Idle Air Control (IAC) System Diagnosis

Diagnostic Chart (Part 1 Of 3):

Diagnostic Chart (Part 2 Of 3):

Diagnostic Chart (Part 3 Of 3):

Schematic:

CIRCUIT DESCRIPTION
The control module controls idle speed with an Idle Air Control (IAC) valve. The desired RPM is based on sensor inputs and the actual engine RPM. Four circuits are used in order to control the IAC valve, allowing a precise amount of air into the intake manifold. In order to increase the idle RPM, the control module will command the valve pintle to move away from its seat, allowing more air to bypass the throttle plate. To decrease RPM, the control module commands the IAC pintle to move closer to its seat, causing a lower airflow to bypass the throttle plate. A scan tool will display the control module commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate. Lower counts indicate that less air is commanded by the control module.

IAC Valve Reset Procedure Without A Scan Tool
1. Turn ON the ignition, with the engine OFF for 5 seconds.
2. Turn OFF the ignition for 10 seconds.
3. Start the engine and check for proper operation.

IAC Valve Reset Procedure With Scan Tool
1. Turn ON the ignition, with the engine OFF.
2. Install a scan tool.
3. Select F4 Misc. Tests.
4. Select F3 IAC System.
5. Select F1 IAC Reset.
6. Use up arrow to reset IAC.

DIAGNOSTIC AIDS
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. The following checks should be made to repair a non-IAC system problem.

IMPORTANT If for some reason the air intake to the engine is restricted, the VCM will attempt to compensate by backing out the IAC pintle to maintain desired idle. If the engine has another source of air to the intake, the VCM will compensate by extending the IAC pintle, scan IAC counts may reach zero as the VCM tries to maintain desired idle.

^ Vacuum Leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) the IAC with tester.

Start the engine. If the idle speed is above 800 RPM, locate and correct a vacuum leak including the crankcase ventilation system. Also, check for binding of the throttle blade or linkage.
^ System lean (high air/fuel ratio) - The idle speed may be too high or too low. The engine speed may vary up and down, and disconnecting the IAC does not help. May set DTC P0171. The scan tool will read a Heated Oxygen Sensor (HO2S) output of less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in the fuel.
^ System rich (low air/fuel ratio) - Idle speed too low. Scan tool IAC counts are usually above 80. The system is obviously rich and may exhibit black smoke exhaust. May set DTC P0172. The scan tool will read a Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (8 volt). Check for high fuel pressure, or leaking or sticking injectors. Silicone contaminated HO2S scan voltage will be slow to respond. Throttle body - Remove the IAC valve and inspect the bore for foreign material.
^ Crankcase ventilation valve - An incorrect or faulty crankcase ventilation valve may result in an incorrect idle speed. Refer to Crankcase Ventilation System Diagnosis. Component Tests and General Diagnostics

Refer to Symptoms. Symptom Related Diagnostic Procedures
^ A/C Compressor - Refer to A/C Compressor Control Circuit Diagnosis if circuit is shorted to ground. If the relay is damaged, an idle problem may exist. Component Tests and General Diagnostics
^ If intermittent reduced driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck the connections, the valve terminal resistance or replace the IAC.

An intermittent may be caused by any of the following conditions:
^ A poor connection
^ Rubbed through wire insulation
^ A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagrams.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagrams.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. This test determines if the Vehicle Control Module (VCM) is capable of closing the Idle Air Control (IAC) valve.
3. This test determines if the VCM is capable of opening the IAC valve.
4. Before you test the IAC system, check all engine mechanical related conditions, and repair any conditions.