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Test Description

WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

  1. 5:  This test will determine the ability of the PCM and IAC valve control circuits to control the IAC valve.
  2. 7:  This test will determine the ability of the PCM to provide the IAC control valve circuits with a ground. On a normally operating system, the test lamp should not flash while the IAC Counts are incriminating.
Idle Air Control (IAC) System Diagnosis

Step Action Yes No
Schematic Reference:  Engine Controls Schematics 
Connector End View Reference:  Engine Controls Connector End Views  or  Powertrain Control Module (PCM) Connector End Views 
1 Did you perform the Diagnostic System Check-Engine Controls? Go to Step 2  Go to Diagnostic System Check - Engine Controls
2
IMPORTANT: Ensure that the Engine Speed parameter stabilizes with each Commanded RPM change to determine if the engine speed stays within 100 RPM less than the Commanded RPM or within 200 RPM more than the Commanded RPM.
  1. Set the parking brake and block the drive wheels.
  2. Start the engine.
  3. Turn OFF all accessories.
  4. Slowly increment the engine speed to 1,700 RPM, then to 600 RPM, then to 1,700 RPM with the scan tool RPM Control function.
  5. Exit the RPM Control function.
Did the Engine Speed parameter stabilize within 100 RPM less than the commanded engine speed or within 200 RPM above the Commanded RPM?		 Go to Diagnostic Aids Go to Step 3 
3
  1. Turn OFF the ignition.
  2. Disconnect the IAC valve.
  3. Connect an J 37027-1A  IAC Motor Driver to the IAC valve.
  4. Start the engine.
  5. Command the IAC valve in until 600 RPM is reached with the J 37027-1A  .
  6. Command the IAC valve out until 1,700 RPM is reached with the J 37027-1A  .
  7. Return the engine speed to the Desired Idle Speed parameter.
Did the Engine Speed parameter steadily decrease to 600 RPM and steadily increase to 1,700 RPM when the IAC valve was commanded in and out?		 Go to Step 5  Go to Step 4 
4 Did you observe an excessively high idle condition that could not be controlled with the IAC motor driver? Go to Step 11  Go to Step 12 
5 
  1. Connect a test lamp between one of the IAC valve control circuits and a good ground.
  2. Start the engine.
  3. Observe the IAC Counts parameter with a scan tool.
  4. Command high RPM with the J 37027-1A  until the IAC Counts parameter starts to increment.
  5. Command low RPM with the J 37027-1A  until the IAC Counts parameter starts to increment.
  6. Return the Engine Speed to the Desired Idle Speed parameter.
  7. Repeat the above procedure for the other three IAC valve control circuits.
Did the test lamp remain ON, never flashing, while the IAC Counts were incriminating for any of the IAC valve control circuits?		 Go to Step 10  Go to Step 6 
6 Did the test lamp remain OFF, never flashing, while the IAC Counts were incriminating for any of the IAC valve control circuits? Go to Step 9  Go to Step 7 
7 
  1. Connect a test lamp between the IAC coil A low control circuit and the IAC coil A high control circuit.
  2. Observe the IAC Counts parameter with a scan tool and the test lamp.
  3. Command high RPM with the J 37027-1A  until the IAC Counts parameter starts to increment.
  4. Command low RPM with the J 37027-1A  until the IAC Counts parameter starts to increment.
  5. Return the Engine Speed parameter to the Desired Idle Speed parameter.
  6. Repeat the above procedure with the test lamp connected between the IAC coil B low control circuit and the IAC coil B high control circuit.
Did the test lamp stay illuminated, never flashing, while the IAC Counts were incriminating?		 Go to Step 8  Go to Step 17 
8 Test for an intermittent and for a poor connection at the IAC valve. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 18  Go to Step 13 
9
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Test the circuit where the test lamp remained OFF for the following conditions:
Did you find and correct the condition?		 Go to Step 18  Go to Step 13 
10
  1. Turn ON the ignition.
  2. Test the IAC valve control circuit where the test lamp remained illuminated for the following conditions:
Did you find and correct the condition?		 Go to Step 18  Go to Step 13 
11
  1. Inspect for the following:
    • Throttle body damage and tampering
    • Skewed TP sensor signal
    • Throttle lever screw tampering, if equipped
    • Vacuum leaks
    • Faulty, incorrectly installed PCV valve and hose, if equipped
    • Throttle shaft binding
    • Throttle linkage or cruise control linkage binding, if equipped
  2. Remove the IAC valve. Refer to Idle Air Control (IAC) Valve Replacement .
  3. Inspect for the following conditions:
    • Debris in the IAC passage
    • Excessive deposits on the throttle plate
    • Excessive deposits in the throttle bore
    • Excessive deposits on IAC valve pintle
Did you find and correct the condition?		 Go to Step 18  Go to Step 15 
12
  1. Inspect for the following conditions:
    • Throttle body damage and tampering
    • Throttle lever screw tampering, if equipped
    • Excessive deposits on the throttle plate
    • Restricted air intake system-Inspect the following conditions:
      • A possible collapsed or clogged air intake duct, before and after the air filter element
      • A restricted air filter element
      • Restriction at the throttle body intake screen, if equipped
  2. Remove the IAC valve. Refer to Idle Air Control (IAC) Valve Replacement .
  3. Inspect for excessive deposits on the IAC valve pintle and in the IAC valve passage.
Did you find and correct the condition?		 Go to Step 18  Go to Step 15 
13 Test for an intermittent and for a poor connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 18  Go to Step 14 
14
  1. Disconnect the PCM.
  2. Test all IAC valve control circuits for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 18  Go to Step 17 
15 Test for an intermittent and for a poor connection at the IAC valve. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 18  Go to Step 16 
16 Replace the IAC valve. Refer to Idle Air Control (IAC) Valve Replacement .
Did you complete the replacement?		 Go to Step 18  -
17 Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement .
Did you complete the replacement?		 Go to Step 18  -
18
  1. Reassemble the vehicle as necessary.
  2. Clear any DTCs with a scan tool.
  3. Start the engine.
  4. Operate the system to verify the repair.
Did you correct the condition?		 Go to Step 19  Go to Step 2 
19 Observe the Capture Info with a scan tool.
Are there any DTCs that have not been diagnosed?		 Go to Diagnostic Trouble Code (DTC) List System OK
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