Your best defense against fuel problems related to E-10 fuel is a 10-micron fuel filter like this one from Yamaha Marine.

Your best defense against fuel problems related to E-10 fuel is a 10-micron fuel filter like this one from Yamaha Marine.


Late last summer I was testing at Alumacraft in Minnesota with a tech-rep from Honda Marine. He'd been calling on dealers in Minnesota and Wisconsin for a week, and was getting an ear-full about a common problem: fuel systems gummed up after the boats had been sitting idle for as little as 10 days. The suspected culprit? Gasoline blended with ethanol.

"I'm telling all my service managers they've got to get their customers to drain the carb floats if they are going to leave the boat at the cottage for more than a week or two," Honda Man told me. "And to use fuel stabilizer in every tank."

Five minutes later, my cell phone rang. It was a reporter from the Milwaukee Journal-Sentinel, who wanted to talk about problems with ethanol fuel in boats. Clearly something was going on.

The Corny Story

Start a conversation about ethanol, and it won't take long for the discussion to veer off into the politics of farm subsidies, global warming and the price of tortillas in Mexico. We'll leave that to another site, avoid the controversy, and stick to the facts on ethanol-blend fuel in boats.

A sign of the times. In most states, pumps dispensing ethanol-blend fuel must have a label like this one.

A sign of the times. In most states, pumps dispensing ethanol-blend fuel must have a label like this one.

Ethanol is grain alcohol, produced mostly from corn and used as an "oxygenate," an additive blended with gasoline to reduce exhaust emissions by introducing additional oxygen molecules to the combustion process. Ethanol has largely replaced another fuel additive, MTBE, a potential carcinogenetic which has been banned in 25 states after it was discovered in ground water supplies, usually from leaks and spills. The farm lobby has been successful in promoting the widespread use of ethanol-blend fuels, even when they are not mandated for air-quality reasons. All motor fuel in Minnesota, for example, is 10 percent ethanol by law.

Ethanol-blend fuels are described by the percentage of alcohol in the blend. "E-10" fuel is 10 percent alcohol, and the EPA presently mandates that standard motor fuel may not contain more than 10 percent ethanol. E-85, which is 85 percent ethanol, is classified as an "alternative fuel" and can be used on in "flex-fuel" vehicles. Never put E-85 in your outboard.

If you are old enough to recall "gasohol," an ethanol-blend fuel that was promoted during the fuel shortages of the late 1970s, you might also remember that the marine industry has had fits with ethanol in the past. Gasohol was incompatible with many rubber and plastic components in the fuel systems of marine engines, and caused parts to swell or just dissolve. The marine industry immediately made fuel systems alcohol-resistant, and offered kits to fix older motors. And since that time, all new outboards have been designed to run on fuel with up to 10 percent alcohol.

So why are we having trouble with E-10 fuel today?

"Ethanol is a fabulous solvent," said Don Schultz, who retired from Mercury Marine in 2001 after serving 14 years as that company's leading expert of fuels and lubricants and today is an independent consultant to the marine industry. "So it's going to dissolve accumulated deposits in the fuel-supply system, and those could end up in a fuel filter or elsewhere in the fuel system if they get past the filter. Ethanol also loves water, it just sucks up moisture. This is not a big problem in the sealed fuel system of a modern car, but most boats have an open-vented fuel tank that breaths and introduces humid air to the fuel tank. This can create the formation of strong organic acids as water combines with ethanol and other elements in the fuel, forming sludge and corrosive compounds."

Other fuel experts I've talked to suspect that ethanol is literally cleaning the fuel-supply line, from the tanker truck to the in-ground storage tank, to your boat's fuel tank. Deposits of all sorts of nasty stuff then find their way, finally, to your outboard motor, where they can clog the fine filters on fuel injectors, or gum up a carburetor. It could be that, with the passage of time and enough E-10 though the delivery system, this situation will clear up on its own because all of the accumulated varnish and sludge will be rinsed away. But ethanol will always attract water, and that will remain an issue for outboard owners.

If you want to avoid ethanol issues, follow this advice from outboard service experts:

1. If it's possible in your area, avoid E-10 fuel like the plague. In most states, a pump dispensing E-10 is required by law to the labeled.

2. Install a 10-micron, water-separating fuel filter on your boat, or switch from the standard 30-micron element to a 10-micron element if you've already got a canister-type filter between your fuel tank and your engine. Suzuki ( has required a 10-micron filter for all of its OEM installations for several years, and Yamaha now offers two 10-micron filters, including one designed for the tight transoms of smaller boats. Mercury Precision Parts ( offers three different Racor 10-micron filter elements.

3. Carry a spare filter on board, and check the filter often for water.

4. Try to keep your boat's fuel tank filled. This will reduce the volume of the air-void space above the fuel in the tank, and reduce the flow of air in and out of the tank through the vent line that happens with changes in temperature (from cool nights to warm days, for example).

5. Add a fuel stabilizer to the fuel tank if you are not going to use the boat for more than a few weeks. This will inhibit oxidation of the fuel, which leads to the formation of deposits - commonly called varnish, gum, or gunk - in the fuel system. Follow the directions on the stabilizer, and then run the outboard for about 10 minutes to get the stabilized fuel all the way through the fuel system.

6, If you have a fishing boat with a kicker, make sure it's also protectedwith stabilizer and a 10-micron fuel filter.

For more information on ethanol-blend fuels and outboard motors, check the "FAQ" section on the Yamaha website; or the Service and Warranty section of the Mercury Marine website at

Editor's Note:Charles Plueddeman is the editor at large for Boating, the nation's largest boating magazine.