Idle Air Control (IAC) System Diagnosis. Oldsmobile Intrigue 1999

For 1990-2009 cars only

Makes 🢒 Oldsmobile 🢒 1999 🢒 Intrigue 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Idle Air Control (IAC) System Diagnosis

Engine Controls Components
MAF, IAC, EGR, and EVAP
OBD II Symbol Description Notice

Circuit Description

The powertrain control module (PCM) controls 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) in order to extend the IAC pintle into a passage in the throttle body to decrease air flow. The PCM reverses the current pulses to retract the pintle, increasing the air flow. This method allows highly accurate control of the idle speed and quick response to changes in engine load.

Diagnostic Aids

Inspect for the following conditions:

•	A faulty connection at the PCM or the IAC motor. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.

•	Inspect the wiring harness for damage.

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

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

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

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

Reviewing the Fail 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(s) below refer to the step number(s) on the Diagnostic Table.

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

Idle Air Control System Check

Step

Action

Value(s)

Yes

Did you perform the On-Board Diagnostic (OBD) System Check?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Start the engine.
Turn all accessories off (A/C, rear defroster, etc.).
Use a scan tool in order to command the RPM up to 1500, down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool.

Does the engine speed remain within the specified range of Desired Idle for each RPM command?

± 50 rpm

System OK

Go to Step 3

Disconnect the idle air control (IAC) sensor.
Install a J 37027-A IAC system monitor.
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 (never off)?

Go to Step 5

Go to Step 4

Test 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

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

Was a low idle symptom or rich condition noticed?

Go to Step 6

Go to Step 7

Visually/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.

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

If a problem is found, repair as necessary.

Was a problem found?

Go to Step 11

Go to Step 8

Visually/physically inspect for the following conditions:

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

•	Any throttle body tampering.

•	The throttle plate or throttle shaft for binding.

•	The accelerator and cruise control cables for misadjustment or for binding. Refer to Accelerator Control Cable Replacement .

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

If a problem is found, repair as necessary.

Was a problem found?

Go to Step 11

Go to Step 8

Inspect for a faulty connection at the IAC harness connector.
If a problem is found, replace the terminals. Refer to Connector Repairs in Wiring Systems.

Was a problem found?

Go to Step 12

Go to Step 9

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

Did you complete the repair?

Go to Step 12

Inspect for a faulty connection at the PCM harness connector.
If a problem is found, replace the faulty terminals as necessary. Refer to Connector Repairs in Wiring Systems.

Was a problem found?

Go to Step 12

Go to Step 11

Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming .

Replace the PCM.

Did you complete the repair?

Go to Step 12

Clear any DTCs.
Start the engine.
Turn all accessories off (A/C, rear defroster, etc.).
Use the scan tool in order to command the RPM up to 1500, down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool.

Does the engine speed remain within the specified range of Desired Idle for each RPM command?

± 50 rpm

System OK

Go to Step 3