Idle Air Control (IAC) System Check

Circuit Description
The Powertrain Control Module (PCM) controls Engine idle speed with the idle air control (IAC) valve. To increase idle speed, the PCM retracts the IAC valve pintle away from its seat, allowing more air to bypass the throttle bore. To decrease idle speed, it extends the AC valve pintle towards its seat, reducing bypass air flow. A Scan Tool will read the PCM commands to the IAC valve in counts. Higher counts indicate more air bypass (higher idle). Lower counts indicate less air is allowed to bypass (lower idle).

Diagnostic Aids
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC Scan Tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem:
- Vacuum leak (high idle) - If idle is too high, stop the Engine. Fully extend (low) IAC with the IAC motor analyzer J 39027-A. Start the Engine. If idle speed is above 800 RPM, locate and correct the vacuum leak, including the Positive Crankcase Ventilation (PCV) system. Check for binding of the throttle blade or linkage.
- Lean heated oxygen sensor signal (high air/fuel ratio) - The idle speed may be too high or too low. Engine speed may vary up and down, and disconnecting the IAC valve does not help. Diagnostic trouble codes P0131, PO151, P0171, or P0174 may be set. Scan tool oxygen (02) voltage will be less than 100 mV (0.1 V). Check for low regulated fuel pressure, water in fuel, or a restricted injector.
- Rich heated oxygen sensor signal (low air/fuel ratio) - The idle speed will be too low. Scan tool AC counts will usually be above 80. The system is obviously rich and may exhibit black smoke in the exhaust. Scan tool 02 voltage will be fixed at about 750 mV (0.75 V). Check for high fuel pressure, or a leaking or sticking injector. A silicon-contaminated heated oxygen sensor will show an 02 voltage slow to respond on the Scan Tool.
- Throttle body - Remove the IAC valve and inspect the bore for foreign material.
- IAC valve electrical connections - IAC valve connections should be carefully checked for proper contact.
- PCV valve - An incorrect or faulty PCV valve may result in an incorrect idle speed. Refer to Diagnosis by Symptom / Rough Idle, Stalling. If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck the connections and valve terminal resistance, or replace the IAC.

Test Description
Number(s) below refer to the step number(s) in the Diagnostic Test:

1. The IAC motor analyzer J 39027-A is used to extend and retract the IAC valve. Valve movement is verified by an Engine speed change. If no change in Engine speed occurs, the valve can be resettled when removed from the throttle body.
2. This step checks the quality of the IAC movement in step 1. Between 700 revolutions per minute (RPM) and about 1500 RPM, the Engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes to extend the IAC valve before Engine speed will begin to drop. This is normal on certain Engines. Fully extending the IAC may cause Engine stall. This may be normal.

6. Steps 1 and 2 verified the proper IAC valve operation. This step checks the IAC circuits. Each lamp on the noid light should flash red and green while the IAC valve is cycled. While the sequence of color is not important, if either light is "OFF" or does not flash red and green, check the circuits for faults, beginning with poor terminal contacts.

Diagnostic Test
1. a. Ignition "OFF".
b. Connect the IAC motor analyzer J 39027-A to the IAC valve.
c. Set the parking brake.
d. Block the wheels.
e. Turn the air conditioning "OFF".
f. Idle the Engine in Park (A/T) or Neutral (M/T).
g. Install the Scan Tool. Display the RPM.
h. Use the IAC motor analyzer J 39027-A to extend and retract the IAC valve.
i. The Engine speed should decrease and increase as the IAC is cycled.
j. Does the RPM change?

[YES] Go to Step 2.
[NO] Go to Step 3.

2. a. RPM should change smoothly with each flash of the IAC motor analyzer J 39027-A light.
b. Does the RPM change within the range specified?

[VALUES] 700 - 1500 RPM.

[YES] Go to Step 6.
[NO] Go to Step 3.

3. a. Check the IAC passages.
b. Are the IAC passages OK?

[YES] Go to Step 4.
[NO] Go to Step 5.

4. a. Clear any obstruction from the IAC passages.
b. Is the action complete?

[YES] Verify repair.

5. a. Replace the IAC.
b. Is the action complete?

[YES] Verify repair.

6. a. Install the appropriate IAC noid light from J 39027-A into the Powertrain Control Module harness.
b. Cycle the IAC motor analyzer J 39027-A and observe the noid lights.
c. Both the lights should cycle red and green, but never "OFF", as the RPM is changed over its range.
d. Do the noid light cycle red and green?

[YES] Go to Step 7.
[NO] Go to Step 8.

7. a. Use the other connector on the IAC motor analyzer J 39027-A pigtail.
b. Check the resistance across the IAC coils.
c. Measure the resistance between terminal A and terminal B.
d. Measure the resistance between terminal C and terminal D.
e. Is the resistance within the specified range?

[VALUES] 40 - 80 ohms.

[YES] Go to Step 9.
[NO] Go to Step 10.

8. a. If the circuits did not test green and red, check the following:
- Faulty connector terminal contacts.
- Open circuits, including connections.
- Circuits shorted to ground or voltage.
- Faulty Powertrain Control Module connection or powertrain control module.
b. Are repairs necessary?

[YES] Go to Step 13.

9. a. Check the resistance between the IAC terminal B and terminal C.
b. Check the resistance between the IAC terminal A and terminal D.
c. Is the resistance infinite?

[YES] Go to Step 11.
[NO] Go to Step 12.

10. a. Replace the IAC.
b. Is the action complete?

[YES] Go to Step 7.

11. a. Check the IAC valve and circuit.
b. Are the IAC valve and circuit OK?

[YES] Refer to Diagnostic Aids.
[NO] Go to Step 12.

12. a. Replace the IAC.
b. Is the action complete?

[YES] Go to Step 9.

13. a. Repair as necessary.
b. Is the action complete?

[YES] Go to Step 6.