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Signs Of A Bad Idle Air Control Valve & How To Test It

You probably know exactly how your engine responds to throttle input. You press the gas pedal, and your car speeds up. Let off on the accelerator, and the engine slows down. This control happens because the throttle plate inside the throttle body opens and closes as you press and release the accelerator pedal. But what happens when you totally release the gas pedal? Why doesn’t the engine stall? The idle air control valve (IAC valve) is responsible for helping the engine maintain an idle speed when there is no throttle input. If this valve gets clogged or goes bad, there are several symptoms you might notice. Here are the most common signs of a bad IAC valve and how to test it.

What Is An Idle Air Control Valve?

An idle air control valve controls the amount of air entering the intake manifold at engine idle. This valve is sometimes called the auxiliary air valve. At idle, the throttle plate is completely closed because the gas pedal is not being pressed. The IAC valve is responsible for bypassing the throttle body and allowing enough air into the intake manifold to keep the engine running.

Most idle air control valves are located adjacent to the throttle body. Some of these valves are integrated into the throttle body itself. Finally, some new cars with electronic throttle control do not have separate idle air control valves. This is because a small electric actuator controls the throttle plate. An IAC valve is unnecessary on these cars because the car’s computer can control the idle speed using the actuator. The computer can open and close the throttle plate even when the gas pedal is not being pressed.

How Does An IAC Valve Work?

First, it is important to note that an IAC valve only operates at engine idle speed. If the engine is running above idle, the valve has no effect on the engine speed at all. At idle, the throttle plate is closed, so no air would typically make it into the intake manifold. This would cause the engine to stall if the idle air control valve were not in place and operational. However, the IAC will open at idle and allow air to bypass the closed butterfly valve in the throttle body.

The car’s computer can adjust the amount of air that flows through the IAC valve to adjust the idle speed higher or lower. Some IAC valves also allow coolant to flow through passages in the valve to warm the air inside the valve. In addition, the IAC valve can become clogged over time due to the high volume of air that flows through the valve. When this happens, you will likely experience one of the symptoms in the next section.

Symptoms Of A Bad Idle Air Control Valve

Idle air control valves can fail for mechanical reasons, but they can also become dirty or clogged over time. Either of these conditions will result in problems with your vehicle. You will likely notice one of the following symptoms, and you will need to either clean or replace the IAC valve. Many symptoms of a bad IAC valve can also mimic the symptoms of a bad throttle body. Here are the things you need to watch for.

— Rough Idle

Since the IAC valve controls the engine idle, a rough idle is often one of the first symptoms you will notice associated with a bad IAC valve. The engine RPMs associated with idle should be flat and consistent. An idle curve that varies up and down is not right, and you know that something is probably wrong with the idle air control valve. When the valve starts to malfunction, it often overcompensates for the problem. This causes swings between a high idle and a low idle, and it feels like your idle is bouncing all over the place. A rough idle can also be caused by a bad throttle position sensor, so you might need to do some additional diagnostics to nail down the specific problem.

— Check Engine Light

In the automotive world, a check engine light can be a sign of a multitude of problems. An issue with airflow is one of these problems, and a failing idle air control valve might be the culprit. The car’s engine control unit (ECU) uses sensors to measure the amount of air coming into the engine. The computer can tell when there is an airflow problem, and the engine control module will trigger the check engine light when this problem occurs. A repair shop can use a scan tool to read the trouble codes from your car’s computer and determine whether the code points to a bad IAC valve.

— Car Starts, Then Dies

In some cases, your car might start but stall immediately afterward. If the valve fails, it will sometimes still work for part of the idle cycle. This could allow your engine to start but then die as the airflow needs to change. If the air conditioner or power steering pump comes on, the engine needs more air at idle to run those accessories. A faulty IAC valve might be unable to adjust to those needs, and the engine will quit running.

— Backfiring

Backfiring can be another sign of a faulty idle air control valve. At the engine’s idle speed, the IAC will control the amount of air coming from the air intake into the manifold. When the valve fails, the wrong amount of air enters the combustion chamber. This can cause the air-fuel mixture to become too rich, and excess fuel will be left in the car’s engine after the ignition spark occurs. This excess fuel makes its way into the exhaust system and ignites. Fuel ignition in the exhaust system causes a backfire. Not only is this bad for the vehicle’s engine, but it can also be scary because it causes a loud boom.

— Difficulty Starting

The throttle plate in your engine is closed at startup, so your engine relies on the idle air control valve to provide a source of air as the engine starts. If the valve is not functioning properly, the engine might not get any air when you try to start it. A lack of air will cause your engine to have trouble starting, or it might not start at all. If you notice that your engine takes longer than usual to start, you might have an idle air control valve that is starting to fail.

How To Test An Idle Air Control Valve

There are a couple of different ways to test an idle air control valve to pinpoint the cause of your problems. First, if you have high or low idle issues, you should first rule out any vacuum leaks on your engine. If any of the vacuum hoses develop a leak, this can cause similar symptoms as a bad IAC valve. Typically, the system will attempt to keep the idle air control valve fully closed if there is a vacuum leak. Once you confirm that there are no vacuum leaks in the system, you can move on to the next step in the testing process.

Next, you can disconnect the idle air control valve to see if it makes a difference in the car’s idling. If the idle improves, reconnect the valve to see what happens. If reconnecting the valve causes problems with the idle again, then you likely have a bad valve.

Next, you can visually inspect the valve. You will need to remove the valve from the vehicle for the inspection. Check the ports for debris or carbon buildup. Any buildup of contaminants inside the valve can greatly affect engine performance, so you should go ahead and clean the valve while you have it removed. We will discuss cleaning the valve in more detail in the next section.

Finally, you can use a multimeter to test the functionality of your idle air control system. This process is usually used by professional mechanics, but you can do this yourself if you have a little mechanical knowledge. You will need to remove the valve and use the multimeter to test the electrical connectors across the valve. If the reading is within spec, the valve does not have an electrical fault. If the reading is outside the normal range, then the valve has a problem. You will need to reference a repair guide for your vehicle to determine the proper ranges for the valve.

How To Clean An Idle Air Control Valve

In some cases, a thorough cleaning is all that is needed to fix your idle air control valve problems. Here is how to clean it properly. First, remove the valve from the engine. You will typically need a screwdriver and a set of wrenches or sockets for this task. Once the valve has been removed, use a can of intake cleaner to remove the buildup on the valve. Spray the cleaner into the ports on the valve and also spray some on the connection point at the throttle body.

Use a soft brush to remove excess dirt or buildup, and repeat the process until all the deposits are removed. Never use a wire brush on the valve, which could damage the valve and cause even bigger problems. Wipe the valve clean and dry with paper towels and reattach the valve to the engine. Always use a new gasket when reattaching the valve. The old gasket will almost always leak, and a leak at the valve will trigger additional symptoms.

Start up the engine and check the idle. It might run rough for a few seconds as the excess cleaner or solvent burns off, but the idle should then smooth out pretty quickly. If the engine is still idling rough, you might have additional problems that the cleaning did not fix. It is usually a good idea to go ahead and perform a throttle body cleaning at the same time as the idle air control valve since you already have access to the throttle body.

Cost To Replace An Idle Air Control Valve

The replacement cost for an idle air control valve varies widely, depending mostly on the make and model of your vehicle. Having a repair shop replace your IAC valve can cost anywhere from $200 to $700. Labor costs are usually pretty consistent for this job, ranging from $100 to $250. However, the idle air control valve itself can cost anywhere from $100 to $600. High-end vehicles typically have the most expensive parts, and idle air control valves are no different. You can sometimes save yourself some money by purchasing a different brand besides the OEM brand.

Tips For Replacing Your Own Idle Control Valve

Many people choose to perform their own auto repairs, and replacing an idle air control valve is a job that many backyard mechanics can do on their own. Performing a replacement of the IAC valve is not extremely complicated in most vehicles. Here is how to do it.

First, you will need to disconnect the car’s battery to avoid damage to the IAC solenoid or other electronic components while working. Next, remove the old IAC valve from the vehicle. The valve is usually attached to the throttle body, although some IAC valves are integrated into the throttle body. Remember that some newer vehicles with electronic throttle control do not have an IAC valve at all. You will typically need a screwdriver and socket set for the removal of the old valve.

Once the old valve has been removed, you should reattach the new valve using a new gasket. Never attempt to reuse the old gasket, as this will lead to leaks. Plug the electrical connector back into the new valve, reconnect the battery, and start up the vehicle. The car’s idle should be smooth, and the throttle response should be immediate. If you still have issues with the car’s engine, you should do additional troubleshooting to determine the root cause of the problems.

The Bottom Line

The most common symptoms of a bad idle air control valve include a rough idle, check engine light, and rough starts. All of these symptoms should be further investigated to determine the cause of the problem. Replacing an idle air control valve is not extremely difficult, although it can cost you as much as $800 in some cases. The valve itself is fairly expensive for some vehicles, but thankfully, a thorough cleaning of the valve is enough to fix the problem in many cases.

Frequently Asked Questions (FAQs)

What happens when the idle control valve goes out?

There are several bad idle air control valve symptoms you might experience when the idle control valve goes out. High or low idling is one of the most common problems, although you might experience backfiring or engine stalling as well. The reason for this is that the proper amount of air is not able to bypass the closed throttle body at idle. The engine might be starved for air, or it could allow too much air into the system. Neither condition is correct, and these symptoms will begin to show up immediately.

How do I reset my idle air control valve?

You can try disconnecting the idle air control valve to reset the solenoid inside the valve. However, this does not often fix the problem. Cleaning the IAC valve often helps with issues, and this should be done by removing the valve and using an intake cleaner to remove the buildup on the valve. Sometimes, a valve replacement is the only way to fix the problem.

Can I drive with a bad idle air control valve?

The answer likely depends on how bad the valve is. Sometimes, you can still drive with a bad idle air control valve. However, you will likely experience a rough idle at the very least. You might even have to deal with engine stalling, especially when engine accessories like the air conditioner or power steering pump put a load on the engine. A stalling engine can present a dangerous situation, so you should get your IAC valve replaced as soon as possible once it goes bad.

What should I do if my idle air control valve is stuck in the open position?

You should visually inspect the valve to determine whether cleaning will solve the problem. Debris or buildup may be lodged in the valve and cause it to become stuck open. A valve that is stuck open usually results in a high idle, and this can cause excess wear on the engine. It can also cause jerking of the transmission when putting the car in gear. Cleaning the valve might fix the problem, but a replacement might be necessary.

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