How to Flush a Mercury Boat Motor

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Every time you bring your boat in, you should flush your Mercury outboard with fresh, clean water to remove aquatic plants, mud, silt and salt from the cooling system. You can't always flush your motor before leaving the boat landing, but you can always flush it in your driveway. The flushing process can be completed in just a few minutes, doesn't require complicated tools, will make a world of difference in your motor's performance and extend the life of your outboard.

Items you will need

  • For Motors Without a Hose Attachment:

  • Flushing attachment ("ear muffs")

  • Hose

  • Source of fresh, clean water

  • Block of wood

  • Adjustable wrench

  • For Motors With a Hose Attachment:

  • Garden hose

  • Mercury Marine hose connector

Mercury Boat Motors Without a Hose Attachment

Use an adjustable wrench to remove the propeller from the engine; this will help prevent propeller-related injuries, which can be severe. Install the flushing attachment (called "ear muffs" because they look like a pair of ear muffs that people wear during the winter) over the water intake holes so that the "ear muffs" are fitted tightly over the cooling water intakes.

Connect one end of your garden hose to an exterior water faucet and the other end to the ear muffs. Turn the water on at the faucet, and turn it up high enough that the water is leaking from around the ear muffs. Make sure that the leak continues throughout the flushing process to ensure that the flow of water will be adequate.

Shift the engine into neutral, and start it. Allow the engine to run at idle with the shifter in the neutral position.

Make sure that a steady stream of water flows out of the water pump indicator hole as you continue flushing the motor for 3 to 5 minutes. This will be sufficient to flush the engine.

Stop your motor, turn off the water, remove the ear muffs from the hose and stow them properly. Inspect your propeller for dings and dents, then reinstall it.

Mercury Boat Motors With a Hose Attachment

Connect one end of your garden hose to an exterior water faucet and one end to the Mercury Hose Adapter. This adapter should have come with your engine, but, if necessary, a replacement can be obtained from the Mercury dealer.

Take the dust cover off of the flush connector that's on the end of the flushing hose on the front of the engine. The dust cover snaps off but is attached to the hose by a plastic tab so that it won't get lost. Push the hose adapter into the flush connector until you feel the snap of the connection being made.

Allow the motor to flush for 3 minutes. Mercury outboard motors with a hose attachment can be flushed without starting the motor.

After the motor has flushed for 3 minutes, release the hose adapter from the flush connector by pressing the release button on the flush connector. When the hose adapter is disconnected from the connector, it will automatically shut off the flow of water.

Turn off the garden hose, and store the hose adapter.


  • Propeller injuries can be severe at best and fatal at worst. Scrupulously observe all safety precautions when working on or around an outboard motor's propeller.


  • Disconnect the spark plug before removing the propeller; reconnect it to flush the engine, but disconnect it before re-installing the propeller. Straighten the locking tabs on the propeller nut retainer with your wrench before you remove the nut. Use a block of wood to hold the propeller while you remove the nut, and after you remove the nut, the propeller should pull straight off the shaft. If it doesn't, you may have a bent shaft; take it to your dealer to have them remove the propeller and inspect the shaft. Mercury outboards equipped with a hose attachment can be flushed without running. If your motor is so equipped, you do not need to remove the propeller.
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