Checking a Car’s Idle Air Bypass Valve

Have you noticed your car running a little rougher lately? Maybe the check engine light has come on. One possible cause for these, and other problems, is the car’s idle air bypass valve. This valve is responsible for maintaining the engine’s idle speed. Age and normal wear and tear can cause the valve’s performance to decrease over time, and sometimes even fail. Fortunately they are usually very affordable and easy to replace yourself. In fact, sometimes a simple cleaning may be enough to keep the valve in service.

What is an Idle Air Bypass Valve?

The idle air bypass valve, often simply called the idle air valve (IAV), or idle air control (IAC) plays an important role in regulating engine speed. Most cars use a butterfly valve for the throttle. When you press the accelerator, the valve opens and allows air into the engine and thereby increases engine speed. If you are not depressing the valve, however, air cannot enter the engine and it will stall. This is where the IAV comes in. It bypasses the butterfly valve, and allows enough air into the engine to maintain idle speed. Most modern cars allow the car’s engine control computers to adjust the IAV and control the amount of air coming in at idle. It needs to adjust for loads, such as air conditioning, and different temperatures.

What are Signs of a Faulty IAV?

An IAV sometimes may fail, or can become troublesome. This can manifest itself through various problems while driving. Following are a few problems that may be caused by a faulty IAV.
A rough running engine can be caused by a faulty IAV. If the valve begins to stick over time, or fails to respond to inputs from the car’s computer, it may not properly adjust for engine loads and operating temperatures. If you notice that your car seems to run rough when it starts up, or once it warms up, it may be a good idea to check the IAV.
Sometimes you may notice that your car stalls at a stoplight or anytime you come to a stop. An IAV that isn’t working correctly can cause that to happen because the engine’s computer can no longer keep the engine idling. Many other things can cause stalls, so be sure to investigate other possibilities, such as bad spark plugs.
A failing IAV may cause the check engine light or service engine soon light to come on. Because the IAV plays a role in emissions control, if the valve fails it may cause problems with the engine’s emissions control system. You can plug a scanner into vehicles equipped with an OBD II port for a quick diagnostic of the car’s check engine light issue.

Removing and Testing an IAV

If you think your IAV may be causing some of the problems listed above, you’ll want to test it. The IAV on most vehicles is easy to get to. You simply need to find the throttle valve. You can follow the throttle cable from the accelerator or from the cruise control unit. The valve is on top or on the side of the throttle. You simply need to remove any bolts holding it on and gently pull it off. There is usually a gasket attached to the valve, so be careful not to lose it. Once you have it off, you can visually inspect it. Does everything look alright? Is there any debris or broken parts on the valve? If you have a volt meter, you can check the resistance of the valve. It should be between 7 to 13 ohms. If the valve is broken or the resistance is too low, replace the valve.

Cleaning or Replacing an IAV

Even if you think the IAV is ok, you should clean it while you have it off the engine. Use throttle body cleaner, which you can find at any auto parts store, and gently spray the pinto and spring. Use a paper towel or clean rag to wipe it off. Simply put it back onto the engine, rebolting it as you found it. Be sure the gasket is in place. If you need to replace your IAV, you can find replacements for most models for $15-30. Be sure to get a new gasket for it as well. Gentle unplug the wiring harness from the old IAV and connect it to the new unit. Rebolt the new IAV onto the throttle body.

If you have followed all of the steps above, your car’s IAV should now be back up and running. A clean or new IAV may significantly improve your car’s performance if you were experiencing a rough idle or decreased performance. Of course, many things can cause poor engine performance, so knowing a good mechanic is always a good idea as well. If you have a little mechanical inclination, however, you can easily replace an IAV yourself.