Mercury Outboard Engine on Deck Boat

How to Flush Your Boat Engine

Learning how to flush your boat engine properly is one of the most important maintenance tasks a boat owner can do to protect their investment. While salt water can leave damaging crystals behind if this process is skipped, even freshwater bodies of water contain impurities that need flushing.

Different model Mercury outboard and MerCruiser sterndrive engines have vastly different procedures for fulfilling this important boat engine maintenance step so always consult your owner’s manual before flushing your engine for the first time.

Four Women Riding on Bayliner Deck Boat, Stern View, Boat Underway

Mercury FourStroke Outboards From 75hp to 300hp

Mercury makes this outboard care procedure easy by including a flush port located on the port side (left) of the engine’s cowling. It’s important to note the engine is never turned on to perform this outboard motor maintenance task.

  • Trim the engine down
  • Unscrew the flush port cap found on the left side of the cowling
  • Twist off the cap from the hose fitting
  • Attach a garden hose to the engine hose fitting
  • Turn on the water supply half-strength
  • Flush for 15 minutes

    • When done, turn the water off, detach the garden hose, reinstall the cap on the hose fitting, and twist it back onto the cowling.

    Man and Woman on Deck of Anchored Bayliner Boat with Mercury Outboard Engine, Palm Tree in Background

    Mercury 40hp to 50hp FourStroke Outboards

    Learning how to flush an outboard engine of this size requires a different procedure than the one used for larger Mercury outboards. The main differences are the flush port on these models is located on the back of the engine’s cowling, the engine is run during the flush process and Mercury recommends taking the prop off anytime an engine is started out of the water for safety.

  • Remove the propeller
  • Trim the engine down
  • Twist on a garden hose to the port found on the rear of the cowling
  • Turn the water supply on half-way
  • Start the engine and run for five minutes at idle speed

    • When done, stop the engine, remove the garden hose, and reinstall the prop.

    Bayliner Boat with Mercury Outboard Engine, Boat Underway

    MerCruiser Sterndrive Engines

    Before flushing a 4.5L or 6.2L MerCruiser sterndrive engine, consult the owner’s manual, which can be found online. There are two ways to attach the water source: by using an “earmuff” attachment that fits over the intakes found on the lower unit or by ordering an optional flush kit from Quicksilver that has a built-in garden hose attachment.

  • Remove the propeller for safety and while it’s off check for wrapped fishing line and lube the driveshaft splines
  • Attach a garden hose to the earmuff or flush kit port and turn on the water full blast
  • Lower the lower unit to the down position
  • Make sure the throttle is in the neutral position and start the engine
  • Press the throttle-only button and advance the throttle until the engine is at 1400 rpm
  • Run the engine for 10 minutes then return the throttle to the idle position and shut off the engine
  • Shut off the water and remove the hose
  • Raise the outdrive and reinstall the prop

    • Faithfully flushing your Bayliner’s engine each time after an outing will increase its longevity and will prevent damage from occurring due to internal corrosion caused by salt or impurities.