Idle Speed/Throttle Actuator - Electronic: Testing and Inspection

Idle Air Control (IAC) Valve Circuit Diagram:

CIRCUIT DESCRIPTION
The PCM controls idle speed by moving the IAC valve pintle which opens and closes the air passage bypassing the throttle valve. A "SCAN" tool displays the commanded pintle position in counts. The greater the number of counts, the farther open the IAC valve is and the greater the commanded idle speed. IAC counts range from 0 (fully closed) to 255 (fully open.) Normal is between 5 - 50 counts.

TEST DESCRIPTION

Chart C-2B, Idle Air Control Valve Circuit Check:

Numbers below refer to circled numbers on the diagnostic chart.
1. The Tech 1 "SCAN" tool rpm control mode is used to extend and retract the IAC valve pintle. the valve should move smoothly within the specified range. If the idle speed is commanded too low (below 700 rpm) the engine may stall. This is normal and does not necessarily indicate a problem. Retracting the pintle beyond its normal control range (above 1500 rpm) will cause a delay before the engine speed drops when commanded low again. This too is normal.
1. This test the Tech 1 "SCAN" tool to command the controlled idle speed. The PCM commands the IAC valve to obtain the specified idle speed. The node lights should flash red and green, indicating a good circuit. While the sequence of flashes is not important, if either light fails to flash, or does not flash red AND green, check the circuits for faults. Look for poor connections, open circuits or shorts to ground.

DIAGNOSTIC AIDS
An idle speed that is too fast, too slow or unstable can be caused by other conditions that the IAC valve cannot compensate enough for.

NOTE: If the air intake to the engine is restricted, the PCM will attempt to compensate by retracting the IAC valve (increasing IAC counts), opening the air passage to maintain the desired idle speed. IAC counts may increase to greater than 60. If there is a vacuum leak or the throttle plates do not fully close, the PCM will try to compensate by extending the IAC pintle. IAC counts may be reduced to zero (air passage fully closed) as the PCM tries to reduce idle speed to the desired rpm.

ALSO CHECK

^ 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 Code 44. "Scan" and/or voltmeter will read an oxygen sensor output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel. A lean exhaust, with an oxygen sensor output fixed above 800 mV (0.8 volt), will be a contaminated sensor, usually silicone. This may also set Code 45.

^ System rich (Low Air/Fuel Ratio) - Idle speed too low. "Scan" counts usually above 80. System obviously rich and may exhibit black smoke exhaust. "Scan" tool and/or voltmeter will read an oxygen sensor signal fixed above 800 mV (0.8 volt). Check for high fuel pressure or possibly an injector leaking or sticking.

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

^ IAC Valve connections - Check electrical connections carefully for proper contact.

^ PCV Valve - An incorrect or faulty PCV valve may cause incorrect or unstable idle.

^ A/C Compressor or Relay failure - If the A/C control relay drive circuit is shorted to ground, or if the relay is faulty, idle problems may result.