How to Replace a Fuel/Water Separating Filter (and Check for Water in Your Fuel)

Turn the key off and remove it. On inboards and sterndrives, run the bilge blower for at least 4 minutes. Open hatches for airflow. Put on gloves and safety glasses.

Trace the fuel line from the tank toward the engine to find the canister. On a portable-tank outboard, disconnect the fuel line at the engine or tank. On a permanent tank, close the fuel shutoff valve if the boat has one.

Lay absorbent pads under the filter and set a drain pan below it.

If your filter has a drain plug or clear bowl, drain a little fuel into a clear glass jar. Let it settle (up to half an hour gives the clearest result) and look:

• Clear, one layer: good.
• Beads or a layer of water at the bottom: water contamination.
• A cloudy or milky layer at the bottom: likely phase-separated ethanol and water.

If you see more than a few drops of water, keep reading Step 11 before you run the boat again.

If there is a wire going to the bottom of the canister, unplug it. You will move the sensor to the new filter.

Turn the canister counterclockwise (looking up at it from below) by hand. If it is stuck, use the filter wrench. Keep it upright as it comes off so it does not dump fuel.

Do not empty the old filter and reinstall it. Dirt and water trapped inside can get pushed into the clean side.

Pour the old filter's contents into your clear jar and look for water, rust or debris. Make sure the old rubber gasket came off with the canister and is not still stuck to the mounting head. Wipe the head's sealing surface clean.

Unscrew the sensor from the old canister and fit it to the new one with a new O-ring if one came in the box. Hand-tight only, unless the filter's instructions say otherwise.

Smear a thin film of clean engine oil on the new filter's gasket. Then fill the new canister about three-quarters full with fresh, clean, stabilized gasoline. Pre-filling makes priming much easier.

Spin it onto the head by hand until the gasket touches, then snug it the extra amount printed on the filter (usually a fraction of a turn). Hand-tight is the norm. Over-tightening can crack an aluminum head or make the next change a fight. Reconnect the sensor wire.

Open the fuel valve or reconnect the fuel line. On an outboard with a primer bulb, squeeze it until firm. A bulb that never firms up points to an air or fuel leak, or a bulb with worn internal valves; find it before starting.

Later, if the bulb sucks flat while the engine is running, the engine is fighting a restriction between the tank and the motor: a closed tank vent, a kinked or internally collapsed hose, a sticking anti-siphon valve, or a clogged or wrong filter. A new filter should never restrict flow. On engines with an electric pump, turn the key to "on" (not start) a few times to let the pump fill the system, per your manual.

Look and sniff around the filter and every fitting you touched. Any drip or fuel smell means stop and fix it.

• A few drops in a new-season filter is common. Replace the filter and keep an eye on it.
• A lot of water, or a milky layer, means the tank has a problem. Additives will not turn bad fuel back into good fuel. Have the tank pumped out and refilled with fresh gas by a shop or fuel polisher. Running the engine on phase-separated fuel can damage it.

With cooling water supplied (in the water, in a test tank or on flush muffs; the built-in flush port on most outboards is for engine-off flushing only, unless your manual allows running on it), start the engine and let it idle a few minutes. Check the filter again for leaks while it runs.

Put fuel-soaked rags and the old filter in a sealed bag and take them to a hazardous waste drop-off or a marina that accepts them. Log the date and engine hours in BoatMend so the next change reminds you on time.