Commonly called an idle air control (IAC) valve or idle speed control (ISC) valve, this device is used on many cable-operated throttle bodies to regulate the engine idle speed. This process is completed by regulating airflow through a bypass circuit around the throttle plate to increase or decrease idle speed. Increasing the volume of air that flows through the bypass circuit around the throttle plate increases idle speed. Reducing the bypass airflow decreases idle speed. The engine control module controls and monitors this process and can command the fuel mixture to compensate for the airflow.
When we refer to the engine idle speed, this is the speed in revolutions of the engine when the driver's foot is not activating the throttle. While the throttle plate is closed, the main intake passage for the engine is closed off so a bypass circuit is necessary to keep from choking the engine.
When the engine's idle speed is above or below the predetermined range in the computer's program, the computer commands the valve to either increase or decrease the bypass air flow. Additional sensor inputs from the coolant sensor, brake switch and vehicle speed sensor may also be used by the computer to regulate idle speed according to various operating conditions. Idle speed may also be increased when the A/C compressor is engaged, the alternator is charging above a certain voltage, and/or the automatic transmission is in gear to prevent the engine from lugging down.
Part 1 of 4: Identifying idle speed problems
Tip: If the engine speed is too high, too low, or stalling, the problem may not be the idle speed control system, but an engine vacuum leak. Check for vacuum leaks first to rule out this possibility.
Considering a vacuum leak, a common condition is to find the IAC valve extended all the way out (closed position). This usually means the engine has an air leak and the engine computer is trying to bring the idle speed back down by closing the idle air bypass circuit. If there is an open or shorted circuit in the idle air control solenoid, driver circuit, or the idle speed is out of range, it will usually set one or more fault codes and turn on the Check Engine light. If the light is on, you must plug a scan tool into the diagnostic connector and read out the codes that set the light.
Note: Many modern vehicles are equipped with a “drive-by-wire” induction system. These vehicles use an angle sensor on the throttle pedal to calculate throttle input. The engine computer will then activate a small motor on the throttle body which also has a position sensor so the computer can match the throttle pedal angle to the throttle plate angle.
“Drive-by-wire” vehicles typically do not have an idle air control valve because the computer will take information from all of the sensors and automatically adjust the throttle plate angle as necessary. An idle malfunction here may require cleaning or the replacement of the throttle body as well as the use of a professional scanner to reset the system.
Materials Needed
- Digital Multimeter
- Professional Vehicle Scanner
- Q-Tips or Pipe Cleaners
- Throttle Body/Intake Cleaner
Vehicle Service Manual
Warning: The difference between throttle body cleaner and brake cleaner is the throttle cleaner does include a bit of lubricant for the moving components of the throttle body. DO NOT use throttle body cleaner on brake system hardware.
Part 2 of 4: Checking the IAC function by disabling it
Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.
Step 2: Disconnect IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.
Step 3: Start the engine. Start the engine and observe how the vehicle reacts. Where a vehicle may have previously been stalling after start-up, disabling the IAC valve my open the bypass circuit and cause the vehicle idle to raise when the valve is disabled.
Step 4: Reconnect the IAC valve. Turn off the ignition and reconnect the IAC valve electrical connector.
Step 5: Start the engine. At this point the engine idle should return to normal. If it does, the IAC valve may be functioning properly. If not, use the next method to check and see if it needs to be cleaned.
Part 3 of 4: Visually inspecting the valve
Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.
Step 2: Disconnect IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.
Step 3: Remove the IAC valve from the vehicle. Use the procedure detailed in the vehicle service manual to remove the IAC valve.
Step 4: Inspect the IAC valve. Inspect the valve and the mounting location for carbon build-up, rust or dirt. Inspect the IAC valve pintle and mounting location for damage.
Step 5: Clean the IAC valve and bypass channel. Use a carbon cleaner or intake cleaner solvent to remove buildup and dirt from the IAC valve. Use the straw that comes with the spray can to clean the IAC valve mounting location and bypass passage as well.
- Warning: Do not use metal wire brushes to clean the valve or bypass circuit. Scratching the walls or the pintle with a metal wire brush can change the IAC valve function. Step 6: Install the IAC valve. Install the IAC valve with a NEW seal. Using the old seal may cause a vacuum leak or a coolant leak on vehicles where coolant runs through the IAC valve. Step 7: Start the engine. If you used a lot of solvent, the engine may run rough momentarily as it takes in and burns off any leftover solvent. After a brief period of running rough, the idle should return to normal.
Part 4 of 4: Using a multimeter to check IAC valve resistance specification
Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.
Step 2: Disconnect IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.
Step 3: Remove the IAC valve from the vehicle. Use the procedure detailed in the vehicle service manual to remove the IAC valve.
Step 4: Inspect the IAC valve. Inspect the valve and the mounting location for carbon build-up, rust or dirt. Inspect the IAC valve pintle and mounting location for damage. Rules these items out before condemning the IAC valve.
Step 5: Check resistance of IAC valve. Use the specification listed in the vehicle service manual for the IAC valve and follow the guideline on how to test the valve using a digital multimeter against the electrical terminal pins on the IAC valve electrical connector. If the reading falls within specification, the valve should be electronically sound and the fault lies somewhere else. If the reading is not within the specification, replace the unit with a new one.
- Note: The new IAC valve may or may not come with a new seal. Remember to replace the seal any time a sealed part is removed from the engine to avoid a vacuum leak or a coolant leak where coolant runs through the IAC valve body.
A professional technician equipped with a professional grade vehicle scan tool may be able to plug into the vehicle and command the IAC valve through the scanner to check operation. If you have followed all methods of testing and are still stumped, consider hiring a professional technician to take a look. The technicians we have to offer here at YourMechanic are happy to make house calls.
