MEFI Fuel Injection
Idle Air Control Valve (IAC)
One of the major benefits of an EFI engine is its ability to maintain proper idle speed. The ECM will immediately compensate for increased load by providing additional fuel, air and spark timing, thereby maintaining proper idle rpm. Adding fuel is easy, the ECM simply holds the injectors open longer. Adding air, however, is a bit more complicated. On GM based EFI systems this is accomplished with a computer controlled motor in the throttle body. This motor is called an Idle Air Control motor (IAC). It is used along with a spark scatter strategy to control idle speed. An IAC is a stepper motor, the ECM controls the movement by sending voltage pulses to the IAC’s 2 coils, thereby causing the motor to take one or more steps in a particular direction. As the IAC opens, more air is allowed to bypass the throttle blade, increasing idle speed. As the IAC is closed, less air bypasses the throttle blade, reducing idle speed. The ECM “tracks” the intended IAC position by counting the step pulses it has sent the motor. Since the IAC does not provide “feedback” to the ECM indicating its actual position the ECM must “reset” the IAC periodically to ensure that it is where the ECM believes it to be. On most MEFI 4 ECM’s, once per key cycle the ECM will “reset” the IAC by driving it in the closed direction for a predetermined length of time followed by driving it in the open direction for another predetermined length of time and then returning it to the desired number of steps. Additionally, each time the the ignition key is turned off, the ECM remains electrically active for several seconds. One of the actions it performs during this period is it “parks” the IAC at a predetermined location in anticipation of the next start. At the next “key on” the ECM will move the IAC either open or closed from the park position based on ECT input. MEFI 4 and 4b engines perform an IAC reset (full closed to full open and back to preset position) the first time 2500 RPM is achieved each key cycle. 95
