Idle Speed/Throttle Actuator - Electronic: Testing and Inspection

Circuit Description:
The Engine Control Module (ECM) controls idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve out, allowing more air to pass by the throttle plate. To decrease rpm, it moves the IAC valve in, reducing air flow by the throttle plate. A "Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle).

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Continue with test, even if engine will not idle. If idle is too low, "Scan" tool will display 80 or more counts, or steps. If idle is high, it will display "0" counts. Occasionally, an erratic or unstable idle may occur. Engine speed may vary 200 rpm, or more, up and down. disconnect IAC. If the condition is unchanged, the IAC is not at fault.
2. When the engine was stopped, the IAC valve retracted (more air) to a fixed "park" position for increased air flow and idle speed during the next engine start. A "Scan" tool will display 100 or more counts. When performing this test, immediately note rpm on start up, because on a warm engine, the rpm will decrease rapidly.
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" and probe terminals to check for this condition.

Diagnostic Aids:
A slow unstable, idle may be caused by a system problem that cannot be overcome by the IAC. "Scan" tool counts will be above 80 counts, if engine speed is too low, and "0" counts, if engine speed is too high.
If idle is too high, stop engine. Ignition "ON". Ground diagnostic terminal. Wait 30 seconds for IAC to seat, then disconnect IAC. Unground diagnostic terminal and start engine. If idle speed is above 450 rpm in drive, locate and correct vacuum leak.
If rpm is less than 450 rpm, refer to "Rough, Unstable or Incorrect Idle", in "Symptoms".
- System too 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 Diagnostic Trouble Code (DTC) 44. "Scan" tool and/or voltmeter will read an oxygen sensor (02S) output less than 300 mV (.3 volt). Check for low regulated fuel pressure or water in fuel. A lean exhaust with an 02S output fixed above 800 mV (.8 volt), will be a contaminated sensor, usually silicone. This may also set a DTC 45.
- System too rich (Low Air/Fuel Ratio). Idle speed too low. "Scan" tool counts usually above 80. System obviously rich and may exhibit black smoke exhaust. "Scan" tool and/or voltmeter will read an 02S signal fixed above 800 mV (.8 volt)
Check:
- High fuel pressure.
- Injector leaking or sticking.
- Throttle Body. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.
- If above are all OK, refer to "Rough, Unstable, Incorrect Idle or Stalling", in "Symptoms".




ECM : Engine Control Module
A/C : Air Conditioning
CLEAR DIAGNOSTIC TROUBLE CODES AND CONFIRM "CLOSED LOOP" OPERATION AND NO "CHECK ENGINE" MALFUNCTION INDICATOR LAMP.