What is an idle air control valve and how does it work

Learn how an idle air control valve works

An idle air control valve bypasses air around a closed throttle plate so the engine can get air at idle. Because it bypasses air, it’s also called an air bypass valve.

Back in the days of carburetors, idle speed was adjusted by way of an idle speed screw that forced the throttle plates open by a crack. In fact, on many carburetors, there were two idle speed adjustment screws; one for hot idle and another for cold. Turning the screw inwards prevented the throttle plate from closing all the way, and the amount the throttle plate remained open determined how much air could flow into the engine.

Fuel-injected vehicles don’t work that way. First off, the throttle body on a fuel-injected engine doesn’t have a venturi because it doesn’t inject fuel into the throttle body. The throttle body’s job is simply to regulate how much air comes into the engine—period.

On start-up, the engine control module (ECM) or powertrain control module (PCM) checks the engine coolant temperature, ambient air temperature, and barometric pressure (on some engines). Then it determines how much air and fuel is required to start the engine. At startup, the throttle plate is completely closed. How does the engine get air? From the idle air valve. The correct term for this part is the idle air bypass valve because its job is to BYPASS air around the throttle plate to provide combustion air at idle.

Car makers use five different styles of idle air bypass valves

• Stepper-motor idle air bypass valve—In this system, a stepper motor adjusts idle air bypass by moving a pintle valve in or out of a valve seat.

This is a typical stepper motor idle air control valve

The stepper motor can position the pintle valve into one of 125 different possible “steps.” The higher the number of steps, the larger the airflow opening. If the stepper motor fails, it will default to its last commanded step position. Since all idle control valves are prone to carbon buildup, the ECM/PCM may perform an idle air control calibration sequence where it commands a full closed and full open position while the engine is running. If the PCM detects more air flow at full closed than expected, it set a check engine light. That would indicate the need for cleaning or valve replacement.

• Duty-control rotary solenoid— A rotary valve, as the name implies, uses a movable rotary valve that blocks or exposes a bypass port based on command signals from the PCM. However, rather than operating in “steps,” the valve has a default spring-loaded closed position. Battery power is run to the valve, and the PCM pulses the ground on and off in rapid pulses to complete power to the solenoid. This method of pulsing a solenoid is called the duty cycle and is usually calibrated in 1/10’s of a second. If the ground path is completed for 5/10’s of a second, it’s a 50% duty cycle.

Incoming air is stopped at the throttle plate. The rotary idle air control valve allows air to bypass the throttle plate based on commands from the PCM

This image shows a typical duty-cycle Ford idle air bypass valve

Duty-control air control valve (ACV) This is the style used in many Ford vehicles. The valve has an internal tapered pintle and solenoid. It uses the same duty cycle circuitry as the duty cycle rotary valve described above.

The PCM pulses ground to the solenoid making the pintle retract from its seat. That allows incoming air to bypass around the closed throttle plate.

• On/off vacuum switching valve (VSV)— In this style valve, a solenoid-operated valve is switched open or closed by the PCM.

• A thermostatically controlled valve is used in some applications. A pellet thermostat in the valve sits in contact with the engine coolant. On a cold start, the thermostat doesn’t obstruct the air bypass port. However, as the engine coolant warms and the wax begins to melt, the wax expansion pushes a pintle to gradually reduce bypass airflow.

As mentioned earlier, idle air control valves can accumulate carbon buildup that can interfere with their operation. The symptoms may include hard cold starting, high idle speed, rough idle, or even a “hunting” or pulsing idle. Many DIYers immediately replace the idle air control valve. That’s understandable, but that usually doesn’t fix the problem. Instead, your first step should be cleaning the tapered seat on the valve and idle air bypass passages. Spray them with throttle body spray cleaner. Then, check for vacuum leaks. A cracked vacuum hose can confuse the PCM, forcing it to issue conflicting commands to the Idle air control valve and cause a hunting idle.

If your vehicle has a high idle or hunting idle, click here to see how to fix it

To see an animation of an idle air bypass valve, click here.

© 2012 Rick Muscoplat