Ethanol Blended Fuel and Boat Engines

Simply put, ethanol is an excellent SOLVENT (dissolves plastic, rubber, fiberglass and more), and unlike MTBE (the current gas additive), ethanol will ABSORB WATER, which can cause serious problems to many marine gas engines (inboards and outboards).

What is Ethanol?

Ethanol is a clean-burning, high-octane fuel additive that is produced from renewable sources. At its most basic, ethanol fuel additive is denatured alcohol, produced from crops such as corn or sugar cane. Adding ethanol to gasoline "oxygenates" the fuel, adding oxygen to the fuel mixture, so that it burns more completely and reduces polluting emissions, such as carbon monoxide, hydrocarbons, and oxides of nitrogen. The U.S. Environmental Protection Agency (EPA) requires the use of oxygenated gasoline to improve air quality. Many regions use Methyl Tertiary-Butyl Ether (MTBE), but it is gradually being eliminated, due to its contamination of ground water systems and soil through leaking underground storage tanks.

Any amount of ethanol can be combined with gasoline, but the most common blends are:

E10 - 10% ethanol and 90% unleaded gasoline
E85 - 85% ethanol and 15% unleaded gasoline.

It is important to note that modern engines can use E10 with no modifications. E85 is for use in a flexible fuel vehicle so some people confuse "ethanol" with the blend of 85% ethanol and 15% gasoline. Currently, there is not a marine engine on the market that will work with E85.

What is the Problem?

Ethanol is a hygroscopic substance, meaning it attracts water. The presence of ethanol in fuels may contribute to decreased fuel surface tension, which in turn may promote increased fuel tank condensation from air moisture.

Another problem with the introduction of ethanol comes from the mixing of old MTBE gas with the ethanol gasoline, especially if water is present in the fuel. If a significant amount of water is present in a fuel tank with gasoline that contains ethanol, the water will be drawn into the fuel, until the saturation point is reached, resulting in the three-component mixture of water + gasoline + ethanol. It is at this point that phase separation may occur. Phase separation is a process that can cause most of the ethanol and water to separate from the fuel and settle to the bottom of the tank. This leaves your gasoline with a significantly reduced level of ethanol in the upper phase, while the lower phase of the gas would contain a gelatinous mixture of water and ethanol. If the lower phase is large enough to reach the fuel inlet, it could be pumped directly to the engine and cause significant problems.

Ethanol is also a very good solvent, which allows it to loosen rust and debris that might lie undisturbed in fuel systems. Since ethanol has such good solvent properties, it can more readily remove plasticizers and resins from certain materials that might not be affected by gasoline alone. Loose debris will plug filters and can interfere with engine operation.

Check out this press release from the Ethanol lobby that clearly outlines these concerns.

Reasons Boat Engines Have More Problems with Ethanol Gas:

Boaters, often store gas in tanks longer than recommended for E10 (90 days). Cars, unlike boats, usually replace fuel every week or two, which will successfully prevent the possibility of water-contamination/phase separation.

Boat engines live in a water environment - Alcohol gas loves to absorb water. Ethanol E10 gas can absorb large amounts of water into the fuel tank, MTBE in conventional gasoline did not.

Plus, boat engines usually last longer than cars. Still owning and using a marine engine from the 1970's or 1980's is not uncommon. * These older engine parts and tanks were not usually designed or tested to withstand the damaging effects of alcohol gas.

* Several older marine engines (made prior to 1992) have plastic and rubber parts, and fiberglass tanks that are NOT compatible with E10 alcohol fuel.

Ethanol is being introduced to my area. What should I do?

There are a lot of stories centering on the introduction of ethanol into the marine environment, but not all are true. Most all problems can be avoided with proper preventive measures.

Before ethanol is introduced to your fuel tank, ask your boat manufacturer if any special precautions should be considered with the use of fuel containing ethanol. If there are no compatibility issues with your boat/engine manufacturer and the use of ethanol, then you may find the following guidelines helpful in your transition.

Before you fill-up with ethanol-based gasoline, empty all of the old MTBE fuel from the tank to help avoid any ill effects of mixing the two fuels.

Check for the presence of water in the fuel tank. If any is found, remove all water and dry the tank completely.

Use a good quality 10 micron water separating fuel filter and carry several spare cartridges for it. The Moeller Clear Site fuel filters (Overton’s item numbers 75846, 75848, 75850) are a great choice. Your first few tanks of the new ethanol based gasoline, may loosen and remove contaminates into your fuel system. This may cause an initial bout of clogged fuel filters, but this will pass as your system is cleared of these contaminates. As a precaution, carry a few extra filters, in case filter plugging becomes a problem during boating.

Twice a year inspect your fuel lines, fittings, and tanks for any signs of leaks, cracking, wear, etc. Replace worn components to insure safe boating.


By Robert Van Brunt
Chief Petty Officer U.S.G.G. ret


Short description:
When the engine compartment becomes hot either by climate or idling, and you use ethanol-blend gasoline it can cause excessive vapors in your fuel line and starve the engine of fuel. The engine can run poorly or stop and will not run until the fuel condenses.


Vapor Lock

Fuel containing 10% ethanol is called E10. If you have ethanol in your gas, you run the risk of creating vapor lock because of excess vapors.

Ethanol “boils” at 87ºF (at normal atmospheric pressure) and turns from a liquid to a gaseous state. By comparison, most automobiles have their fuel pump in the gas tank, so the whole system remains under pressure unlike boats whose fuel tanks are vented. In a closed system, the higher pressure raises the flash point of the ethanol reducing the amount of vapor that is produced. In addition, most automobile fuel lines are outside of the vehicle allowing them to stay cooler.

Since most boat fuel lines are in the enclosed space (sometimes even insulated) of the engine compartment, normal ventilation will not cool the fuel significantly enough to avoid the potential problems of vapor lock. Furthermore, since the fuel pump in a boat is mounted on the engine (versus a car where the pump resides in the tank) the action of the pump can reduce pressure in the tank to below atmospheric pressure and further reduce the flash point.

Boat engineers are aware of this problem and are reducing the likelihood of this occurring by reducing the suction required by the fuel pump, minimizing hose fittings and bends, and including a quality anti-siphon valve. In existing boats, fuel lines and filters should be kept as low in the boat as possible and tank vents should be cleaned and open.

Heat Soak

Most boats have “forced” ventilation. Air moves through the engine compartment when the boat moves forward. Heat soak happens after you have been at high RPM and then stop or drift on idle for a while. Because of heat soak the engine compartment will rise to a point where the ethanol will boil


To prevent vapor lock (i.e. boiling ethanol):

1. Make sure the engine compartment has adequate ventilation.
2. Relocate fuel lines to be low in the bilge. (The bilge is cooler because it is in direct contact with the water.)
3. Monitor the engine compartment temperature.
4. Add (or turn on) engine room blowers.
5. Keep the tank vent clean and unobstructed.



Ethanol is a solvent and can damage fiberglass tanks. It can dissolve old sludge in tanks which mixes with the gasoline and clogs filters. This can cause the engine to run very poorly.

Since ethanol molecules bond with water molecules the fuel can be contaminated by water. Fuel/water separators (filters) are supposed to separate out this water, but the amount of water trapped by the ethanol can quickly overwhelm the filter and allow water to pass through to the engine. Moist air enters the fuel tank by the tank vent (remember automobiles do not have this problem due to their closed system designed to help reduce emissions). Water can also settle to the bottom of the tank and cause the engine to run poorly and corrode fuel system components especially if the tank is not kept close to full.

E10 fuel is not as stable as past formulas. Older formulations would stay “fresh” for about 6 months. E10 can go stale in about 2 – 3 weeks. This is not a problem for an automobile with a 15 gallon tank, but if you have a boat with a 250 gallon tank, watch out!


1. The best way to keep water from the engine is to install a good water separator with a 10 micron filter to remove sludge and other contaminants. It is also crucial to drain the filter and check it more frequently than you may have in the past.
2. Volvo Penta recommends adding Sta-bil ™ to the fuel system. This must be done with new fuel. The Sta-bil will not treat stale fuel.

Express author Robert Van Brunt (Chief Petty Officer, U.S.C.G. ret) is a member of the American Boat and Yacht Council and a member of the International Association of Marine Investigators.

UPDATE: A comment asked about a treatment... while I don't write about this stuff to sell you parts or supplies and don't have ANY affiliation with anyone who does, I have looked into this for my own boat, which takes 250US Gallons of the ol' liquid gold at every fill-up... never mind the tender....

Anyway, it looks like a product that I have used in the past for marine gas stabilization has been adapted to meet the Ethanol challenge... Sta-Bil (marine formula). It is available at West Marine, WalMart etc and purports to solve some of the symptoms but cannot cure the problem.

Here is a link to their website...

As far as fuel filters go, just twist off your old one and replace it with a new filter that 10-30 microns. Make sure you fill it with gas and wet the gasket with gas (well ventilated folks... no smoking... no ignition sources!). You can get these at any marine supply place...

Here is an interesting development... it appears that there is now a massive class action lawsuit on the go in the USA about ethanol's effects on older boats.

Here is a link to an article on the Canadian Boaters Alliance website.

Thanks for this insightful

Thanks for this insightful article, you explained everything accurately. I am currently working in the rebuilt marine engines field and this kind of information helps me do my job in a better way. I didn't had anything to deal with ethanol so far but knowing about it is an advantage for me.

Geremy (not verified) | Tue, 01/27/2009 - 13:57

Ethanol & HPDI boat motors

Are there any additives that we can add to the ethanol gas to help it stay stable and not cause these condensation issues?

Guest (not verified) | Wed, 10/29/2008 - 22:11

You cannot get away from the

You cannot get away from the condensation properties in the Ethanol... but there are more and more products out there that temper this and help stabilize the fuel (not just for extended storage) and help keep things running smoothly.

I have been using a StarBrite product called StarTron... I put this stuff in at every fill-up (once a month for me). You can visit their site at for more details and can get the stuff online and at most marina stores.

Don't forget to change your water separating fuel filters to meet the ethanol challenges. You might find that you are draining them more often or changing them twice a year instead of once a season. Hope this helps.

admin | Wed, 10/29/2008 - 23:12

Scary... I think my marina is switching

I got a newsletter this winter indicating that my marina was switching to this stuff this spring. I am going to have to look into this further. Thanks!

Guest | Sat, 04/19/2008 - 00:18