Chart C-2B Idle Air Control (IAC) System Check

Chart C-2B:

Idle Air Control (IAC) Wiring Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) controls idle RPM with the Idle Air Control (IAC) valve. To increase idle RPM, the PCM retracts the IAC pintle from the seat, allowing more air to bypass the throttle plate. To decrease RPM, it extends the IAC pintle valve in towards the seat, reducing air flow through the IAC valve port in the throttle body. A Tech 1 will read the PCM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air allowed (lower idle).

CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. The Tech 1 is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change.
2. This test checks all wires, connectors, and PCM.
3. This test checks for shorted or open IAC valve coils.

DIAGNOSTIC AIDS
A slow, unstable idle may be caused by a system problem that cannot be overcome by the IAC.

NOTICE:
If for some reason the air intake to the engine is restricted, the PCM will attempt to compensate by backing out the IAC pintle to maintain desired idle. This may result in IAC counts above 60. If the engine has another source of air to the intake, the PCM will compensate by extending the IAC pintle - scan IAC counts may reach zero as the PCM tries to maintain desired idle.

System lean (high air / fuel ratio)
Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help. May set DTC P0131.
Tech 1 will read Heated Oxygen Sensor output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel.

System rich (low air / fuel ratio)
Idle speed too low. Scan IAC counts are usually above 80. System obviously rich and may exhibit black smoke exhaust.
Tech 1 will read Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (0.8 volt).

Check:
- High fuel pressure.
- Injector leaking or sticking.

Throttle Body
Remove IAC and inspect bore for foreign material or evidence of IAC pintle dragging the bore.

Refer to "Rough, Unstable, Incorrect Idle or Stalling," in "Diagnosis by Symptom". Rough, Unstable, or Incorrect Idle, Stalling