Chart C-2B Idle Air Control (IAC) System Check

Trouble Code 35 Chart:

Idle Air Control:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) controls engine idle speed with the Idle Air Control (IAC) valve. To increase idle speed, the PCM retracts the IAC valve pintle away from the 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 Tech 1 (or equivalent) scan tool, or equivalent, will read the PCM commands to the IAC valve in counts. Higher counts indicate more air bypass (higher idle). The lower the counts indicates less air is allowed to bypass (lower idle).

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. The Tech 1 scan tool 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 RPMs start dropping. This too is normal.
2. This test uses the Tech 1 scan tool to command the IAC controlled idle speed. The PCM issues commands to obtain commanded idle speed. The node lights each should flash red and green to indicate a good circuit as the PCM 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 range IAC counts will be above 60 if idle is too low and zero counts if idle is too high. If idle speed is above 600-700 RPM in drive with an A/T, locate and correct vacuum leak. If RPM is below spec., check for foreign material around throttle plates. 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 IAC with tester. Start engine. If idle speed is above 800 RPM, locate and correct vacuum leak including crankcase ventilation 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. Diagnostic Trouble Code (DTC) 44 may set. The Tech 1 scan tool Heated Oxygen Sensor (HO2S) voltage reading will be less than 300 mV (0.3 volt). Check for low regulated fuel pressure, water in the fuel or a restricted injector or filter.

System Too Rich (low air/fuel ratio)
The idle speed will be too low. Tech 1 (or equivalent scan tool) IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. DTC 45 may set. Tech 1 (or equivalent) scan tool HO2S voltage will be fixed above 800 mV (0.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S will cause sluggish voltage variations on Tech 1 scan tool.

Throttle Body
Remove IAC valve and inspect bore or foreign material.

The IAC Valve Electrical Connections
The IAC valve connections should be carefully checked for proper contact. If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.

Crankcase Ventilation Valve
An incorrect or faulty valve may result in an incorrect idle speed. For further diagnosis OR if above are OK, refer to System Diagnosis/Procedures/ Diagnosis by Symptom/Rough, Unstable or Incorrect Idle, Stalling. Rough, Unstable, or Incorrect Idle, Stalling