Troubleshooting Your IAC Motor

Updated on July 27, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Ford IAC valve.
Ford IAC valve. | Source

IAC motor or valve problems can show symptoms like:

  • erratic idling
  • engine stalling
  • surging
  • high idle
  • low idle

Although practically any problem that affects engine operation may affect idle speed, the two most common causes of idle speed problems are:

  1. throttle position sensor (TPS) issues, and
  2. carbon buildup in the IAC motor air passages and throttle body.

Before checking or testing the IAC motor, let’s take a brief look at how the valve operates, so that you know what to look for when checking the motor.

Index
1. How the IAC Works
2. Cleaning Carbon Buildup
3. Testing the IAC Motor
4. Replacing the IAC motor
You can find the IAC attached to the throttle body, which connects to the intake manifold.
You can find the IAC attached to the throttle body, which connects to the intake manifold. | Source

1. How the IAC Works

Your car’s computer controls idle speed using information from various sensors and vehicle operating conditions. With this information, the computer either allows air to bypass the throttle plate or keeps it from doing so. The computer controls the airflow using either an idle speed control (ISC) motor (usually found in throttle body injection (TBI) systems), or an idle air control (IAC) motor or valve (In multiport electronic fuel injections (EFI) systems). You may see these names ISC and IAC used interchangeably. On some models, manufacturers may call this mechanism the air bypass valve.

IAC operation is simple. The computer commands the pintle in the ISC motor or the IAC motor to move in or out to allow more or less air to flow through when the engine is at idle.

Some older vehicle models may have a "fast idle" thermo valve that works without a computer to preform the same function as the ISC or IAC motor. The thermo valve uses a thermowax plunger that expands with heat and contracts when cool. Eventually, the valve’s wax may leak and start to cause high idle problems. In some cases, you may be able to adjust the valve. But usually, a valve replacement is necessary. Consult your vehicle repair manual to find out which system you have.

If you don’t have this manual yet, you can buy one for your make and model from Amazon. Haynes manuals come with step-by-step procedures for many troubleshooting, replacement and maintenance projects you can do at home. So you’ll get your investment back in a short period of time.

Careful When Cleaning the IAC Valve!

When cleaning the IAC motor, be careful not to allow cleaning fluid into the motor. It could damage the motor.

Use carburetor cleaner to remove carbon buildup from IAC valve and throttle body air passages.
Use carburetor cleaner to remove carbon buildup from IAC valve and throttle body air passages. | Source

2. Cleaning Carbon Buildup

After a few years of operation, air passages inside the throttle body and the IAC motor valve may accumulate carbon buildup. Inspecting and cleaning passages is a simple procedure.

Before you start, though, be aware that on some models the throttle body and plate come with a protective coating that makes it difficult for carbon to accumulate. In fact, trying to clean a throttle body and plate with special chemicals on these models can remove this protective layer and interfere with proper throttle body operation and engine performance. If throttle bore looks clean already, don’t try to make it shine. If necessary, consult your vehicle repair manual.

To clean the IAC motor:

  1. Set the transmission to Park (automatic) or Neutral (manual).
  2. Set the ignition to the "off" position.
  3. Engage the parking brake and pop the hood open.
  4. Remove any accessories, if necessary, to gain access to the IAC motor attached to the throttle body.
  5. Unplug the IAC motor electrical connector.
  6. Detach the IAC motor from the throttle body. On some models, you need to remove two or three mounting bolts. On other models, you need to unthread the motor itself from the throttle body. If necessary, consult your vehicle repair manual.
  7. Once removed, position the device so the motor is pointing upwards and the air passages are facing you.
  8. Carefully spray the air passages with throttle body cleaner and wipe the orifices with a clean rag. Use a swab to reach into the valve passages for cleaning. Then, clean the rest of the IAC valve housing, including the mating surface. Repeat this step until the cleaning fluid comes out clean from the device.
  9. Then, spray throttle body cleaner into the throttle body bypass passage.
  10. If it is safe, clean the throttle bore and plate with the solution as well. Make sure to consult your vehicle repair manual for this.
  11. When reinstalling the IAC, replace the valve’s gasket.
  12. Reinstall any components you may have removed to gain access to the IAC valve.

The following video shows the location of the throttle body and IAC motor, and some tips about cleaning both the throttle bore and plate and the IAC motor.

Use a digital multimeter to troubleshoot the IAC motor.
Use a digital multimeter to troubleshoot the IAC motor. | Source

3. Testing the IAC Motor

You can use a digital multimeter to check the electrical condition of the IAC motor and feed circuit. If your engine is equipped with a throttle body injection (TBI) system, it may have an idle speed motor. Check your vehicle repair manual, if necessary, for the testing procedure.

Here, we’re using an IAC motor with a simple electrical configuration. Consult your vehicle repair manual, if yours is different.

Checking the IAC valve internal circuitry:

  1. Set the ignition key to the "off" position.
  2. Unplug the IAC motor electrical connector.
  3. Turn the ignition key to the "on" position; don’t start the engine.
  4. Set your multimeter to 20 volts on the DC volts scale.
  5. Connect your meter’s black lead to the battery negative post.
  6. With your meter’s red lead, probe the wires on the harness connector, the terminal side that leads to the computer.
  7. One of the wires should read between 10.5 to 12.5 volts; otherwise, there’s something wrong on that side of the circuit and the IAC motor is not getting any signal from the computer. Consult your vehicle repair manual for the correct specifications for your particular model.
  8. Turn off the ignition key.
  9. Now switch your multimeter to the Ohms scale.
  10. Measure the resistance across the terminal of the IAC valve.
  11. You should get between 6 to 14 ohms of resistance, depending on your particular application; otherwise, replace the IAC motor.

Consult your repair manual for the correct specs for your particular model.

Checking for an IAC motor short circuit:

  1. Set your digital multimeter to the Ohms scale and choose a setting higher than 10,000 ohms.
  2. Connect one of the meter leads to the IAC motor case, and the other meter lead to each of the IAC valve pins at the terminal.
  3. Both readings should be about 10,000 ohms or more; otherwise, the valve’s electrical circuit is shorted to the case. If necessary, replace the IAC motor.

Consult your repair manual for the correct electrical value specifications for your particular vehicle make and model.

Checking for IAC motor operation:

You can also check IAC motor operation using battery voltage.

  1. Unplug the IAC motor electrical connector.
  2. Remove the IAC motor from the throttle body.
  3. Connect battery voltage to the corresponding terminals on the IAC motor using jumper wires.
  4. The motor plunger should extend.
  5. Reverse the jumper wire connections on the IAC motor terminals.
  6. The plunger should retract.

If the motor doesn’t respond properly to battery voltage, the device is faulty. If your IAC motor passes all the tests and you still suspect the motor is not operating as it should, you may want to check the harness connector and wiring for a possible short or open. Another possibility is a faulty control module or relay, depending on your particular vehicle make and model.

Keep in mind that there are many other sensors or systems faults that can affect idle. Check this other post for other potential problems that can affect idle.

When replacing the IAC motor, use the part number or your vehicle make, model and engine size to get the correct replacement.
When replacing the IAC motor, use the part number or your vehicle make, model and engine size to get the correct replacement. | Source

4. Replacing the IAC Motor

If you need to replace the IAC motor, there are a few simple recommendations you can follow to make sure you are getting the correct replacement for your application and installing it properly.

  • Look for a part number on the IAC motor you want to replace and use that to order the new one.
  • Compare the pintle shape from the old motor to the new one.
  • If necessary, adjust the pintle distance on the new device by measuring the distance on the old motor from the tip of the pintle to its base. Follow the instructions that come with your new IAC motor.
  • If necessary, verify pintle diameter using a vernier caliper.
  • Install a new gasket or O-ring with your new IAC motor.
  • If your IAC motor uses an O-ring, lubricate the ring with a thin layer of transmission oil before installation.

Use the repair manual for your particular make and model to get the correct specifications for your tests.
Use the repair manual for your particular make and model to get the correct specifications for your tests. | Source

5. Using the Correct Tools for the Project

Checking and testing an IAC motor is not difficult, and you can do it with a few simple tools. For most applications, you only need a digital voltmeter or a test light, or both. Often, it is necessary to have the manual for your particular model to identify wires and pins you may need to probe during your troubleshooting procedure. Besides, your manual may have tests for your particular IAC configuration.

Having the correct tools for the job is essential for a fast and good repair. In this case, you only need a few tools you may already have in your toolbox. And having the manual will help you confirm circuit and device operation and, if necessary, proper repair of your IAC for your particular application.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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    © 2019 Dan Ferrell

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