GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Yukon XL - 4WD 🢒 Engine 🢒 Engine Controls - 4.8L, 5.3L, and 6.0L 🢒 Idle Air Control (IAC) System Diagnosis

Circuit Description

The idle air control (IAC) valve is located in the throttle body of both sequential multi-port fuel injection (SFI) and multi-port fuel injection (MFI) systems. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor, which is called a stepper motor. The IAC valve motor is a 2 phase bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time the polarity of a winding is changed. This change in polarity can be seen when observing a test lamp connected between ground or B+ and an IAC valve circuit while the powertrain control module (PCM) is attempting to change engine RPM. The test lamp will flash ON or OFF each time the polarity is changed.

The PCM does not use a physical sensor to determine IAC pintle position. The PCM uses a predicted number of counts, 1 count represents 1 change in polarity which equals 1 step of the stepper motor. The PCM counts the steps that have been commanded to determine IAC pintle position. The PCM uses the IAC valve to control engine idle speed by changing the pintle position in the idle air passage of the throttle body. This varies the air flow around the throttle plate when the throttle is closed.

To determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs:

    • The engine RPM
    • The battery voltage
    • The air temperature
    • The engine coolant temperature (ECT)
    • The throttle position (TP) sensor angle
    • The engine load
    • The vehicle speed

When the ignition key is turned OFF after an ignition cycle, the PCM will first seat the IAC pintle in the air bypass bore. Next, the PCM will retract the pintle a predetermined amount of counts. This retraction allows the proper amount of air to bypass the throttle plate in order for the engine to start-up. This procedure is known as an IAC Reset.

Diagnostic Aids

Inspect for the following conditions:

    • A skewed high 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. 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.

Step

Action

Yes

No

Schematic references: 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
  1. Set the parking brake and block the drive wheels.
  2. Connect a scan tool.
  3. Start the engine.
  4. Turn OFF all accessories.

    5. Important: Verify that the engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 50 RPM of the commanded RPM.

  5. With the scan tool RPM control function, slowly increment engine speed to the following RPMs:
    6. • 1,800 RPM
    • 600 RPM
    • 1,800 RPM
  6. Exit the RPM control function.

Did the engine speed stabilize within 50 RPM of the commanded RPM 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 an J 37027-A IAC Motor Driver to the IAC valve.
  4. Start the engine.
  5. With the IAC motor driver, command the IAC valve IN until near 600 RPM is reached.
  6. With the IAC motor driver, command the IAC valve OUT until near 1,800 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 1,800 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 10

Go to Step 11

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 IAC motor driver, command low RPM while observing a scan tool until the IAC counts start to increment.
  4. With the IAC motor driver, 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 where incrementing at any of the IAC valve circuits during the above test?

Go to Step 9

Go to Step 6

6

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

Go to Step 8

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 IAC motor driver, command low RPM while observing a scan tool until the IAC counts start to increment.
  3. With the IAC motor driver, 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 Diagnostic Aids

Go to Step 15

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connectors.
  3. With a DMM, test for an open or short to ground on the IAC valve circuit where the test lamp remained OFF. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 12

9
  1. Turn ON the ignition.
  2. With a DMM, test for a short to voltage on the IAC valve circuit where the test lamp remained illuminated. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 12

10
  1. Visually and physically inspect for the following components:
    2. • Evidence of damage or tampering of the throttle body
    • 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 binding
    • Cruise control linkage binding, if equipped
  2. Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
  3. Inspect for the following conditions in the IAC components:
    4. • Debris in the IAC passage
    • Excess deposits on the throttle plate
    • Excess deposits in the throttle bore
    • Excess deposits on IAC valve pintle
  4. Repair any of the above conditions as necessary. Refer to the appropriate repair procedure.

Did you find and correct the condition?

Go to Step 16

Go to Step 13

11
  1. Visually and physically inspect for the following conditions:
    2. • Evidence of damage or tampering in the throttle body, and the throttle lever screw, if equipped
    • Excess deposits on the throttle plate
    • Excess deposits in the throttle bore
  2. Inspect the following components of the air intake system for restrictions:
    3. • A possible collapsed or clogged air intake duct--Inspect before and after the air filter element
    • A restricted air filter element
    • Restriction at the throttle body intake screen, if equipped
  3. Remove the IAC valve. Refer to Idle Air Control Valve Replacement
  4. Inspect for excess deposits on the IAC valve pintle and in the IAC valve passage.
  5. Repair any of the above conditions as necessary. Refer to the appropriate repair procedure.

Did you find and correct the condition?

Go to Step 16

Go to Step 13

12

Inspect for a poor connection at the PCM harness connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 15

13

Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 14

14

Replace the IAC valve. Refer to Idle Air Control Valve Replacement .

Did you complete the replacement?

Go to Step 16

--

15

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 16

--

16
  1. Use the scan tool in order to clear any DTCs that may have set.
  2. Turn OFF the ignition for 30 seconds.
  3. Operate the vehicle within conditions to verify the repair.

Did you verify the repair?

Go to Step 17

Go to Step 2

17

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

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

Go to Diagnostic Trouble Code (DTC) List

System OK