How to Test a Mercruiser IAC

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The IAC valve of an engine helps control your vehicle's idle speed, or the rpm of the engine when the vehicle is idling. If you are unsure about the position of the IAC valve of your Mercruiser outboard engine, you can test it using a scanner tool.

The scanner tool, a multipurpose diagnostic device, interfaces with the engine to determine and display the position of the valve and the rpm of your engine. To increase the rpm, you must increase the IAC position.

Open the cover of the outboard engine and locate the four-pin diagnostic connector. Typically, this connector is located to the left of the engine module. Remove the cover from the diagnostic connector and slide the pins of the scan tool into the diagnostic connector.

Turn the outboard engine on and move the gear shifter so that the engine is in neutral. Allow the engine to rise to its normal temperature by idling for about five minutes.

Turn on your Mercruiser scan tool by pressing and holding the power button. Use the up and down arrow keys to scroll to "Marine EFL" and press "Yes."

Scroll to the "PCM Functions" menu by pressing the down arrow key. Press "Yes" to open the menu and scroll to "IAC Test." Press "Yes."

Read over the advisory messages that pop up on the scan tool and press "Yes." Allow the scanner to test the position of the IAC valve as well as the engine's rpm.

Move the IAC valve, if desired. Press the up key to move it up and the down key to move it down. The possible positions range from negative to positive 100 and each press of the key moves it by 10. For example, if the scanner displays the current position as "-10" and you wish to move it to "+30," you must press the up key four times.

Exit the test by pressing the "No" key.