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

For 1990-2009 cars only

Makes 🢒 Buick 🢒 1999 🢒 Century 🢒 Engine 🢒 Engine Controls - 3.1L 🢒 Idle Air Control (IAC) System Diagnosis

Engine Controls Components
Cell 20: Engine Data Sensors-EGR, EVAP Purge, EVAP Vent and MAF
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:

•	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.

Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

•	Poor terminal connection.

Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Intermittents and Poor Connections Diagnosis , and Connector Repairs Wiring Systems.

•	Damaged harness.

Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems.

•	Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.

Test Description

The number below refers to the step number 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 .

Step

Action

Value(s)

Yes

Was the Powertrain OBD System Check performed?

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.)
Using a scan tool, command RPM up to 1500, 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 Motor Analyzer.
With the engine running, command RPM up to 1500, down to 650, and then up to 1500 while observing the J 37027-A IAC System Motor Analyzer lights.

Does J 37027-A IAC System Motor Analyzer light cycle red and green (never off)?

Go to Step 5

Go to Step 4

Test 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/physically inspect for the following conditions:

•	Throttle body tampering.

•	Restricted air intake system.

•	Collapsed air intake duct.

•	Restricted air filter element.

•	Foreign objects blocking the air intake system.

•	Throttle body. Test 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 11

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 .

•	Faulty, missing, or incorrectly installed PCV valve. Refer to Crankcase Ventilation System Description .

If a problem is found, repair as necessary.

Was a problem found?

Go to Step 11

Go to Step 8

Test for a poor connection at the IAC harness connector.
If a problem is found, replace faulty terminals as necessary. Refer to Testing for Continuity Intermittents and Poor Connections Diagnosis Repairing Connector Terminals and Connector Repairs .

Was a problem found?

Go to Step 12

Go to Step 9

Replace the IAC valve. Refer to IAC Valve Replacement .

Is the action complete?

Go to Step 12

Test for a poor connection at the PCM harness connector.
If a problem is found, replace faulty terminals as necessary. Refer to Testing for Continuity , Intermittents and Poor Connections Diagnosis , Repairing Connector Terminals , and Connector Repairs .

Was a problem found?

Go to Step 12

Go to Step 11

Improtant: : The replacement PCM must be programmed.

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

Is the action complete?

Go to Step 12

Clear DTCs.
Start the engine.
All accessories off (A/C, Rear defroster etc.)
Using a scan tool, command RPM up to 1500, 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