How to install a water-separating fuel filter

Clean fuel = better boating. Here's how to get a bit greener

By David HarrisDavid Harris

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One thing on which most mechanics can agree is that the majority of outboard “problems” are fuel related. Failure to start or poor starting, rough running, misfiring, and loss of power can certainly be blamed on other culprits, but ensuring that your engine has clean fuel to burn and a clean fuel system will help eliminate the most likely suspect. This is especially true now that increased ethanol content is changing the chemistry of today’s gasolines and increasing their ability to retain water.

Enter the water-separating fuel filter, a simple solution that’s easy to install on small outboard-driven boats.

In addition to removing water from marina gas or condensation buildup in the tank, water-separating fuel filters remove sediment to help keep the fuel clean. This supplements the small in-line fuel filter just in front of the fuel pump on most engines, or the screen filter that’s part of the pump. (Note that the makers of a few larger outboards with electric fuel pumps discourage adding additional filters, which could reduce fuel pressure and cause vapour locks or fuel starvation. Check your manual before you start.)

1. For this job, pick up a water-separating fuel-filter kit for gasoline engines and two brass, aluminum, or stainless steel barbed hose fittings of appropriate size (widely available at auto-parts stores and larger hardware stores). You’ll also need pipe sealant suitable for use with gasoline, hose clamps, stainless steel screws or bolts to secure the housing to the boat, and waterproof bedding compound for mounting the filter housing. The size of filter will depend on your outboard’s fuel-flow capacity, which is based on engine size.

2. Find a location on the boat that is along the natural path of the fuel hose and is accessible for servicing. Make sure you have enough clearance to remove and replace the filter cartridge, which threads into the bottom of the housing, and to fit a catch tray underneath when you replace it so you don’t spill fuel. In addition, the filter should be installed away from direct sunlight and somewhere it won’t be whacked by water skis, fuel tanks, rambunctious dogs or small children, and other cockpit hazards. Good locations include the inside of the transom or below the rear bench seat on a tiller-driven outboard.

3. Double-check engine clearance and filter accessibility and install the barbed fittings to the filter before you mount the unit to the boat. The size of these fittings, which are threaded on one side to fit the filter housing and barbed on the other side to slip into the fuel hose, is determined by the specifications of the filter kit and the inside diameter of your hose. Coat the threads with pipe sealant. Turn the fittings finger tight, then give them another gentle turn by wrench. Be careful not to overtighten or you might crack the housing.

4. Hold the filter housing in place, note the arrows that indicate the direction of fuel flow, and check that the fuel hose will line up easily with these ports. Then mount the unit with the appropriate-sized fasteners and bedding compound.

5. Coat the threads and sealing ring of the filter cartridge with a smidgen of engine oil and screw it into the housing.

6. Now you’re ready to cut the fuel hose. Mark the hose where it lines up with the centre of the fuel filter, ensuring it still has enough slack that the engine can fully turn and tilt. Remove the hose from the tank and engine and take it off the boat somewhere you can collect the small amount of fuel that will spill. (This is a good time to replace the hose if it is showing signs of wear.) Use a sharp, clean knife to cut the hose at a 90° angle. Reconnect your fuel line to the tank and engine, slide a hose clamp over each of your freshly cut ends, and push the ends over the barbed fittings on the in and out ports of the filter housing: “in” from fuel tank and “out” to engine. Tighten the hose clamps, squeeze the prime bulb of the fuel hose to fill the filter, and check for leaks.

7. Change the filter cartridges at least once a year and keep a few spares on hand. If you detect deteriorating engine performance or have recently filled up with fuel from a suspect source, such as old jerry cans or a highway discount gas station, change the cartridge and hope for improvement. Since water collects in the bottom of the filter, a bad load of fuel may require more than one cartridge change until the system has been purged. (Higher-end filters have a plug in the bottom to drain off the water, which prolongs the cartridge’s life and lets you monitor the water uptake.)

That’s it — you’re done. For extra insurance against breakdowns, consider carrying a spare uncut fuel hose on the boat; though it’s not ideal to route bad gas directly to the engine, if the filter should clog or the fuel line springs a leak, the spare bypass hose is a band-aid solution that could just save the day.

This article was originally published on September 17, 2009


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