How to Change a Marine Fuel Filter

How to Change a Marine Fuel Filter

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Marine fuel filters function as critical components on marine vessels, regardless of engine size. They resemble automotive fuel filters in some basic ways, but can be more complex and arranged in multiple configurations. Marine filters have to separate water from fuel while also removing harmful particles and contaminants in the micron (very small) size. Marine filters have designations for the size of the particles they can remove in the micron scale, with the lower numbers being more fine. The also have gallons per hour ratings.

Items you will need

  • Fuel Filter wrench (marine)

  • Socket set and wrench

  • End wrenches

  • Oil

  • Screwdrivers

  • Drain pain

  • Rags

Remove the battery from the boat and store it on the ground or dock. Look at your fuel tank inlet (with cap open) for a cone-shaped sleeve that sits inside the inlet. Pull the sleeve out and clean the small screen on the end by turning the sleeve upside down and using water pressure to blast the screen clean. Dry it off thoroughly and replace it back in the tank inlet.

Trace your carburetor fuel line back toward the tank. If you find a small canister in-line filter near the engine, it will be a secondary filter. Use a slotted screwdriver to loosen the hose clamps and pull the old filter off the hoses. Replace it with a new canister filter.

Look for any large bracket-mounted fuel filter between the carburetor and the fuel pump, and between fuel pump and the tank--this will be the primary filter. If the primary filter has a screw-on configuration, loosen it with the fuel filter wrench by turning it counterclockwise. Let the fuel drippings drain into a pan. Lubricate the O-ring on the new filter and screw it in by hand until snug. Tighten 1/4 turn further with the filter wrench. Change out any additional primary filter that has the screw-on configuration.

Determine if you have a drop-in primary fuel filter that has a water separator bowl attached. If so, release the fastener on the separator bowl and drop it, letting the fuel drain into a catch pan. Unlatch the top lid flange on the fuel filter and pull out the old element. Clean out the inside of the canister with a rag. Drop in the new filter element and secure the lid flange. Reattach the separator bowl and clip it in place.

Find the primary fuel filter on your diesel engine. It should be a drop-in style with a water separator bowl. Change it as you would for the drop-in gasoline type filter. Clean the inside of the fuel filter canister. Before you refasten the top lid flange, prime the canister up to the top with diesel fuel, then refasten the lid flange.


  • Remove the battery completely from the craft. No open flames or smoking during a fuel filter change.


  • Purchase a stock fuel filter for your engine, or one that has a higher gallons per hour rating. Never get a filter below your prescribed GPH requirements.
  • Overly high vacuum readings from your manifold can indicate a clogged fuel filter.
  • Ten percent of your maximum horsepower will give you the GPH rating of your engine. Use 18 percent times your maximum horsepower for a GPH on a diesel engine
  • Fuel filters come in micron ratings--which is 1/1000 of a millimeter. Make sure you match your filter with the correct micron rating for your boat.
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