Idle air control valve stuck open symptoms

An engine idles when you turn it on and leave it in the park with the engine running. An ideal idle should have the AC on though most people still call it idling if you have the AC and other electronics running. This is technically right as long as you have no throttle input.

Since you are not controlling the throttle, the engine has to figure out how much air and fuel to feed the engine to keep it running at the lowest possible RPM without stalling.

The idle air control (IAC) valve is part of the throttle body in most older fuel-injected vehicles. It works with the PCM to achieve smooth idle. Since it bypasses the throttle body and pipes air straight to the engine, it can modulate how much air the engine gets without working the throttle body.

• When the car is in park, and the throttle is in a neutral position, the ECU sends a signal to transfer idle management to the IAC valve.
• The valve will keep adjusting how much air it feeds the engine depending on the engine load and other electrical or mechanical demands on the system.

Since the valve is smaller than the throttle body intake, there’s a limit to how much RPM it can achieve. 

Newer drive-by-wire vehicles that don’t use a cable to control the throttle body don’t have an idle air control valve.

Most Modern Vehicles Don’t Have an IAC Valve

Since their throttle body is fully electronic, they can still achieve the minuscule controls of a traditional IAC valve by working the throttle plate over very small distances. It will constantly be working your throttle position throughout the idling process.

While the electric throttle pedal and the entire CAN bus acceleration might sound complicated to some, the fact that your throttle body doesn’t need extra valves to manage to idle makes it easier to idle the vehicle at whatever speed the engine load demands.

What Will Happen if It is Stuck Open?

Your car will have trouble idling perfectly if the valve is stuck open. Here are the top things you should look out for to pick out a faulty IAC valve.

A Check Engine Light

A check engine light could pop up when the PCM detects a failure in the IAC circuitry or senses an unexpected idle. The error code will likely be P0505, meaning there is a fault in the Idle Control System. 

When it happens, the idle speed won’t be the same as what the PCM was shooting for. Note that this error doesn’t necessarily mean the IAC valve is faulty. It could also mean other issues affect how well the car can idle.

You still have to do further digging or bench testing to confirm whether the idle control valve is the problem.

Higher than Normal Idle Speed Even after the Engine Warms Up

Normally, your engine should start by idling at a high RPM before later settling down to something between 500 and 1100 RPM, depending on your vehicle. 

If your idle air control valve is stuck open, your engine will keep running at a higher RPM, even if your engine is warm. 

Check your manufacturer’s specified idle RPM for your engine trim and confirm that it is idling as expected. A high idle that doesn’t dip hints at a valve that is stuck open.

Erratic Idling Speed

Since your power train control module can’t modulate airflow on the IAC valve, your engine can’t seamlessly adapt its idling speed to engine load.

This means the RPMs will dip as a heavy load kicks in, for instance, the AC compressor, and jump high again when the load is off. 

While this might not be a problem if the idle speed is stuck high enough to provide enough power while making the engine noisier and less power efficient, it will be a bother if it is stuck halfway open.

A Jerky Engine and Further Failure

While your engine might not have trouble turning on with the idle valve stuck open, you could still end up with an engine that runs lean when idle.

Even though it will run, a lean engine is never good news:

• It produces way less power; hence it will be jerky and erratic
• Lean engines can run hotter than usual, exposing your motor to the risk of overheating problems
• You could end up with damaged gaskets, valves, and pistons

How to Test Your Idle Air Control Valve

The best way to test your Idle Air control valve if your ECU isn’t throwing an error code is by observing how the engine behaves when idling.

• Turn on the engine and let it warm up to normal operating temperature
• Check your user manual to find out the ideal idle RPM for your engine (you can sometimes find this figure in a sticker under the hood)
• Once the car warms up, check out what RPM it is idling at. This should be within the given range for your engine.
• Add some load to the engine by turning on the AC and setting it to the lowest possible temperature so that the AC compressor kicks on.

If your RPMs get erratic when idling, jumping up and down now and then, you have an issue with idle control.

If they remain at a steady high level at the maximum of the idling speed, you could be dealing with a valve stuck open, letting in too much air.

It is normal for the RPMs to dip a bit when you add a load, for instance, turning the AC on. The RPMs might ramp up again or remain there. You shouldn’t be worried as long as it is within the ideal idling RPM for your engine.

Can Cleaning My Idle Air Control Valve Get it Unstuck?

Though very rare, cleaning your idle air control valve might get it unstuck. The thing is, the valve isn’t exposed to that many contaminants. It is hard to get dirty to the extent of sticking open. 

Getting a replacement IAC valve is always better once you realize that yours is stuck open.

Can You Keep Driving With an Idle Air Control Valve Stuck Open?

An IAC valve stuck open isn’t as bothersome as one stuck closed. Your engine will idle only that it will idle at a very high RPM.

The result is a noisier ride, less fuel efficiency (especially in traffic jams), and potential failure, especially if your fuel injection system can’t compensate and the engine runs lean.

Though negligible, keeping your car running is never a good idea if you know something is faulty. Fixing it as soon as possible could prevent other related problems from occurring.