Your vehicle’s idle air control valve is a component which manages the revolutions per minute (RPM) of the engine while it is idling. The idle engine speed needs to be adjusted based on certain conditions, which we will see shortly.
Connected to the throttle body and controlled by the engine control unit, the idle control valve is a vital tool for managing your car’s engine. Unfortunately, all too often, the IAC will fail and lead to some frustrating performance problems.
In this guide, we will cover the major symptoms of a failing idle air control valve so you can try and diagnose whether it is affecting your vehicle. Then, depending on the cause of the problem, we will show you how to fix it.
Failing Idle Air Control Valve Symptoms
There are five major symptoms of a bad idle air control valve. It should be relatively simple to diagnose the problem if you notice any of the following in your car.
Check Engine Light
Your car’s check engine light will light up for a variety of reasons – most of them unrelated to the IAC valve. However, if the light is on it does mean “something” is wrong with your car. By combining this with other symptoms below, it will help in establishing what the problem is.
When it is the IAC vavle that is causing the check engine light, it will be because the ECU has noticed abnormal RPMs while the car is at idle. A bad idle air control valve is one of the prime suspects for a check engine light.
Unstable RPMs While the Car is Idling
It should probably be pretty obvious to you, but if the IAC is broken, it will cause your car to idle incorrectly. Usually this will present to the driver as the engine speed fluctuating up and down at random while idling.
You should be able to hear the engine speeding up and slowing down in the range of 100-200 RPM. All for seemingly no reason and without touching any of the vehicle’s controls.
In addition to simply being a little aggravating, this fluctuation in idle speeds can lead to performance problems in your car’s engine. This is the most common symptom of a bad idle air control valve.
Engine Doesn’t Idle Smoothly
If your vehicle doesn’t idle smoothly, then this is another strong symptom of a bad IAC valve. The roughness is caused by the ECU not being capable of adjusting the engine speed because the broken IAC valve is throwing it off.
You should never get rough idling in a car where the idle air control valve is working properly. Whether you are just starting up or sitting at the lights, the engine should purr like a kitten.
If you’re not sure what rough idling is exactly, it is most notable as rough, sporadic vibrations. You will be able to feel your car shaking unnaturally and the engine noise will simply sound wrong.
Vehicle Stalling For No Reason
Like an illuminated check engine light, constant vehicle stalls can be caused by a large variety of underlying problems. A failing idle air control valve is no exception, it should be one of the prime suspects when your car starts stalling for no apparent reason.
There are two reason a bad IAC valve can cause a car to stall. The first is that the ECU cannot cope by itself trying to maintain the engine speed. Over time the problem will get worse and the ECU will keep letting the RPMs drop so low that the engine stalls.
The second reason is that without a functioning IAC valve, the engine may struggle to get enough oxygen while at idle. Without enough air, the combustion process simply fails, leading to an engine stall.
This symptom is pretty hard to miss! Another likely sign of a bad idle air control valve is when you hear a loud backfire from the exhaust. These startling “bangs” are a general sign of a few different engine problems, so you will need some other evidence to diagnose the cause.
If in combination with the backfiring, the car lurches suddenly to a stop as you lift off the accelerator, then you almost certainly are dealing with an IAC valve problem.
How to Clean the Idle Air Control Valve
If you’ve had a look through the symptoms listed above and think that your idle air control valve is the likely cause of the problem, then follow these steps to resolve it.
Time needed: 2 hours.
Clean and Replace Idle Air Control Valve
- Prepare the Vehicle
Park your vehicle in a suitable, well lit location so you can see what you’re doing. Make sure the engine has had time to cool down completely.
- Confirm the Problem
First you should note down the RPM that your vehicle idles at with the IAC valve in place. Then repeat with the valve disconnected and see whether there is any difference. If the idle RPMs are the same then the idle air control valve is not working.
- Disconnect the Battery
For the safety of yourself and your vehicle’s electrical components, make sure to disconnect the negative battery terminal before proceeding further.
- Locate the Valve
In most cases the IAC valve is located right next to the the throttle body. If it’s not there you will have to do a quick search to find the exact spot on your vehicle.
- Remove the Valve
Take off any screws or clips attaching the the IAC valve to the engine. Gently remove and inspect it for any damage.
- Clean the Valve
There is a good chance your idle air control valve is simply too clogged with dirt and debris to function. Spray it thoroughly with throttle body cleaner and allow to soak for 5 minutes.
Wipe it down with a rag, ensuring you remove all the gunk and carbon build ups.
- Replace the Valve
If there is no obvious damage to your cleaned valve, then replace it and reattach the fastenings securely.
If the valve is damaged then you should put the new one in at this point.
- Re-connect the Battery and Test
After reattaching the negative terminal of your vehicle’s battery, you need to test whether the problem is resolved. Start up the engine and listen to it idling.
Frequently Asked Questions
Depending on your vehicle brand and model, the cost for the part alone will range between $50 and $400. If you choose to have a mechanic replace it, add an additional $80 in labour costs.
A bad throttle body sensor will cause symptoms very similar to a bad IAC valve. You should check your TPS at the same time since they are located right next to each other.
Secure your car with the parking brake and start the engine. Rev it above 2000 rpm for 10 seconds then switch off. Check to see if the problem has resolved.
So long as your car is fuel injected, the IAC valve will be connected to the throttle body. This itself is in between the intake manifold and the air cleaner.
Your vehicle’s engine will fail to get the right amount of air, leading it to run inefficiently. You will notice rough idling with sudden changes in RPM.
In the short term there is no great danger of damage to your engine. Your vehicle will run less efficiently and may stall sometimes, but I wouldn’t classify it as an emergency.
Don’t put off the repair for too long though, if you leave it, the problems will just get worse.