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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 Idle Air Control (IAC) System Diagnosis

Circuit Description

The idle air control (IAC) valve is located in the throttle body of both the SFI and the MFI Systems. The IAC consists of a movable pintle that is driven by a gear that is attached to an electric motor called a stepper motor. The IAC valve motor is a two-phase bi-polar permanent magnet stepper motor. The motor is capable of highly accurate rotation or movement, every time the polarity of a winding changes. You can see this change in polarity when you observe a test lamp that is connected between ground or B+ and an IAC valve circuit while the PCM is attempting to change engine RPM. The test lamp will flash ON or OFF each time the polarity changes. The PCM does not use a physical sensor to determine IAC pintle position, but uses a predicted number of counts. One count represents one change in polarity, which equals one step of the stepper motor. The PCM counts the steps it has commanded in order to determine IAC pintle position. The PCM uses the IAC valve in order to control the engine idle speed. It does this by changing the pintle position in the idle air passage of the throttle body. This position varies the air flow around the throttle plate when the throttle is closed. In order to determine the desired position of the IAC pintle during idle or during deceleration, the PCM refers to the following inputs:

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

When you turn OFF the ignition after an ignition cycle, the PCM will seat the IAC pintle in the air bypass bore, and then retract the pintle a predetermined amount of counts in order to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as an IAC Reset.

Diagnostic Aids

Inspect for the following conditions:

    • A skewed high TP sensor
    •  A restricted air intake system
    • A blocked IAC passage or a blocked throttle bore
    • A PCV valve that is incorrect, improperly installed, and improperly operating
    • Air intake components that are improperly installed and improperly operating
    • A MAF sensor that is improperly installed or improperly operating, if the vehicle has this equipment
    • A throttle stop screw that has evidence of tampering or damage
    • Evidence of tampering or damage to the following components:
       - The throttle plate
       - The throttle shaft
       - The throttle linkage
       - The cruise control linkage, if the vehicle has this equipment
    • Excessive deposits in the following components:
       - The IAC passage
       - The IAC pintle
       - The throttle bore
       - The throttle plate
    • Vacuum leaks
    • Excessive load on the engine, from the following components:
       - The transmission
       - The power steering
       - The alternator
    • A high or unstable idle condition can be caused by a non-IAC system problem that the IAC valve cannot overcome. Refer to Symptoms - Engine Controls .
    • A low or unstable idle condition can be caused by a non-IAC system problem that the IAC valve cannot be overcome. Refer to Symptoms - Engine Controls .
    • If the problem 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 Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic Starting Point - Engine Controls

2

Important:  Ensure that the engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 100 RPM of the commanded RPM.

  1. Set the parking brake, and block the drive wheels.
  2. Install a scan tool.
  3. Start the engine.
  4. Turn OFF all of the accessories.
  5. With the scan tool RPM control function, slowly increment engine speed to 1,800 RPM, and to 600 RPM, and to 1,800 RPM.
  6. Exit the RPM control function.

Did the engine speed stabilize within 100 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 IAC motor driverJ 37088-A 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-A.
  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, and 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, and never flashing, while the IAC Counts were 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, and 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 for a 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
    • Inspect for the following conditions:
       - Evidence of throttle body damage and tampering
       - A TP sensor signal that is skewed
       - Evidence of throttle lever screw tampering, if the vehicle has this equipment
       - Vacuum leaks
       - A PCV valve and hose that are faulty, or incorrectly-installed hose
       - Evidence of throttle shaft binding
       - Evidence of throttle linkage binding, or Cruise Control linkage binding, if the vehicle has this equipment
    • Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
    • Inspect for debris in the IAC passage.
    • Inspect for excessive deposits on the throttle plate.
    • Inspect for excessive deposits in the throttle bore.
    • Inspect for excessive deposits on IAC valve pintle.
    • 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. Inspect for the following conditions:
    2. • Evidence of throttle body damage and tampering and evidence of throttle lever screw tampering, if the vehicle has this equipment
    • A restricted air intake system. Inspect for the following conditions:
       - A collapsed air intake duct, in front of and behind the air filter element
       - A clogged air intake duct, in front of and behind the air filter element
       - A restricted air filter element
       - A restricted throttle body intake screen, if the vehicle has this equipment
    • Excessive deposits on the throttle plate
    • Excessive deposits in the throttle bore
    • Remove the IAC valve and inspect for excessive deposits on the IAC valve pintle and in the IAC valve passage.
  2. 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 Connector Repairs 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 Connector Repairs 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, Capture Info.

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

Go to Diagnostic Trouble Code (DTC) List

System OK