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.
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.
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.
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