To flush an outboard, connect fresh water to its cooling system, either with flushing muffs over the water intakes on the lower unit or through the flush port that many modern outboards have, and run fresh water through it until the salt, sand and silt are washed out. With muffs, turn the water on before starting the engine, run it at idle in neutral, and turn the engine off before the water. Flush after every trip in salt or brackish water.
How to flush an outboard with muffs
- Remove the propeller or make sure no one can contact it, and put the motor in neutral.
- Fit the muffs over the water intakes on both sides of the lower unit, centered on the intake grilles.
- Turn the hose on fully so water flows out of the muffs before starting.
- Start the engine and let it idle; never rev it on muffs.
- Check the tell-tale stream to confirm water is circulating; if it stops, shut the engine off.
- Run it for the time your manual recommends, usually several minutes, until it reaches operating temperature.
- Shut off the engine first, then the water.
How to flush an outboard without muffs
Many four-stroke outboards have a garden hose flush port that flushes the cooling passages with the engine off. Connect the hose, run water for the time the manual specifies, then remove it and replace the cap. Do not start the engine while using the flush port unless your manual says it is allowed. Small motors can also be flushed by running them in a barrel or tank of fresh water deep enough to cover the intakes.
How to flush an outboard in the water
If the boat lives on a mooring or lift, tilt the motor up and use the flush port, or run the motor in fresh water when you can. Some owners use flush kits that attach to the flush port with the motor tilted.
Salt water and salt-removal products
Salt crystals left in the cooling passages cause corrosion and blockages that lead to overheating. Flush after every saltwater trip; some owners add a salt-removal solution through the flush system, following the product directions.
What flushing does not fix
A weak or absent tell-tale stream after flushing points to a clogged tell-tale tube, a worn water pump impeller or a blockage; see outboard water pump impeller and outboard overheating.
Routine care
Flushing is one part of regular care alongside gear oil changes and spark plugs; see outboard motor maintenance, how to change lower unit oil and how to winterize an outboard. If your motor is past saving, see our outboard motors.
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