Code 35, Idle Air Control (IAC)
The ECM controls engine idle speed by moving the Idle Air Control (IAC) valve to control airflow around throttle valve plate. It does this by sending voltage pulses called "counts" or "steps" to proper motor winding. The motor shaft and valve move a given distance for each pulse received.
- To increase idle speed, the ECM sends enough counts to retract IAC valve and increase airflow until idle speed reaches proper RPM.
- To decrease idle speed, the ECM sends enough counts to extend IAC valve and reduce airflow. This will reduce ECM counts.
The ECM "learns" to correct valve position to maintain proper idle RPM. If the "learned" IAC valve is correct, the ECM will command a reset. The reset will occur after the next engine start and vehicle speed is greater than 35-45 MPH.
The following are diagnostic aids:
- Park/Neutral switch (auto. trans. only). If the ECM thinks the vehicle is always in Neutral, idle speed will not be correct in Drive range. See CHART C-1A.
- A leaking injector will cause poor idle quality due to excess fuel.
- A sticking injector causing too lean or too rich condition can cause a poor idle condition. A Code 44 or Code 45 may be stored.
- A sticking throttle shaft or binding linkage causes a high TPS voltage (open throttle indication). The ECM does not recognize a closed throttle and will not control idle. Monitor TPS voltage, voltage should read less than 1.2 volts with throttle closed.
- EGR on while idling will cause roughness, stalling and hard starting. See CHART C-7A.
- Battery cables and ground straps should be clean and secure, erratic voltage will cause IAC to change its position resulting in poor idle quality.
- ECM should compensate for power steering loads. Loss of this signal will be most noticeable when parking and steering loads are high.
- IAC valve will not move if system voltage is below 9 or greater than 17.8 volts.