Sailboat Engine Fuel Filter Change in Four Steps

A fuel filter change in four steps

Last month, I explained how to change your engine’s oil. Like oil changes, fuel filter changes are also essential for keeping our iron gennies running smoothly. Fuel quality is fundamental to preventing unexpected breakdowns and extending the life of your engine, but it turns out that boats are not the most hospitable environment for fuel. Condensation forms and microbes and other debris can accumulate in fuel tanks, but luckily our fuel filters prevent all that gunk from reaching our engines—as long as we take care of them.

Your fuel filter needs will depend on your engine. Most inboards have two: the first is a primary fuel filter that is larger, separate from the engine, and that also serves as a fuel-water separator. There might be a secondary fuel filter on the engine itself or between the primary filter and the engine. Outboard motors could have one or more fuel filters, all somewhere on the engine itself. Fuel filters should be changed every time you do an oil change, although secondary and tertiary filters can often go longer, perhaps every other oil change.

fuel filter
A recently removed two-micron fuel filter that is relatively clean, which is what you are hoping for!

What You’ll Need

New fuel filter, including O-rings, gaskets, etc.
Possibly a strap wrench, depending on the filter
Small flathead screwdriver
Rags and oil absorbing pads
Disposable gloves
Possibly a socket wrench

Step One: Find Your Old Filters

As previously mentioned, the number and location of your fuel filters will depend on your engine. Primary fuel filters for inboards will likely be mounted somewhere between the engine and the fuel tank, while secondary fuel filters and outboard engine fuel filters will most likely be on the engine itself. If you’re changing a primary fuel filter, you won’t need a strap wrench. For a secondary filter or an outboard engine filter, you likely will need one because it may be screwed onto the side of the engine the same way an oil filter would be.

Step Two: Find Your New Filters

New fuel filters can be found at most marine supply stores and chandleries. Look for the appropriate physical size and micron rating, which refers to the smallest particle size the filter can capture. Lower micron ratings mean it can capture smaller particles. Many experts favor 10-micron filters or lower to trap even the smallest particles; although these will need to be changed more often.

Step Three: Replace the Filters

The exact process for filter removal will, again, depend on the specific filter. For the most common type of primary fuel filter for an inboard engine, start by laying some oil-absorbing pads underneath it, since it should be filled to the brim with fuel and will likely spill some. Unscrew the T-bolt handle on top and lift off the lid. Now you should be able to see the top of the filter. On either side, it should have some flexible pieces of plastic. Use your flathead screwdriver to grab them and gently lift the filter up and out, being mindful of fuel spillage. The next thing to remove will be the main O-ring. This will likely be located inside the lid that you’ve already removed, and you can use your flathead screwdriver or a fingernail to take it out.

To put the new filter in, just drop it into where the old one was. It may come with a thick rubber ring that fits into the bottom. If so, put that into the bottom of the filter before dropping it in. The new filter should also come with a new O-ring; press this into where the old one was in the lid, ensuring there are no twists. Set the lid back on top. There should be one last small O-ring that came with the fuel filter, and this one is for the T-bolt that you removed first. Roll the old one down and off, and roll the new one on, again ensuring there are no twists. Lastly, screw the T-bolt back on; hand tight is plenty. If this process doesn’t align with your fuel filter, YouTube is a great source for the particularities of every fuel filter out there.

Step Four: Bleed Your Fuel Line

Fuel systems are closed systems free of air, and since you just introduced air into the system by opening the fuel filter, the last step will be getting all of that air out. The general idea is to open the fuel line somewhere downstream of the filter and force fuel through the line until all the air is out. This is where you’ll want to consult your engine manual. Most engines will have a bolt somewhere on the engine that you should loosen (but not completely remove), using your socket wrench and a small pump nearby. You should be able to operate the pump by pressing it repeatedly with a few fingers, and it will force fuel through the line until it comes out through the hole where the bolt you just loosened is. Be sure to lay some oil absorbing rags underneath the engine to catch the fuel. 

Keep pumping until you see a steady stream of fuel coming out, with no bubbles. Continue pumping while you retighten the bolt to ensure no air comes back in. Wipe up any remaining fuel on and around the engine. If you also changed the secondary fuel filter, you may need to crack open some other points after the secondary fuel filter, such as the injectors, and pump again to fully bleed that portion of the line. Start the engine. If it sputters or dies, there is still air in the system, and it needs to be bled again.

Congratulations, you just changed your own fuel filter! Not only did you save time and money doing the work yourself, you also have a better understanding of your engine and its fuel system if you ever need to troubleshoot a problem. 

by Kelsey Bonham

About the Author: Kelsey Bonham is based in Norfolk, VA, where she keeps her 50-year-old, 30-foot steel boat, Little Wing, which she restored with her dad during the pandemic.

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