Idle Air Control (IAC) System Diagnosis. Buick GL7 (Export) 2000

For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) controls the engine idle speed by adjusting the position of the idle air control (IAC) motor pintle. The IAC is a bi-directional motor driven by 2 coils. The PCM pulses current to the IAC coils in steps (counts) to extend the IAC pintle into a passage in the throttle body in order to decrease the air flow. The PCM reverses the current pulses to retract the pintle and increase the air flow. This method allows a highly accurate control of the idle speed and a quick response to changes in the engine load.

### Diagnostic Aids

Inspect for the following conditions:

• | A poor connection at the PCM or IAC motor. Inspect the harness connectors for the following conditions:

- | Backed-out terminals

- | Improper mating

- | Broken locks

- | Improperly formed or damaged terminals

- | Poor terminal-to-wire connections

• | A damaged harness. Inspect the wiring harness for damage.

• | A restricted air intake system. Inspect for a collapsed air intake duct, a restricted air filter element, or any foreign objects blocking the air intake system.

• | The throttle body. Inspect for objects blocking the IAC passage or the throttle bore, any excessive deposits in the IAC passage and on the IAC pintle, and any excessive deposits in the throttle bore and on the throttle plate.

| Check for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which will not allow the IAC pintle to fully extend.

• | Any vacuum leaks. Inspect for any condition that causes a vacuum leak, such as disconnected or damaged hoses, leaks at the EGR valve and EGR pipe-to-intake manifold, leaks at the throttle body, a faulty or incorrectly installed PCV valve, leaks at the intake manifold, a disconnected brake booster hose, etc.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

### Test Description

The number below refers to the step number on the diagnostic table.

1. This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read-only memory (EEPROM). If the PCM is replaced, the new PCM must be programmed.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check

2

1. Start the engine.
2. Turn all accessories OFF (A/C, rear defroster etc.).
3. Using a scan tool, command the engine RPM up to 1500, down to 650, and then up to 1500 while monitoring the engine speed on the scan tool.

Does the engine speed remain within the specified value of the desired idle for each RPM command?

+/- 50 RPM

System OK

Go to Step 3

3

1. Disconnect the idle air control (IAC) valve.
2. Install a J 37027-A IAC system motor analyzer.
3. With the engine running, command the RPM up to 1500, down to 650, and then up to 1500 while observing the IAC system monitor node lights.

Does each node light cycle red and green, and never off?

--

Go to Step 5

Go to Step 4

4

1. Check the following circuits for an open, a short to voltage, or a short to ground:

• | The IAC A Low circuit

• | The IAC A High circuit

• | The IAC B Low circuit

• | The IAC B High circuit

2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 12

Go to Step 10

5 | Was a low idle symptom or a rich condition noticed? | -- | Go to Step 6 | Go to Step 7

6

1. Visually and physically inspect for the following conditions:

• | Any throttle body tampering

• | A restricted air intake system

• | A collapsed air intake duct

• | A restricted air filter element

• | Any foreign objects blocking the air intake system

• | Inspect the throttle body for objects blocking the IAC passage or the throttle bore, any excessive deposits in the IAC passage and on the IAC pintle, or any excessive deposits in the throttle bore and on the throttle plate.

2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 11

Go to Step 8

7

1. Visually and physically inspect for the following conditions:

• | Any vacuum leaks. Refer to Emission Hose Routing Diagram .

• | Any throttle body tampering

• | A binding throttle plate or throttle shaft

• | A misadjusted or binding accelerator or cruise control cable. Refer to Accelerator Control Cable Replacement .

• | A faulty, missing, or incorrectly installed PCV valve. Refer to Crankcase Ventilation System Description .

2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 11

Go to Step 8

8

1. Check for a poor connection at the IAC harness connector.
2. If a problem is found, replace any faulty terminals as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 12

Go to Step 9

9 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Is the action complete? | -- | Go to Step 12 | --

10

1. Check for a poor connection at the PCM harness connector.
2. If a problem is found, replace any faulty terminals as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go to Step 12

Go to Step 11

11 | Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 12 | --

12

1. Clear the DTCs.
2. Start the engine.
3. Turn all accessories OFF (A/C, rear defroster etc.).
4. Using a scan tool, command the RPM up to 1500, down to 650, and then up to 1500 while monitoring the engine speed on the scan tool.

Does engine speed remain within the specified value of the desired idle for each RPM command?

+/- 50 RPM

System OK

Go to Step 3