Idle Air Control Valve (IAC) – Function – Failure Symptoms – Testing

Idle Air Control Valve (IAC) - Function - Failure Symptoms - Testing
Idle Air Control Valve (IAC) - Function - Failure Symptoms - Testing

So, The idle air control valve (IAC) is responsible for managing the engine’s idle speed.

Furthermore, the idle air control valve (IAC) is a major management component of the engine.

Consequently, decreasing or increasing the (RPM) of the engine, depending on what the current operating conditions call for.

The idle air control valve (IAC) is usually connected to the throttle body next to the intake manifold. Consequently, it is the engine control unit (ECU) that manages all the functions of the valve.

Idle Air Control Valve (IAC) - Function - Failure Symptoms - Testing
Idle Air Control Valve (IAC) – Function – Failure Symptoms – Testing

Based on the information it receives like engine load and temperature, it will change the speed of the idle accordingly. If a vehicle idles erratically and surges, that could be the sign of a failing idle air control valve (IAC).

Common Symptoms Of A Bad Or Failing Idle Air Control Valve (IAC)

When the idle air control valve (IAC) fails it can cause all sorts of issues; and in some cases may even render it undrivable. Usually, a bad or failing idle air control valve (IAC); will produce a few symptoms that can alert the driver of a potential issue:

Irregular Idle Speed

So, one of the most common symptoms associated with a problematic idle air control valve (IAC) is irregular idle speed. Because, the (IAC); is programmed to regulate and maintain the engine idle speed at a constant rate. If the valve fails or has any issues, it can cause the idle speed to be thrown off. This may result in an unusually high or low idle speed; or in some cases a surging idle speed that repeatedly climbs and falls.

Check Engine Light Illuminated

Another symptom of a potential issue with the idle air control valve (IAC) is; an illuminated Check Engine Light. If the engine control module detects an issue with the idle air control valve circuit or signal; it will set off the Check Engine Light to notify the driver that there’s an issue.

Check Engine Light
Check Engine Light

But, the Check Engine Light can also be set off by a wide variety of issues; so having the computer scanned for trouble codes is highly recommended.

Rough Engine Idle

A normal healthy idle air control valve (IAC) will provide smooth idling in your vehicle. But if the valve goes bad for any reason, the idling will go from smooth to rough. A rough idle will result in intense vibrations forming whenever your vehicle is stopped with the engine running. Since less air will be going into the engine during its idle state, the car will react by shaking profusely.

Engine Stalling

Another more serious symptom of an issue with the (IAC) is engine stalling. If the idle air control valve (IAC) fails completely; it may leave the vehicle without a source of air to maintain a proper idle. This may result in the engine stalling while operating; and in some cases may result in an engine that will not idle at all; and stalls as soon as it is started.

Stalling Under Load

Sometimes the engine stalling will happen on its own while other times; increasing the load on the engine will cause it to stall. For example, if you turn on your heater or air conditioner; when you have a bad idle air control valve (IAC); then your engine will probably stall immediately afterward. Consequently, the steering wheel might also feel like it’s being dragged to one side too.

How To Clean The Idle Air Control Valve (IAC):

One common problem is a buildup of carbon or fuel varnish deposits in the idle speed control valve. The cure for this condition is to clean the valve, with some aerosol throttle cleaner or engine top cleaner.

Follow these common instructions:

  1. Disconnect the air intake ductwork from the throttle body.
  2. Start the engine, then increase and hold the idle speed to 1,000 to 1,500 rpm.
  3. Spray the throttle cleaner or engine cleaner into the throat of the throttle body; aiming for the idle air bypass port.
  4. Turn the engine off to allow the cleaner to soak into the (IAC) passageway.
  5. Wait about three minutes.
  6. Restart the engine, rev and hold at 1,000 to 1,500 rpm, and repeat the cleaning process again.
  7. Turn the engine off again, and reattach the air intake duct work to the throttle body.
  8. Start the engine and rev and hold to 1,500 to 2,000 rpm; until no white smoke is coming out of the exhaust pipe.
  9. If the idle speed still surges after this, the (IAC) valve is defective and needs to be replaced.

Related Engine Code P0505:

If the code P0505 appeared soon after the battery has been replaced or disconnected; it’s a good idea to try performing the idle relearn procedure. The procedure varies for different cars.

How To Do Basic Testing

In the event of an electrical fault, a spring maintains the rotary valve in a fixed position; and the idling speed is increased to approximately 1200 RPM. The (ECM) incorporates a self-learning system which enables it to learn the ideal idling speed. Because of this feature, the (IAC) valve is not adjustable.

First, Disconnect the negative battery cable and unplug the (IAC) connector.

How To test the coil driver circuits in the (IAC) valve:

Using an ohmmeter, measure the resistance between pins 3 and 2 on the (IAC) valve.
Resistance should be 10-14 ohms. If resistance is ok, proceed to the next step. Finally, if resistance is out of range, replace the (IAC) and retest.

Digital Multimeter
Digital Multimeter

Using an ohmmeter, measure the resistance between pins 1 and 2 on the (IAC) valve.
Resistance should be 10-14 ohms. If resistance is ok, proceed to the next step. Finally, if resistance is out of range, replace the (IAC) and retest.

How To test the voltage lead:

Using an voltmeter, measure the voltage at terminal 2. Turn the ignition ON , and measure the voltage between terminal 2 and ground.
Voltage should be battery voltage. If voltage is not present or out of range, repair the circuit. If voltage is ok, proceed to the next step.

How To test the signal return:

Using an voltmeter, measure the voltage at terminal 1. Turn the ignition ON , and measure the voltage between terminal 1 and ground.
Voltage should be around 6.0-8.5v. If voltage is not present or out of range, repair the circuit. If voltage is ok, proceed to the next step.

How To test the ground circuit:

Using an ohmmeter, measure the resistance at terminal 3. Turn the ignition ON , and measure the resistance between terminal 1 and ground.
Resistance should be 0 ohms. But, if continuity is not present or out of range, repair the circuit. Finally, if continuity is ok, replace the (CMP) sensor and retest.

Crank And Cam Sensors (CKP)(CMP) Required By Distributorless Ignition
Crank And Cam Sensors (CKP)(CMP) Required By Distributorless Ignition

Conclusion

So, If your vehicle is experiencing any of the symptoms above; or you suspect that your idle air control valve (IAC) may be having a problem, do the testing first. Finally, If it fails just replace it.

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