Power to Win

KB1032 - Tuning Idle Air Controls (IAC)

IAC Tuning

All of the EFI Technology ECU products are capable of controlling a wide range of idle air devices (IAC's). The most common for marine applications is the 4 wire stepper motor type. By energizing the coils in sequence the stepper motor is able to rotate smoothly and move the idle screw backwards and forwards to satisfy the programmed idle setting. The idle setpoint is usually adjustable as a function of temperature to assist with cold start conditions.

To assist the IAC control in handling large load changes on the engine, such as moving in and out of gear, the ECU has an input to detect these conditions. When moving in or out of gear the IAC is moved at full speed for a programmable period of time to help adjust to the new load condition. In addition the idle setpoint can also be offset by a certain number of rpm.

Idle Speed Constants


  1. Start the Editor with the ECU connected and make sure that the correct map is loaded.
  2. Open the Idle Speed function group and constants window.
  3. Enter the required idle speed values at each of the temperature sites.

    The idle speed setpoint table is usually a function of water temperature. In marine applications where the water temperature is not always a good indicator of the true engine operating condition you can use the optional head temp sensor. (part number 60-110). Contact your EFI representative for availability and further information.

  4. Observe the throttle angle at idle and set the IAC Throttle Limit approx 2 degs higher.
  5. Set the RPM Limit approx 1000 rpm above the highest idle setpoint in your map.
  6. Adjust the Gain settings to obtain the best response of the IAC without inducing oscillation.

    Increasing the gain will speed up the overall response of the IAC control. Using an excessive amount of gain may lead to rpm overshoot and can induce idle speed oscillation. A general rule is to increment the proportional gain by a quarter of the previous value until the output becomes unstable and then back down the gain a small amount.

  7. Set the Kick RPM to the amount you want to offset the idle when the stop switch is grounded.

    If for example you want the engine to idle at 1200 rpm out of gear but want a lower setting such as 800 rpm, when in gear to keep the boat from moving too fast at idle enter -400 as the rpm offset which will be applied when the gear detection switch is grounded. When the switch is released the idle setpoint will return to the default table setting.


If you suspect that the IAC is not functioning as required it may be because the motor is either fully open or closed and cannot make any further movement to adjust the idle speed. In this situation it is necessary to move the mechanical idle stop to allow the IAC to have some range of motion to control the speed.

An unstable idle or excessive oscillation is usually asscociated with using too much gain. Try reducing the gain settings in small steps until the idle becomes smooth. If you are running closed loop lambda this can amplify oscillation of the idle speed. In this situation try temporarily turning off the lambda control while tuning the IAC.


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