Powertrain Controls - ECM/PCM

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 bypass 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. Higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air is allowed to bypass (lower idle). Code 35 will set when the closed throttle engine speed is 225 rpm above or below the desired (commanded) idle speed for 20 seconds. Review the general description of the IAC operation in "Fuel Metering System."

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. The Tech 1 rpm control 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 (IAC extended) too low (below 700 rpm), the engine may stall. This may be normal and would not indicate a problem. Retracting the IAC beyond its controlled range (above 1500 rpm) will cause a delay before the rpm's start dropping. This too is normal.
2. This test uses the Tech 1 to command the IAC controlled idle speed. The ECM issues commands to obtain commended idle speed. The node lights each should flash red and green to indicate a good circuit as the ECM issues commands. While the sequence of color is not important if either light is "OFF" or does not flash red and green check the circuits for faults, beginning with poor terminal contacts.

DIAGNOSTIC AIDS
A slow, unstable, or fast idle may be caused by a non--IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC "Scan" tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem:
^ Vacuum Leak (High Idle).
If idle is too high, stop the engine. Fully extend (low) IAC with 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 too lean (High Air/Fuel Ratio).
The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. Code 44 may be set. "Scan O2 voltage will be less than 300 mV (0.3 volts). Check for low regulated fuel pressure, water in the fuel or a restricted injector.
^ System too rich (Low Air/Fuel Ratio).
The idle speed will be too low. "Scan" tool IAC Counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust.
"Scan" tool O2 voltage will be fixed above 800 mV (0.8 volt).
Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 "Scan" voltage will be slow to respond.
^ Throttle Body
Remove IAC valve and inspect bore for foreign material.
^ IAC Valve Electrical Connections
IAC valve connections should be carefully checked for proper contact.
^ PCV Valve
An incorrect or faulty PCV valve may result in an incorrect idle speed.
^ Refer to "Rough, Unstable, Incorrect Idle or Stalling" in "Symptoms." Rough, Unstable, or Incorrect Idle, Stalling
^ If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.