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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2005 🢒 LeSabre 🢒 Engine 🢒 Engine Controls - 3.8L (L36 and L67) 🢒 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 IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by 2 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) sets.

Diagnostic Aids

Inspect for the following conditions:

    • High resistance in an IAC valve control circuit
    • The correct positive crankcase ventilation (PCV) valve, properly installed and 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
    • A tampered with or damaged throttle stop screw
    • A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped
    • A skewed high throttle position (TP) sensor
    • Excessive deposits in the IAC passage or on the IAC pintle
    • Excessive deposits in the throttle bore or on the throttle plate
    • Vacuum leaks
    • A high or unstable idle condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls .
    • If the problem is determined to be 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 control circuits to control the IAC valve.

    2.

  2. 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 incrementing.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Powertrain Control Module Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

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 175 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 175 RPM above the Commanded RPM?

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Disconnect the idle air control (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 3 IAC valve control circuits.

Did the test lamp remain ON, never flashing, while the IAC Counts were incrementing 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 incrementing 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 incrementing?

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 powertrain control module (PCM).
  3. Test the circuit where the test lamp remained OFF for the following conditions:
    4. • An open circuit
    • A short to ground
    • A short to another IAC valve control 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. Test the IAC valve control circuit where the test lamp remained illuminated for the following conditions:
    3. • A short to voltage
    • A short to another IAC valve control 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. Inspect for the following:
    2. • Throttle body damage and tampering
    • Skewed throttle position (TP) sensor signal
    •  Throttle lever screw tampering, if equipped
    • Vacuum leaks
    • Faulty, incorrectly installed positive crankcase ventilation (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

Did you find and correct the condition?

Go to Step 18

Go to Step 15

12
  1. Inspect for the following conditions:
    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.

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 Valve Replacement .

Did you complete the replacement?

Go to Step 18

--

17

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

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 - Vehicle in Vehicle DTC Information

System OK