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Home › Pontiac › 1987 › Firebird Formula, 5.7 8 › Repair and Diagnosis › Engine Performance › System › 2.8L PFI Tests W/Codes › Component Check (C Charts) › Chart C2C - Idle Air Control › Notes

Chart C2C - Idle Air Control: Notes

The ECM will control engine idle speed by moving the Idle Air Control (IAC) valve to control airflow around the throttle plate. It does this by sending voltage pulses to the proper motor winding for each internal IAC motor. This will cause the motor shaft and valve to move in or out of the motor a given distance for each pulse received. The IAC position is measured in counts. Zero counts is a fully extended valve (no airflow), 255 counts is a fully retracted valve (maximum airflow).

To increase idle speed, the ECM will send a signal to retract the IAC valve and allow more air to flow through the idle air passage and by-pass the throttle plate. This will increase the IAC counts. To decrease idle speed the ECM will send a signal to extend the IAC valve and reduce airflow through the idle air passage around the throttle plate. This will reduce the IAC counts.

Each time the engine is started and then the ignition is turned off, the ECM will reset the IAC valve. This is done by sending enough extend pulses to seat the valve (to zero counts). The fully seated valve is the ECM reference zero. A given number of counts are then calculated by the ECM. This is how the ECM knows what the motor position is for a given idle speed.

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. Continue with test even if engine will not idle. If idle is too low, "Scan" tester will display 80 or more counts. If idle is high it will display zero counts. Occasionally, an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up or down. If this condition exists, disconnect IAC motor. If the condition is unchanged, the IAC is not at fault. There is a system problem. Proceed to DIAGNOSTIC AIDS.
  2. When the engine was stopped, the IAC valve retracted to a fixed "Park" position for increased airflow and idle speed during the next engine start. A "Scan" tester will display 40 or more counts.
  3. Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
  4. There is a remote possibility that one of the circuits is shorted to voltage which would have been indicated by a steady light. Disconnect ECM and turn the ignition on. Probe terminals to check for this condition.
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