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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Yukon - 2WD 🢒 Engine 🢒 Engine Controls - 4.8L, 5.3L, and 6.0L 🢒 Idle Air Control (IAC) System Diagnosis

Circuit Description

The engine idle speed is controlled by the idle air control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The valve consists of a movable pintle, driven by a gear attached to a two phase bi-polar permanent magnet electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has two separate windings that are called coils. Each coil is fed by two circuits from the powertrain control module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed diagnostic trouble code (DTC) may set.

Diagnostic Aids

Inspect for the following conditions:

    • A skewed high throttle position (TP) sensor
    • Restricted air intake system
    • Objects blocking the IAC passage or throttle bore
    • The correct positive crankcase ventilation (PCV) valve
    • A properly installed PCV valve
    • Proper operation of the PCV valve
    • Proper operation and installation of all air intake components
    • Proper installation and operation of the mass air flow (MAF) sensor, if equipped
    • Evidence of damage or tampering of the following components:
       - The throttle stop screw
       - The throttle plate
       - The throttle shaft
       - The throttle linkage
       - The cruise control linkage--if equipped
    • Excess deposits in the IAC passage or on the IAC pintle
    • Excess deposits in the throttle bore or on the throttle plate
    • Vacuum leaks
    • Excess load on engine, such as energy-draining conditions of the transmission, power steering, or alternator
    • A high or unstable idle condition--This condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls .
    • A low or unstable idle condition--This condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls .
    • An intermittent condition--If the condition is intermittent, refer to Intermittent Conditions .

Test Description

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

  1. This test will determine the ability of the PCM and IAC valve circuits to control the IAC valve.

    2.

  2. This test will determine the ability of the PCM to provide the IAC valve circuits with a ground. On a normally operating system, the test lamp should not flash while the IAC counts are incrementing.

Idle Air Control (IAC) System Diagnosis

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: Verify engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 100 RPM less than commanded engine speed or within 200 RPM above commanded engine speed.

  1. Set the parking brake and block the drive wheels.
  2. Install a scan tool.
  3. Start the engine.
  4. Turn OFF all accessories.
  5. With the scan tool RPM control function, slowly increment engine speed to 1,700 RPM, then to 600 RPM, then to 1,700 RPM.
  6. Exit the RPM control function.

Did the engine speed stabilize within 100 RPM less than commanded engine speed or within 200 RPM above commanded engine speed during the above test?

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Disconnect the IAC valve.
  3. Connect a J 37027-1A IAC Motor Driver to the IAC valve.
  4. Start the engine.
  5. With the J 37027-1A , command the IAC valve in until near 600 RPM is reached.
  6. With the J 37027-1A , command the IAC valve out until near 1,700 RPM is reached.
  7. Return engine speed to desired idle as indicated on the scan tool data list.

Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 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 during the last test?

Go to Step 11

Go to Step 12

5

  1. With a test lamp connected to ground, probe one of the IAC valve circuits at the IAC valve harness connector using the J 35616 Connector Test Adaptor Kit .
  2. Start the engine.
  3. With the J 37027-1A , command low RPM while observing a scan tool until the IAC counts start to increment.
  4. With the J 37027-1A , command high RPM while observing a scan tool until the IAC counts start to increment.
  5. While the IAC counts are incrementing observe the test lamp.
  6. Return engine speed to desired idle as indicated on the scan tool data list.
  7. Repeat the above procedure for the other three IAC valve circuits.

Did the test lamp remain ON, never flashing, while the IAC counts were incrementing at any of the IAC valve circuits during the above test?

Go to Step 10

Go to Step 6

6

Did the test lamp remain OFF, never flashing, while the IAC counts were incrementing at any of the IAC valve circuits during the above test?

Go to Step 9

Go to Step 7

7

  1. Connect a test lamp between the IAC coil A low circuit and the IAC coil A high circuit at the IAC valve harness connector using the J 35616 .
  2. With the J 37027-1A , command low RPM while observing a scan tool until the IAC counts start to increment.
  3. With the J 37027-1A , command high RPM while observing a scan tool until the IAC counts start to increment.
  4. While the IAC counts are incrementing observe the test lamp.
  5. Return engine speed to desired idle as indicated on the scan tool data list.
  6. Repeat the above procedure with the test lamp connected between the IAC coil B low circuit and the IAC coil B high circuit at the IAC valve harness connector.

Did the test lamp stay illuminated, never flashing, while the IAC counts where incrementing during the above test?

Go to Step 8

Go to Step 17

8

Inspect for a poor connection at the IAC valve harness connector. 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 harness connectors.
  3. With a DMM, test the circuit where the test lamp remained off for the following:
    4. • An open circuit
    • A short to ground
    • A short to another IAC valve circuit--Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 18

Go to Step 13

10
  1. Turn ON the ignition.
  2. With a DMM, test the circuit where the test lamp remained illuminated for the following:
    3. • A short to voltage
    • A short to another IAC valve circuit--Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 18

Go to Step 13

11
  1. Visually and physically inspect for the following:
    2. • 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 Valve Replacement .
  3. Inspect for the following conditions:
    4. • Debris in the IAC passage
    • Excessive deposits on the throttle plate
    • Excessive deposits in the throttle bore
    • Excessive deposits on IAC valve pintle
  4. Repair any of the above conditions as necessary. Refer to Throttle Body Cleaning .

Did you find and correct the condition?

Go to Step 18

Go to Step 15

12
  1. Visually and physically inspect for the following:
    2. • 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 Valve Replacement .
  3. Inspect for excessive deposits on the IAC valve pintle and in the IAC valve passage.
  4. Repair any of the above conditions as necessary. Refer to Throttle Body Cleaning .

Did you find and correct the condition?

Go to Step 18

Go to Step 15

13

Inspect for a poor connection at the PCM harness connectors. 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 14

14
  1. Disconnect the PCM harness connectors.
  2. With a DMM, test all IAC valve 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

Inspect for a poor connection at the IAC valve harness connector. 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 Valve Replacement .

Did you complete the replacement?

Go to Step 18

--

17

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 18

--

18
  1. Reconnect any disconnected components.
  2. Use the scan tool in order to clear any DTCs that may have set.
  3. Turn OFF the ignition for 30 seconds.
  4. Operate the vehicle within conditions to verify the repair.

Did you verify the repair?

Go to Step 19

Go to Step 2

19

With a scan tool observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK