Consider a Dual Fuel-Filter System

Posted: October 1, 2013

Avoid the worst-possible-moment engine shutdown.

By: Story and photo by Capt. Frank Lanier

A fuel filter always seems to clog at the worst possible moment, such as when you’re running an inlet in rough weather and the wave action stirs up long-dormant crud from the bottom of your tank. It’s situations like these where the extra running time provided by a dual primary filter installation can make the difference between getting through that rough inlet or not.

Fuel filters are important from both a maintenance and a safety standpoint. Contaminated fuel can do more than shut an engine down; it can cause expensive damage over the long run. Most vessels come equipped with an external primary fuel filter of 20 to 30 microns (10 microns is standard for gasoline filters) mounted between the fuel tank and the engine to strain out the bulk of gunk and goo your fuel system accumulates — solids, water, microorganisms, etc. — prior to reaching the finer filter, which is typically 2 to 10 microns, mounted on the engine itself.

Technically, this means you already have a dual filter system in place. In reality, however, since both filters are in series, once the primary filter clogs, the finer engine filter will be fuel-starved, meaning you’ll have to shut down the engine (if it hasn’t already shut itself down) to change the now-useless primary filter. The addition of a second primary fuel filter gives you the option of quickly switching to a clean filter without the need to shut down the engine, change filters and purge the fuel system of air.

Choosing, Adding Second Filter

You can convert to a dual fuel-filter system by adding a second filter to your existing system or by installing a pre-engineered system, such as the FilterBoss by KTI Systems (, which can be purchased as a manifold or a complete dual filter system.

Adding a second filter to an existing system is a fairly straightforward affair. When planning your system, be sure to install shutoff valves upstream at each filter, so you can shut off fuel to the filter prior to changing it. The shutoff valves are in addition to the diverter valves, which allow you to switch between filters. Clearly labeling the valves and other system components will help ensure the valves are properly positioned during operation, while changing filters and during other maintenance.

Follow your engine manufacturer’s recommendations to choose filters that are the right size. A filter that’s too small won’t provide sufficient fuel flow, while an excessively large one wastes money and may have difficulty separating water from your fuel.

Filters are sized by fuel-flow rate, which is typically expressed in gallons per minute or hour. Features to look for when purchasing a filter unit range from advanced water separation designs to built-in alarms indicating the presence of water or restricted fuel flow. Spin-on filters are popular due to the ease with which they can be changed; however, the entire fuel system must be bled of air each time a filter is changed. Filters with top-loading elements typically are a bit more expensive but allow you to simply add fuel to the filter once it is changed, and they may eliminate the need to bleed air from the system.

When mounting your filter or manifold (as with any project), measure twice, drill once, and make sure you know what you’re drilling into. Punching holes into wire runs and hoses hidden behind a bulkhead is always a bad way to start an installation. If space is limited, bear in mind the new filter can be mounted in a different area than the existing filter.

When to Change?

In addition to the classic symptoms accompanying a fully clogged filter — engine surging followed by a complete shutdown — many engine problems that initially appear to be mechanical in nature (e.g., missing, surging, reduced power, excessive black smoke) can often be traced to a clogged filter.

Fuel filters should be changed regularly and logged (along with engine hours) as part of your vessel’s preventive maintenance program. Another way to monitor a fuel filter’s condition is by installing a vacuum gauge. Little or no vacuum means the filter is clean, while high readings on the vacuum gauge indicate the engine is “sucking” harder to get fuel through the filter, an indication it’s clogged and needs replacing (bearing in mind the gauge is accurate only with the engine running).

Finally, it’s important to remember that if the number-one primary filter clogs in heavy weather, the second one may follow suit in short order, depending on the level of gunk in your tank, or fuel contamination. After switching to your number-two primary, try to swap out the clogged number-one filter element as soon as possible, so it’ll be in the ready position should the other one clog. Sailboats have the additional option of making sail if the primary clogs in rough water, saving the number-two primary until the boat reaches calmer waters, or when the engine is needed for docking or maneuvering.

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