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Idle Air Control Valve Symptoms – A Full Technical Guidance

Any drivers would be familiar with idle speed mode of a car. But when the idle speed doesn’t feel right, there are copious reasons that call for idle air control valve symptoms. Improper caring and skipping the repairs are the vital reasons that cause car idling.

Let us talk about the symptoms of a bad idle air control valve by putting forward the question: What happens when the idle air control valve goes bad?

But first, to understand the symptoms, we need to get the hang of the patient, right? In this case, we need to understand the idle air control valve before digging into idle air control valve symptoms.

What Is Idle Air Control Valve (IAC valve)?

In any car, an idle air control valve or more commonly known as an idle air control actuator is a small device that controls the idling rotational speed of the engine. The idling rotation speed of the engine is how fast the engine run when the car stay still. It has the unit as revolutions per minute/revs per minute/RPM.

idle air control valve symptoms
The Idle Air Control(IAC) Valve for Cadillac Oldsmobile Deville Eldorado Aurora 4.6L

Why does this valve control air? Because it precisely adjusts the amount of air allowed into the intake during idle to regulate the engine speed. The intake mentioned here is just an opening for the air to come into the engine.

The IAC valve is an electrically controlled device, which in turn controlled by the engine computer, and the exact the computer name is engine control unit (ECU). The valve is fitted somewhere in the engine to either bypass the throttle or operate the throttle butterfly valve. Throttle here is just a mechanism to manage air flow by the use of constriction or obstruction.

Regarding the its construction, the IAC valve includes a linear servo actuator namely servomotor, whose task is to control a plunger to vary air flow through the throttle. This servomotor is a combination of a digital optical encoder, lead screw and a DC motor. What really control the airflow is the position of the servomotor. This position is exactly managed digitally by the engine computer.

We have gone through the nature of IAC valve including definition, purpose, structure and its operating overall principle. The next part will guide us through detailed steps on the way it work.

How Does The IAC Valve Work?

Please be noted that each manufacturer designs a different type of IAC valve but they share the same operating principles. How the IAC work are divided into 4 steps.

Step 1: The IAC valve in most cases is installed on the throttle bore to adjust the air intake into the engine at idle mode. The power control module got feedback data from sensors to command the the air passage open or close. Remember that the valve will not be used and have nothing to do to the engine at speed over idle mode. Idle speed of the engine is designed to be constant and fixed.

Step 2: When airflow blows through the IAC valve, the volume of air will be adjusted by the opened/closed ports inside the valve. These ports are sealed by a gasket to prevent air leak. Some valves have a part using coolant to cool down the air at 85 Celcius degree to improve the air intake volume.

Step 3: When the IAC valve is running, the airflow bypasses the valve through the passage in front of the butterfly. Then it comes into the throttle bore by the passage next to that, at this time the butterfly is closing completely.

Step 4: Due to regular huge volume of air passing through the valve, a condition called coking augments and hamper the operation of the valve. Therefore, cleaning the valve periodically is a must to avoid idle air control valve symptoms.

Symptoms Of A Bad Or Failing IAC Valve

Having understood the way the IAC valve works, it would be much easier for us to get to know the natures of some symptoms that call for idle air control valve, which are:

1# Fluctuating Speed

The air control valve manages the idle speed of a car engine. A damaged air valve will wallop the power resulting in fluctuation of the idle speed. One might not be able to control the speed anymore. Staying at one constant speed becomes more than difficult when the power starts to fluctuate. Alteration of speed is one of the imperative idle air control valve symptoms.

List of idle air control valve symptoms
Pay attention to the unstable idle speed

2# Engine Freezing

Stalling or freezing of engine is another one out of idle air control valve symptoms that leads to “not moving” car. You might wonder what happens when the idle air control valve goes bad. The vehicle cannot move in this situation but the engine runs. You will not be able to drive the car if stalling happens. This is because the whole valve system will malfunction just after starting the ignition. However, maintenance tips from experts might assist in curing these symptoms.

3# Deeper Step On The Brake Pedal

At idle speed when no foot force is applied on the accelerator, we all know that braking is very subtle to prevent any forward motion. Any excessive effort on the brake to keep the car stand still at red light could be thez symptoms of a bad IAC valve. This is due to the engine idling faster than normal, and thus the car is running at a faster idle speed. As an additional sign in idle air control valve symptoms, driver can also try to listen to the sound of the idling engine to see if it is spinning strangely fast and harsh, which is also a good way to help clarify the feel on the brake foot.

4# Damaged Valve

A good-condition or healthy valve smoothens the rides. But a damaged valve can result in fluctuations and vibrations causing the vehicle to stop at once. This is because less air will pass through the valve and the engine will start shaking vigorously. That is all about what happens when the idle air control valve goes damaged. Hope you have the answer for some idle air control valve symptoms until now.

Different idle air control valve symptoms
The valve can be damaged for some reasons

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5# Overloaded Engine Causing Freeze

There is another reasons for engine stall. When the IAC valve has already been malfunctioned, the engine can still run but will bear an increase in load. Then if the driver turn on the heater or air conditioner, the engine may become overloaded and come to a halt at once. Another sign to look for is the steering wheel dragged to one side.

To get on the road again, the “first aid” steps are to turn off the heater or air conditioner, wait for a few minutes and then the cooled engine can be turned on again. However, this time please stay away from the heater and air conditioner in terms of idle air control valve symptoms dodge.

6# Warning Light For Checking Engine

Central computer will activate the “check engine” warning light on the dashboard whenever something wrong happens related to the engine, including the failed idle air control valve. To explain, in case the engine control unit detect the varying number of revolutions per minute, it will send commands to the dashboard to enable the warning light. Though idle air control valve symptoms are not always due to the failure of the valve, you had better drive to the nearest auto shop for maintenance.

idle air control valve symptoms
Engine malfunction warning light control in car dashboard.

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Three Ways To Check The Idle Air Control Valve After Identifying Symptoms

This section is just for mechanical reference only, we advise you to take your car to a professional garage for quality check-up services. However for whoever are confident enough to experiment with their own cars and for any mechanic apprentices, we hope these three ways will be of great help. 

1# Check Whether The IAC Valve Can Still Work

After confirming idle air control valve symptoms, get a code scanner to the port under the dashboard to bring up the error codes that make the engine warning light to turn on. Looking up the description of the code to find out the reason.

idle air control valve symptoms
What a code scanner looks like

Then you start the engine and note down the settled idle speed. Shut off the car, remove the car key, open the hood and find the Idle Control Motor in the valve and disconnect it. Follow the instruction to disconnect the motor properly according to the car’s service manual.

Now, start the engine again and note down the settled idle speed this time. No difference between the two idle speed means the Idle Control Motor is not working.

This method for idle air control valve symptoms confirm if there is a problem with the idle control motor, not the problem itself.

2# Identifying What Is Wrong With The IAC Valve

A poorly idling engine may run too fast or too slow or stall. Start the engine and look at the tachometer on the dashboard.

  • A high engine idle:

1000 RPMs is the normal limit for idle speed. Compare the current idle speed of your car to 1000 RPMs. If it is higher than 1000 RPMs, it is too high. If you know the normal idle speed of the car, compare it with the current idle speed and it should be lower then .

  • A low engine idle or stalling:

Is the idle on the tachometer lower and more inconsistently than you normally feel and remember? Is the car sporadically stall? If yes, it may be the consequence of a the faulty IAC valve.

idle air control valve symptoms

Keep an eye on the tachometer while checking idle speed. It’s time to diagnose any leaks on the vacuum lines in the engine bay for cracks or worn areas by connecting the line to a vacuum gauge to measure the level and the consistency of the vacuum or spraying a mixture between soap and water on the lines to look for bubbling at the leaking point. If there are leaks, at the same time check the “check engine” warning light to confirm the IAC valve problem.

3# Detecting issues for specific makes

i. General Motor

Connect the negative lead of a test light to the body of the car and press the test light into each of the four circuits on the GM idle control motor. The test light should flash or go from bright to dim through each circuit while the engine is running.

  • If there is no flash, ECU is having problems instead of idle control motor.
  • If there are flashes, you need to replace the idle control motor in the valve.
idle air control valve symptoms
A test light to check the IAC valve

ii. Ford

Find two electrical pins on the solenoid in the idle control motor and connect an Ohm meter to measure the resistance between the two. The resistance should be between 7.0 to 13.0 ohms or else the idle control motor needs to be replaced.

iii. Chrysler

Connect a bidirectional scan tool to the same OBD port you use to plug the code scanner into. Increase the idle speed with the scan tool. If there is no increase, the idle control motor definitely got an issue in its circuit preventing the signal from changing the idle.

iv. Other popular makes

Other popular car models, including Ford Ranger, Honda Civic, Chevrolet, BMW, especially Nissan Maxima, also appear to be susceptible to this issue within the aforementioned content. After confirming idle air control valve symptoms, common check-up recommendations would be to clean the valve, or else it may be the computer’s error, faulty electrical wiring or circuits, air leak or vacuum leak. Check the leaking problem before replacing the valve. Though replacing the valve is pricey but it is quite worth all your trouble.


Finally, in case of some idle air control valve symptoms, your beloved car needs appropriate care at the right time. Again, know all of this for quick detection and temporary fix before opting for professional service and repairing at the given time to enjoy those long drives for long.

1 Comment
  1. Steve says

    Thanks for sharing my video!

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