Idle Air Control (IAC) System Diagnosis Non European Emissions
Circuit Description
The electronic control module (ECM) controls the engine idle speed with the idle air control (IAC) valve. To increase the idle speed, the ECM pulls the IAC pintle away from its seat, allowing more air to pass by the throttle bore. To decrease idle speed, it extends the IAC valve pintle toward its seat, reducing bypass airflow. A scan tool will read the ECM commands to the IAC valve in counts. The higher counts indicate more air bypass, higher idle. The lower counts indicate less air is allowed to bypass, lower idle.
Diagnostic Aids
If the idle it too high, stop the engine. Fully extend the IAC valve with a IAC tester. Start the engine. If the idle speed is above 800 RPM, locate and repair the vacuum leak. Also, inspect for a binding throttle plate, throttle linkage, or an incorrect base idle setting.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Was the Diagnostic System Check - Engine Controls performed? | -- | Go to Step 2 | |
2 |
Does the engine RPM change? | -- | Go to Step 5 | Go to Step 3 |
3 |
Is the condition found? | -- | Go to Step 4 | Go to Step 19 |
4 | Clean the IAC passages. Is the repair complete? | -- | System OK | -- |
5 |
Does the RPM change smoothly within the value specified with each flash of the IAC driver? | 700-1,500 RPM | Go to Step 6 | Go to Step 3 |
6 |
Do both lights cycle red and green but never OFF as the RPM is changed? | -- | Go to Step 7 | Go to Step 9 |
7 |
Does the resistance measure within the value specified? | 40-80 ohms | Go to Step 8 | Go to Step 19 |
8 |
Does the ohmmeter show the specified value? | ∞ | Go to Diagnostic Aids | Go to Step 19 |
9 | Inspect the IAC connector terminals. Is the problem found? | -- | Go to Step 10 | Go to Step 11 |
10 | Repair or replace the IAC connector terminals as needed. Is the repair complete? | -- | System OK | -- |
11 | Inspect for an open or short in the wire between the IAC connector terminal 3 and the ECM connector terminal A4. Is the problem found? | -- | Go to Step 15 | Go to Step 12 |
12 | Inspect for an open or short in the wire between the IAC connector terminal 4 and the ECM connector terminal A3. Is the problem found? | -- | Go to Step 15 | Go to Step 13 |
13 | Inspect for an open or short in the wire between the IAC connector terminal 1 and the ECM connector terminal A1. Is the problem found? | -- | Go to Step 15 | Go to Step 14 |
14 | Inspect for an open or short in the wire between the IAC connector terminal 2 and the ECM connector terminal A2. Is the problem found? | -- | Go to Step 15 | Go to Step 16 |
15 | Repair the wire as needed. Is the repair complete? | -- | System OK | -- |
16 | Inspect the ECM connector terminals. Is the problem found? | -- | Go to Step 17 | Go to Step 18 |
17 | Repair the ECM connector terminals as needed. Is the repair complete? | -- | System OK | -- |
18 | Replace the engine control module (ECM). Refer to Engine Control Module Replacement . Is the repair complete? | -- | System OK | -- |
19 | Replace the idle air control (IAC) valve. Refer to Idle Air Control Valve Replacement . Is the repair complete? | -- | System OK | -- |
Idle Air Control (IAC) System Diagnosis European Emissions
Circuit Description
The engine control module (ECM) controls the engine idle speed with the idle air control (IAC) valve. To increase the idle speed, the ECM pulls the IAC pintle away from its seat, allowing more air to pass by the throttle body. To decrease the idle speed, it extends the IAC valve pintle toward its seat, reducing bypass airflow. A scan tool will read the ECM commands to the IAC valve in counts. The higher counts indicate more air bypass, higher idle. The lower counts indicate less air is allowed to bypass, lower idle.
Diagnostic Aids
If the idle is too high, stop the engine. Fully extend the IAC valve with an IAC driver. Start the engine. If the idle speed is above 800 RPM, locate and repair the vacuum leak. Also, inspect for a binding throttle plate or throttle linkage or an incorrect base idle setting.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
The IAC valve is extended and retracted by the IAC driver. IAC valve movement is verified by an engine speed change. If no change is engine speed occurs, the valve can be removed from the throttle body and tested. Connect the IAC driver to the removed IAC valve and turn the ignition ON. Do not start the engine.
-
This step inspects the quality of the IAC valve movement in step 2. Fully extending the IAC valve may cause an engine stall. This may be normal.
-
Steps 2 and 5 verify proper IAC valve operation. This step inspects the IAC circuit for a wiring or ECM fault.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Perform the Diagnostic System Check - Engine Controls. Was the check performed? | -- | Go to Step 2 | |||||||||
Is the condition found? | -- | Go to Step 3 | Go to Step 4 | |||||||||
3 | Clean the IAC passages. Is the repair complete? | -- | System OK | -- | ||||||||
4 | Measure the resistance between the following terminals of the IAC valve:
Is the resistance equal to the value specified? | 40-80 ohms | Go to Step 6 | Go to Step 5 | ||||||||
Replace the IAC valve. Is the repair complete? | -- | System OK | -- | |||||||||
Is the problem found? | -- | Go to Step 8 | Go to Step 7 | |||||||||
7 | Repair an open or short circuit as needed. Is the repair complete? | -- | System OK | -- | ||||||||
8 | Inspect the IAC connector terminals and the ECM connector terminals. Is the problem found? | -- | Go to Step 9 | Go to Step 10 | ||||||||
9 | Repair or replace the throttle body assembly and/or the ECM connector terminals as needed. Is the repair complete? | -- | System OK | -- | ||||||||
10 | Replace the ECM. Refer to Engine Control Module Replacement . Is the repair complete? | -- | System OK | -- |
