Skylark L4-138 2.3L DOHC VIN D MFI QUAD 4 (1989)
Idle Air Control (IAC) Valve Wiring Diagram
CIRCUIT DESCRIPTION:
The ECM controls idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve out, allowing more air to bypass the throttle plate.
To decrease rpm, it moves the IAC valve in, reducing air flow bypassing 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). The lower the counts, the less air allowed (lower idle).
TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.
1.
Continue with test, even if engine will not idle. If idle is too low, "Scan" will display 80 or more counts, or steps. If idle is high, it will display "5"
counts or less. 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 now and idle speed during the next
engine start. A "Scan" will display 100 or more counts.
3.
The IAC valve should extend as the ECM commands idle speed to decrease to desired rpm.
4.
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.
5.
There is a remote possibility that one of the circuits is shorted to voltage, which would have been indicated by a steady light. Disconnect the ECM
and turn the ignition "ON" and probe terminals to check for this condition.
DIAGNOSTIC AIDS:
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A slow unstable idle may be caused by a system problem that cannot be overcome by the IAC.
^
"Scan" counts will be above 60 counts, if idle is too low, and "5" counts or less, if idle is too high. If idle speed is excessively high, check for
and correct any trouble code problem or vacuum leak.
^
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 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 probably be caused by a contaminated sensor (usually
silicone). This may also set a Code 45.
^
System too 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:
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If high fuel pressure
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Injector leaking or sticking
^
Throttle Body - Remove IAC and inspect bore for foreign material or evidence of IAC valve seat or pintle damage.
^
Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/STALLS-ROUGH UNSTABLE OR INCORRECT IDLE..
