Rodeo S 2WD L4-2.2L (1999)
Idle Air Control Valve: Testing and Inspection
Idle Air Control (IAC) System Check
Idle Air Control (IAC) System Check
Circuit Description
The powertrain control module (PCM) controls engine idle speed with the idle air control (IAC) valve. To increase idle speed, the PCM 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 PCM commands to the IAC valve in counts. Higher counts indicate more air bypass (higher
idle). Lower counts indicate less air is allowed to bypass (lower idle).
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:
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Vacuum leak (high idle) If idle is too high, stop the engine. Fully extend (low) IAC with the AC motor analyzer J 39027-A. Start the engine. If idle
speed is above 800 RPM, locate and correct the vacuum leak, including the PCV system. Check for binding of the throttle blade or linkage.
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Lean heated oxygen sensor signal (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. Diagnostic trouble codes P0131, P0151, P0171, or P0174 may be set. Scan Tool oxygen (02) voltage
will be less than 100 mV (0.1 V). Check for low regulated fuel pressure, water in fuel, or a restricted injector.
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Rich heated oxygen sensor signal (low air/fuel ratio) - The idle speed will be too low. Scan Tool IAC counts will usually be above 80. The system
is obviously rich and may exhibit black smoke in the exhaust.
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Scan Tool 02 voltage will be fixed at about 750 mV (0.75 V). Check for high fuel pressure, or a leaking or sticking injector. A
silicon-contaminated heated oxygen sensor will show an 02 voltage slow to respond on the Scan Tool.
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Throttle body - Remove the IAC valve and inspect the bore for foreign material.
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A/C valve electrical connections - IAC valve connections should be carefully checked for proper contact.
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PCV valve - An incorrect or faulty PCV valve may result in an incorrect idle speed. Refer to Diagnosis, Rough Idle, Stalling. Refer Diagnosis By
Symptom.
If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck the connections and valve terminal
resistance, or replace the IAC.
