Fuel Filters: Gas and Water Never Mix

From the pages of Trailering Magazine

When Dr. Nikolaus Otto built the first gasoline engine in 1876, it quickly became apparent the fuel he used was going to have to be filtered before it got into the engine. A few years later, Rudolph Diesel realized the same thing when he invented a variation of Otto's idea. Fuel filtering has been around for a long time.

Water Separators

Because gasoline (at the top of the glass) is lighter than water (at the bottom of the glass) and because the chemical makeup of ethanol attracts water (through condensation as well as poor fuel storage facilities), water has a good chance of getting into the fuel system. This Racor filter has a clear bowl at the bottom allowing you to easily see if water is in the fuel supply. This filter has a drain plug allowing the water to be easily removed.

Enter ethanol, the mix of 90% gasoline and 10% alcohol created from distilled cornstarch that is now common at your gas station and at marinas throughout the United States. It's the first step, and a somewhat awkward one at that, toward less dependence on oil. It has changed the way we look at fuel systems.

"Now more than ever before, a boat owner needs a good fuel filter and water separator on board," says Robin MacDonald of Racor, a leading manufacturer of marine filters. "We used to say, change the filter at least once a year. Now we say change it twice a year, more if you are on the water a lot. And always carry a spare."

If your boat is out of the water right now, most agree the fuel filter/water separator should have been changed. Check your engine manufacturer recommendations but most suggest filling the tank about 95% full (including a fuel stabilizer) with ethanol and don't close the vents in an effort to keep water out because fuel needs to expand with temperature changes and a closed vent has the potential of being dangerous.

Electronically controlled marine engines require cleaner fuel for efficient operation, making the job of filters and separators all the more important. But finding "clean fuel" has become something of a gamble.

Finding Clean Fuel

First of all, in the scientific community, ethanol is called "hygroscopic"-it has an attraction to moisture. Because ethanol may sit in a storage tank over a long period of time and go through changes in temperature, condensation is going to occur. When it is delivered to a gas station and then put into your boat's gas tank, water is going to get in through the vent system as well as go through the same temperature changes that have already occurred in the storage tank. As the fuel is used in the boat's gas tank, ethanol is pumped to a fuel filter/water separator where most of the water is removed, then into the fuel pump and then into the engine. But if there's no water separator in the system, the water-while it won't clog the filter-will be pumped into the engine, where it won't be well received.

If you have symptoms of engine troubles-loss of power or inability to run smoothly under load within ten minutes of leaving the fuel dock-"bad fuel" is going to be the cause a majority of the time.

"Not every manufacturer has put a filter/separator combination in their marine engines," notes Racor's MacDonald. "So it's best to do some legwork and check with the engine manufacturer to see if it is a good idea to add one."

As can be seen on the preceding page, gasoline (with ethanol or without ethanol) is lighter than water. As the fuel tank drains through use, eventually the gasoline is going to be used up, and the fuel indicator may be showing only the remaining water. When that water is pulled through the fuel line and into the engine, the cell phone may be pulled out for a call to TowBoatUS.

It isn't just water that can harm an engine. As both Dr. Otto and Mr. Diesel learned so long ago, a variety of debris is naturally part of fuel. With names like "microbial growth," found in gasoline tanks, and "abrasive dust," found in diesel fuel, the fuel filter is an essential part of the engine.

While these filters are designed to ensure clean fuel is flowing from the gas tank into the engine, oil that is used (as is the case with blended fuels in two-stroke engines) won't be removed because of the filter's presence.

"It's important to understand that you may indeed have a fuel filter on your engine-almost all do-but whether a fuel filter/water separator is part of that system is an entirely different matter," says McDonald. "All fuel filters are not created equal. You need to be aware of this. If you only have a fuel filter and not a water separator, upgrade to a combination of both."

Engine Size Doesn't Matter

This isn't just an issue for boaters with large outboards (or inboards or i/o's). Even a two horsepower outboard can be affected by water in the fuel supply (especially if the fuel sits in the engine's tank over a long period of time). Ethanol has a shorter shelf life than regular gasoline so this alone, is all the more reason why fuel filter/water separator combinations are available for any size engine. Just know the engine, its flow rate and a West Marine associate will be able to find the right one.

Fuel filter/water separators are measured two ways: by the density of the filter paper used-i.e. most marine engines use either 10 or 20 microns-the larger the number, the more debris is coming through to the engine. The average size for marine engines is 10 microns (a micron=1/1,000 of a millimeter for those taking notes). Diesel engines use smaller numbers; 2 microns is a common size. A filter is also measured by flow rate-i.e. how many gallons per hour will pass through. Marine engines can have a flow rate of 120 gph to 60 gph and even lower, depending on the size. Because diesel engines return the unused fuel to the tank, a higher flow rate is common. They consume, however, less fuel per hour on average than gasoline engines.

Racor makes a popular filter/separator for use on trailer boats, with a clear bowl at the bottom. This way, if water has been collected, you'll see it. If it's been installed properly, you'll be able to fit a paper cup beneath the bowl and drain the water. Other user-friendly features include a screw-on filter, which can be easily removed for inspection or replacement (Racor 320R and 660R filter/separators have both the clear bowl and screw-on features). Some boats, as shown in the photographs, have the fuel filter in a Mercury outboard (no water separator here) that is held in the fuel system by a series of clamps for easy access, inspection and removal.

Your tow vehicle isn't as sensitive to "bad fuel" as is your boat (though there are numerous cases of really bad fuel that have left many trucks and cars along the side of a highway). So it will be a good use of time to do an inspection when the boat is opened up this year that includes taking a look at the fuel filter/water separator. If you don't have one, you've been lucky. And should you decide to keep that streak going, chances are good, that your luck (and boat engine) will come to a stop. If you have both the fuel filter and water separator already attached, this is the time to do an inspection. Replacing them now means you'll probably not have to replace them while adrift out on the water.

Do You Have a Fiberglass Tank?

If your boat has a fiberglass gas tank built before 1991, don't use ethanol. As reported in the January 2006 edition of Seaworthy, these older tanks have clogged fuel as a result of ethanol's effect on fiberglass, resulting in sludge formed by a chemical reaction in the material.