Idle Air Control (IAC) System
Idle Air Control Valve Circuit:
C-2C Idle Air Control Valve Diagnostic Chart:
CIRCUIT DESCRIPTION:
The ECM controls engine idle 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 allowed to bypass (lower idle).
TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.
1. The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by engine rpm change. If no change occurs, the valve can be re-tested when removed from the throttle body.
2. This step checks the quality of the IAC movement in step 1. Between 700 and about 1500 rpm, the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 rpm), it may take many flashes in the extend position before engine speed begins to drop. This is normal in some instances, fully expanding the IAC closes the passage causing engine stall.
3. Steps 1 and 2 verified proper IAC operation. This step checks IAC circuits. the IAC driver and "node" light should flash red and green while the IAC is cycled. While the sequence of color is not important, if either light is OFF or does not flash red and green, check the circuit 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 60 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 PCV system. Also check for binding or throttle blades or linkage.
^ System too lean (high air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down and disconnecting IAC does not help. Code 44 may be set. Scan O2 voltage will be less than 300 mV (0.3 volts). 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 80. System is obviously rich and may exhibit BLACK smoke exhaust. Scan tool O2 voltage will be fixed above 800 mV (.8 volt). Check for a high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensor will Scan an O2 voltage slow to respond.
^ Throttle Body - Remove IAC and inspect bore for foreign material.
^ IAC valve - Electrical connections IAC valve connections should be care fully checked for proper contact.
^ PCV valve - An incorrect or faulty PCV valve may result in an incorrect idle speed.
^ If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.