How do I adjust the IAC on a MerCruiser 8.2 MPI Gen VI?

You cannot manually adjust the idle air control valve on a MerCruiser 8.2 MPI: where a separate IAC is fitted, the engine computer operates it to maintain the programmed idle speed. Identify the engine by its serial number first, because an older Gen VI-based 502 MPI and a later 8.2 MAG can have different throttle and idle-control arrangements.

Identify your 8.2 engine

Check the serial-number label and engine identification before ordering parts or following an adjustment procedure. "Gen V1" usually means Gen VI, the Chevrolet big-block generation; it does not identify the MerCruiser injection system or calibration.

With the engine stopped, look at the throttle body beneath the flame arrestor:

• Cable-operated throttle: a mechanical cable moves the throttle lever. Depending on the injection package, a separate IAC controls air passing around the closed throttle plate.
• Electronic throttle: an electric actuator moves the throttle plate. That actuator can also control idle, so do not assume there is a separate, adjustable IAC.

The serial number determines the correct parts diagram, service procedure and idle specification. Do not apply a 5.0 or 5.7 MPI IAC procedure simply because the components look similar.

Check the idle fault

Leave the throttle-stop screw alone. It sets the throttle's base position. Turning it to cure stalling or high idle can upset the relationship between throttle position, airflow and the computer's idle control.

Before replacing parts, establish whether the engine stalls, hunts, idles too fast or only misbehaves when engaging gear. Note whether the fault occurs cold, fully warm or both.

1. Check battery connections, engine earths and charging operation. Poor electrical supply can interfere with actuators and sensor signals.
2. On a cable-throttle installation, check that the lever returns freely to its proper idle position and the cable is not holding it open. Follow the correct engine procedure for any cable adjustment.
3. Inspect intake hoses, vacuum connections and manifold joints for leaks. Air entering downstream of the throttle can cause high idle even when the IAC is commanded closed.
4. Check the flame arrestor and inspect the throttle bore and idle-air passages for contamination.
5. Where an IAC is fitted, inspect its connector for corrosion, loose terminals and damaged wiring.

Ventilate the engine compartment before starting, provide the correct cooling-water supply and keep hands clear of belts and throttle linkages.