The dirty word in marine fuel

Most outboards have a fuel filter mounted on the side of the engine that is easy to see. Check it regularly for sediment, discoloration and the presence of water and change at the first sign of any of these problems.

Like most of you who haven’t experienced problems caused by ethanol, I really hadn’t thought much about the now-common fuel I burn in my outboard engines.I remember reading something stating they were manufactured to handle gasoline with up to 10-percent ethanol, commonly called E-10, but hadn’t paid much more attention.

A mistake.

I’ve been lucky and haven’t yet experienced a breakdown related to ethanol in my gas. Hopefully you’ll read and heed this in time to head off any issues that haven’t arisen.

Fortunately I am on several mailing lists of industry professionals and sometimes am forewarned of things that might happen and issues that might arise. While I won’t mention his name (but I will thank him for waking me from my complacency), I realized there might be something serious to this ethanol thing when an official of one premier outboard manufacturer sent me information that he said “is from the competition but is good information, and you should read it.”

The information is from Mercury Marine and is available at the Service and Warranty section of Mercury Marine’s web site ( It’s entitled “Ethanol.”

The FAQ section of the Yamaha Outboards’ web site ( also has information regarding ethanol at its Service and Maintenance subtitle.

Simply put, ethanol isn’t a good thing for outboards and marine fuel systems. But if it’s managed correctly, ethanol fuel can be used with a minimum of worry.

Managing ethanol requires a more rigorous preventive maintenance approach, but that’s certainly preferred to repairs and extensive down time.

Before setting up a program to use ethanol safely, let’s understand what it is. Ethanol is an alcohol often blended with gasoline, a highly-refined beverage (grain) alcohol, approximately 200 proof, that can be produced from natural products such as corn, sugar cane and wheat. Ethanol used for fuel has been “denatured” or rendered unsafe to drink by the addition of a hydrocarbon (usually gasoline).

In the past, when it was mostly an option, this blend of gas and alcohol was generally referred to as “gasohol.” MTBE (methyl tertiary-butyl ether) once was used to replace leaded gasoline, but scientists discovered it had a tendency to work its way into ground water systems and was a contaminant. MTBE once worked its way into ground water at Wrightsboro, near Wilmington, and Conoco Oil had to pay millions to area residents because of contaminated wells.

Most engines made during the last 10 years will operate fine on fuel with up to a maximum blend of 10-percent ethanol as long as they have good fuel filters. This is the fuel labeled E-10. The biggest problems seem to come from simple engines, like lawn mowers, weed eaters, tillers and such, which do not use good filter systems. The Flex Fuel Vehicles made by several car companies have special components that allow them to operate on fuel blends of up to 85 per cent ethanol. This is the fuel labeled E-85 and it is not readily available in North Carolina.

Unfortunately, ethanol has some characteristics that make it less than ideal for use as a marine fuel. While ethanol boosts the octane rating of unleaded gasoline and provides beneficial clean-burning combustion characteristics that help improve emissions, it is hygroscopic (it has an attraction for water) and will more readily mix with water than with gasoline. Because of the alcohol, E-10 is more of a solvent than MTBE or straight gasoline, which causes it to loosen fuel varnish, rust and other gunk and debris that have collected in fuel tanks and systems over time. These same solvent tendencies cause it to separate some resins and plasticizers from fiberglass and plastics that are resistant to pure or MTBE gasolines.

Ethanol is also corrosive to some metals, all of which are not known yet, and this is sometimes enhanced when it combines with water. While gasoline has a low conductivity for electricity, ethanol’s conductivity is much higher. This is never good on a boat for a multitude of reasons, with one of the more common being increased galvanic corrosion.

In the North Carolina climate, the warm days, cool nights and high humidity create a combination that produces an over-abundance of condensation in fuel systems. It is an issue with gasoline and is exaggerated with E-10 fuels. Ethanol draws the water created by the condensation into the fuel until the saturation point is reached for the three-component mixture of water, gasoline and ethanol.

Once the amount of water in the fuel exceeds this level a nasty thing called “phase separation” occurs. Phase separation causes most of the ethanol and water to separate from the bulk fuel and sink to the bottom of the tank. This leaves gasoline with a significantly reduced level of ethanol in the upper part (phase) of the tank and a concentration of water and ethanol in the lower tank.

If the lower phase of water and ethanol reaches the fuel inlet, it could be pumped directly to the engine and cause significant problems. Even if the ethanol water phase at the bottom of the tank is not drawn into the fuel inlet, the separation reduces the ethanol level of the fuel above it and reduces the octane rating by as much as 3 octane numbers, which could result in engine problems. This is exaggerated in marine fuel systems as most draw from a point at the rear bottom of the tank. The level at which phase separation can occur is determined by a number of variables, including the amount of ethanol, the composition of the fuel, the temperature of the environment and the presence of contaminants.

Before switching to gasoline with ethanol, run the tank to as near empty as possible and check it and the filters for water. Change the filters and remove any water that was found in the tank.

Yamaha recommends using a 10-micron water-separator filter and offers two sizes, mini and regular. Several mechanics suggested using one of the filters with a see-through section so you can easily see if water is collecting. Once using ethanol fuel, it is suggested to limit the exposure of the fuel tank to excess water. In our climate, one of the key things is to keep the tank full, so condensation can’t occur. This is especially important if the boat won’t be used for a while.

While the general thought has been there was not a fuel additive that prevents phase separation in E-10 fuels, the folks at Blackbarry Marine (910-457-0667 or in Southport said they have seen good results, including some increased performance and economy, from one named PRI-G.

Steve Smith said Blackbarry had been pleased with Power Research Inc. products and there was also one for diesel (PRI-D). Smith will be glad to discuss their satisfaction with the PRI fuel additives or you can visit the PRI website at

“Our service department has seen a couple of real horror stories with E-10 gas,“ Smith said. “What we have seen comes mainly from the increased solvency and the E-10 gas breaking down the resins in older fiberglass gas tanks or dissolving the varnish and gunk deposits in the gas tanks and fuel lines and plugging the fuel filters. You have to keep a good watch on it too. The Yamaha fuel filters are designed so if the filter plugs, it goes into a bypass mode to keep from starving the engine. That can let whatever is plugging the filter go directly into the engine.

“We had one real nightmare where the ethanol dissolved a combination of resin and general fuel tank crud from the fiberglass tank in an old classic Boston Whaler. This gentleman was meticulous about his maintenance and noticed his performance was off, so he immediately brought the boat in. Our mechanics found a soft, tar-like substance all but filling the fuel lines and filters. Fortunately he noticed in time and the filters had prevented it from getting into his motor, or it could have been a lot worse.”

Smith said the deposits they often found in filters were dirt and sludge. He said sometimes in addition to the water there was material that looked like grains of pepper and rust caught in the filters. He suggested running a water-separator filter between the tank and the motor and keeping a close eye on the filter on the side of most outboards. He said the filters on the sides of engines are generally see-through, and you can see if any particles collect.

Smith said to also look for color changes in the engine filter. He said the filament in these filters is white, but as it starts collecting the varnishes and such in the fuel it turns from tan to brown. When you see the engine filter changing color or collecting particles, it is time to change both filters.

With the banning of MTBE as an oxygenating agent in gasoline, it appears ethanol is here to stay. By paying attention to filters, the presence of water and following the steps above, we should be able to use the E-10 gasoline with few problems. However, it is important not to lose sight of the fact that becoming complacent can cause significant problems. While most motor manufacturers are more than willing to offer support to prevent problems originating with ethanol fuels, problems caused by phase separation, clogged fuel filters and other issues created by using E-10 fuels are generally not covered by warranty.

There are also issues with materials used and some components of older boats, motors and fuel systems. Older seems to have a different timetable depending on whom you speak to, so check with the manufacturer for advice about using E-10 gasoline in any boat, motor or fuel system more than a few years old.

Owners manuals of most newer products have this information. Remember, when in doubt or if you have questions, a phone call or e-mail is a whole lot less expensive and time consuming than major repairs.

About Jerry Dilsaver 1171 Articles
Jerry Dilsaver of Oak Island, N.C., a full-time freelance writer, is a columnist for Carolina Sportsman. He is a former SKA National Champion and USAA Angler of the Year.

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