Regal V6-191 3.1L (1991)
Chart C-2B Wiring Diagram
CIRCUIT DESCRIPTION:
The ECM controls engine idle speed with the IAC valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its seat, allowing
more air to pass by 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 ECM commands to the IAC valve in counts. The higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air
is allowed to bypass (lower idle).
TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.
1.
The scanner rpm mode is used to extend and retract the IAC valve. The valve should move smoothly within the specified range. If the idle speed
is commanded to low (IAC extended) the engine may stall. This may be normal and would not indicate a problem. Retracting the IAC beyond it's
control range will cause a delay before the rpm's start dropping. This too is normal.
2.
This test uses the Scan tool to command the IAC controlled idle speed. The ECM issues commands to obtain commanded idle speed. The node
lights should each flash a green and red light to indicate the circuit is good as the ECM issues commands. While the sequence of commands is not
important, if the either light is "OFF" or does not flash red and green, check the circuits for faults begining with poor terminal contacts.
DIAGNOSTIC AIDS:
^
A slow, unstable idle may be caused by a system problem that cannot be overcome by the IAC. Out of control range IAC "SCAN" tool counts will
be above 60 if idle is to low, and zero counts if idle is to high. The following checks should be made to repair a non-IAC system problem.
^
Vacuum Leak (High Idle) - If idle is to high, stop the engine. Fully extend the IAC tester. Start engine. If idle speed is above 800 rpm , locate
and correct vacuum leak including PCV system. Also check for binding of throttle blade or linkage.
^
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 Electrical Connections - IAC Valve electrical connections should be checked for proper contact.
^
PCV Valve - An incorrect or faulty PCV valve may result in an incorrect idle speed.
^
Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/STALLS-ROUGH,UNSTABLE OR INCORRECT IDLE.
