Idle Air Control (IAC) System Diagnosis. Buick LeSabre 1999

For 1990-2009 cars only

Makes 🢒 Buick 🢒 1999 🢒 LeSabre 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Idle Air Control (IAC) System Diagnosis

Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The 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 two coils. The PCM pulses current to the IAC coils in steps (counts) 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 air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load.

Diagnostic Aids

Check for the following conditions:

•	Poor connection at PCM or IAC valve. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

•	Damaged harness. Inspect the wiring harness for damage.

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

•	Throttle body. Check 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 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.

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

•	Missing oil filler cap.

•	Missing oil level indicator.

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 numbers below refer to the step numbers 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 being replaced, the new PCM must be programmed. Refer to PCM Replacement/Programming .

Idle Air Control System Check

Step

Action

Value(s)

Yes

Was the On-Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

Start the engine.
All accessories OFF (A/C, Rear defroster etc.)
Command RPM up to 1500 using a scan tool, then down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool.

Does Engine Speed remain within the specified value of Desired Idle for each RPM command?

± 50 RPM

System OK

Go to Step 3

Disconnect the IAC.
Install a J 37027-A IAC System Monitor.
Command RPM up to 1500, down to 650 with the engine running, 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

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

•	IAC A Low.

•	IAC A High.

•	IAC B Low.

•	IAC B High.

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

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 and physically inspect for the following conditions:

•	Throttle body tampering (throttle lever stop screw turned).

•	Restricted air intake system.

•	Collapsed air intake duct.

•	Restricted air filter element.

•	Foreign objects blocking the air intake system.

•	Throttle body. Check 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 on the throttle plate.

If a problem is found, repair as necessary.

Was a problem found?

Go to Step 12

Go to Step 8

Visually/physically inspect for the following conditions:

•	Vacuum leaks. Refer to Emission Hose Routing Diagram .

•	Throttle body tampering.

•	Throttle Plate or throttle shaft for binding.

•	Accelerator and Cruise Control cables for being misadjusted or for binding. Refer to Accelerator Control Cable Replacement .

•	Malfunctioning, 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 12

Go to Step 8

Check for a poor connection at the IAC harness connector.
If a problem is found, replace loose terminals as necessary. Refer to Repairing Connector Terminals .

Was a problem found?

Go to Step 12

Go to Step 9

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

Is the action complete?

Go to Step 12

Check for a poor connection at the PCM harness connector.
If a problem is found, replace loose terminals as necessary. Refer to Repairing Connector Terminals .

Was a problem found?

Go to Step 12

Go to Step 11

Important:: The replacement PCM must be reprogrammed.

Replace the PCM. Refer to Powertrain Control Module Replacement/Programming .

Is the action complete?

Go to Step 12

Clear the DTCs.
Start the engine.
All accessories OFF (A/C, Rear defroster etc.)
Command RPM up to 1500 using a scan tool, down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool.

Does Engine Speed remain within the specified value of Desired Idle for each RPM command?

± 50 RPM

System OK

Go to Step 3