms that the Idle air control valve (IAC) is Bad?

The typical symptoms of a bad or failing IAC valve are much the same across all applications, and could include one or more of the following, but note that bad or failing IAC valves very rarely produce drivability issues when the engine runs at speeds above idling speed-

  • An illuminated CHECK ENGINE light and one or more stored trouble codes
  • The engine may not run at idling speed
  • The idling speed may be lower than usual
  • The engine may stall at low engine speeds, or when the vehicle comes to a stop
  • Depending on the nature of the problem, the vehicle may be hard to start, or the idling speed may be higher than usual

How do you test the Idle air control valve (IAC)?

NOTE:  Issues like unmetered air that enters the engine after the throttle body can, and often does mimic the effects of a bad or malfunctioning IAC valve. Typical leak sites include damaged inlet manifold gaskets/seals, perforated or dislodged hoses almost anywhere in the engine vacuum system, or malfunctioning vacuum check valves, also almost anywhere in the engine vacuum system.  However, assuming that there no air leaks exist in the engine vacuum system, there are a few things you can do to test the operation of an IAC valve. Here is how-

Disconnect the IAC valve

If the engine idles roughly, or not at all, disconnect the IAC valve’s electrical connector. If the idle quality improves, there is likely a previously undiscovered air leak present.

If the poor idle quality remains the same when the IAC valve is disconnected, the IAC valve is almost certainly bad.

Inspect the IAC valve

Due to its position in the inlet tract, carbon and other residues often accumulate on or around the valve’s pintle. In some cases, the build-up can obstruct the airflow through the dedicated idle air passages, which will mimic the effects of failed IAC valve.

Thus, remove the IAC valve from its position, and check for the presence of carbon build-up or other residues. This issue accounts for a very large percentage of IAC valve failures, but in most cases, carbon and other residues can be removed with approved solvents that are available in aerosol cans. Simply spray the solvent into the relevant passages and onto the valve pintle until all surfaces are clean and free of any kind of deposits or residues. In fact, in nine out of every ten cases, simply removing carbon deposits will restore the IAC valve’s operation.

However, do NOT insert wires and/or other hard objects into the throttle body, or use hard wire brushes to clean off carbon deposits, as any of these methods could cause irreparable damage to both the IAC valve and the throttle body.

If one application of solvent does not work, apply more solvent until the deposits are removed. Bear in mind though that since most of these kinds of solvents are not flammable, the engine may idly roughly or may even misfire afterward until normal engine operation clears the excess solvent from the cylinders.

Test resistance and continuity

Since the stepper motor that controls the movement of the IAC valve is a kind of coil, it must have a specified electrical resistance. However, while it is easy to measure the resistance, the actual resistance value is application-specific, so you need the have access to OEM-level service information to compare your findings with values specified in the service information.

If your obtained reading agrees with specified values, the IAC valve is good, and the problem lies elsewhere.

How do you replace the Idle air control valve (IAC)?

In a few rare cases, the IAC valve is incorporated into the throttle body, meaning that the entire throttle body must be replaced to replace the IAC valve.

However, in almost all applications, it is possible to remove the IAC valve from the throttle body. Note that you may need special tools to remove the retaining screws, but assuming you have access to these tools, replacing the IAC is as simple as removing the old IAC valve from the throttle body, and inserting the replacement- using new gaskets and/or seals in all cases. In terms of difficulty, this process should fall well within the abilities of even novice DIY mechanics.

Nonetheless, it should be noted that while the PCM on most applications will learn the characteristics of the new IAC valve automatically over a few drive cycles, you might need access to a scan tool to perform a prescribed idle relearn procedure in some cases.

These procedures are designed to integrate the new IAC into the overall engine management system. In simple terms, the procedure effectively reprograms some circuits in the PCM to recognize the new IAC valve’s torque characteristics, as well as the new open and closed positions, which always differ from these positions on the old IAC.

Unless this relearn procedure is performed successfully, the engine may not idle and it may be impossible to extinguish warning lights and erase fault codes until the PCM recognizes the new IAC valve.

Thus, while it may be possible to do the actual replacement of an IAC valve on a DIY basis, we recommend that you seek professional assistance with performing relearn and/or integration procedures to ensure the proper operation of new IAC valves.