Idle Up Control Valve: Testing and Inspection

Chart C-2C Idle Air Control System Check:

IAC Circuit:

CIRCUIT DESCRIPTION:
The ECM controls engine idle speed with the IAC valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its seat, allowing more air to bypass the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards its seat, reducing bypass air flow. A scan tool will read the ECM commands to the IAC valve in counts. Higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air is allowed to bypass (lower idle).

TEST DESCRIPTION: Number(s) below refer to circled numbers on the diagnostic chart.

1. The scan tool RPM control mode is used to extend and retract the IAC valve. The valve should move smoothly within the specified range. If the idle speed commanded (IAC extended) too low (below 700 RPM), the engine may stall. This may he normal and would not indicate a problem. Retracting the IAC beyond its controlled range (above 1500 RPM) will cause a delay before the rpm start dropping. This too is normal.

2. This test uses the scan tool to command the IAC controlled idle speed. The ECM issues commands to obtain commanded idle speed. The node lights each should flash red and green to indicate a good circuit as the ECM issues commands. While the sequence of color is not important if either light is "OFF" or does not flash red and green, check the circuits for faults, beginning with poor terminal contacts.

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. Out of control range IAC scan tool counts will be above 50 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem:

^ Vacuum Leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 RPM, locate and correct vacuum leak including crankcase ventilation system. Also check for binding of throttle blade or linkage.

^ Crankcase Ventilation Valve - If a high idle condition exists (800 to 1000 RPM), check for vacuum leaks and proper crankcase ventilation valve operation. All throttle bodies are preset at the factory and do not need adjustment. A missing crankcase ventilation valve or grommet or stuck valve can cause this condition.

^ System Too Lean (High Air/Fuel Ratio) - The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. DTC 44 may be set. Scan oxygen voltage will be less than 300 mV (.3 volt). Check for low regulated fuel pressure, water in the fuel or a restricted injector.

^ System Too Rich (Low Air/Fuel Ratio) - The idle speed will be too low. Scan tool IAC counts will usually be above 50. System is obviously rich and may exhibit black smoke in exhaust. Scan tool oxygen voltage will be fixed above 800 mV (.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated oxygen sensors scan voltage will be slow to respond.

^ Throttle Body - Remove IAC valve and inspect bore for foreign material.

^ IAC Valve Electrical Connections - IAC valve connections should be carefully checked for proper contact.

^ Rough, Unstable, or Incorrect Idle, Stalling

^ If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.