Click on Any Question to Learn More

About E10 Fuels

What is E10 Fuel?
E10 fuel is a blend of ethanol (10%) and gasoline (90%)

What is ethanol?
Ethanol is highly refined alcohol that is made from grain (typically corn) or the cellulose from other plants.

Why is ethanol added to gasoline?
EPACT2005 (The Energy Policy Act), Public Law 109-058 requires the increased use of renewable motor fuels. Ethanol is one of the only renewable fuel types that can be run in conventional motors.

Problems with E10 Fuels

What are the negative properties of ethanol?
Ethanol has several properties that contribute to fuel system issues.
  • Ethanol is a strong cleaner (solvent)
  • Ethanol is hygroscopic (it attracts water)
  • E10 fuel's usable lifespan is much shorter than conventional fuels.

Wouldn't the cleaning properties of ethanol be good for a fuel system?
No, fuel systems that have been used for non-oxygenated gasoline will have varnish deposites and surface corrosion (rust and aluminum oxides). This includes both your own fuel tank and the tanks at the gas station.

Ethanol will clean the varnish as well as the surface corrosion from any surface it contacts.
Ethanol may also disolve many platic resins used in some fiberglass tanks.

The amount of material cleaned by the Ethanol may rapidly exceed the filtration capacity of the fuel system resulting in restricted fuel flow, and potentially causing damage to the engine.

What issues are caused by ethanol's hygroscopic properties?
Ethanol molecules have a stronger bond to water molecules than to gasoline molecules. In the absence of water, ethanol and gasoline molecules will bond. However, when water is added to E10 fuels, the bond between the ethanol and the gasoline will weaken.

When the percentage of water in E10 fuel eceeds .5% the bond between the ethanol and gasoline will break down resulting in phase seperation. Ethanol will continue attracting water at this point up to the amount of Ethanol in the fuel (10%).

Phase Seperation

What happens when phase seperation occurs?
Several things happen:
  • The ethanol and water molecules settle to the bottom of the fuel tank, forming a distinct layer of water and ethanol on the bottom and pure gas on top.
  • Fuel for the engine is drawn from the bottom of the tank. An engine will not run properly (if at all) on a mixture of ethanol and water. The ethanol and water mixture is very corrosive to some metals and can damage internal components.
  • The remaining gasoline, without ethanol will have an octane level below the origional E10 fuel's octane level, approximatly 2-3 octane lower. This octane level may be below the requirements of the engine.


Can Phase seperation be reversed?
No, there are no additives or processes short of a major refinery that can recombine phase seperated ethanol and gasoline. This is a very stable molecular configuration and can't be undone without a refinery.

Can I use the gasoline after phase seperation occurs?
No, the remaining gasoline will have a lower octane level that may not be compatible with your engine.

How long can E10 fuel be stored?
There are a variety of opinions on that question. Generally you should never store your boat for any period without a stabilizer and never store it longer than a year with stabilizer. However the exact time it takes fuel to go bad is dependant on too many variables to predict how long you can store it accuratly.

Other Questions

Why do Boats seem to have more issues with E10 fuel than cars?
Cars also have some issues with E10 fuel, however they are somewhat more tolerant due to the typical usage paterns of cars and certain structural diffrences in their fuel systems.
  • Modern Automotive fuel systems are closed (non-vented). Once fuel is pumped into the tank there is very little circulation of outside air. This helps to protect the fuel from moisture in the air.
  • Boats are operated and often stored in a very wet environment further increasing the risk of water directly entering the fuel system.
  • Boats have vented fuel systems which allow moisture from the air to circulate into the fuel tank as the fuel is drawn out of the tank.
  • Automotive fuel tanks are typically much smaller and are refilled more frequently. Typically automotive fuel tanks range from 10-40 gallons and are refilled every 1-2 weeks.
    By contrast boat fuel tanks are typically 30-90+ gallons and are typically refilled once per month durring the boating season and often sit for months at a time.
  • Some boats use fiberglass fuel tanks. The Polyester resins can be dissolved by Ethanol.


Is my motor compatible with E10 fuel?
All motors made since the late 1980' are designed with components that are tollerant of fresh E10 fuel. That is not to say that motors can't still be affected by water, dissolved gum, varnish, corrosion, and disolved resins.

Can I use a higher percentage of Ethanol Fuel such as E15?
No, all the problems with E10 fuel grow much worse with higher concentrations. Phase seperation becomes almost unavoidable.

What should I do when I store my boat for the winter?
There are two competing recomendations on how to store your boat with E10 fuel.
  • YAMAHA: Store it with the tank completely filled and Stabilized.
  • MERCURY: Store your tank as close to completly empty as you can get it and stabilize the remainder
  • MARKS MARINE: We like Mercury's recomendation the best. In our experiance it's not possible to get your tank full enough to prevent the ethanol from stripping moisture from the air and our long winters makes it highly likely that the gas will seperate before you go to use it the next year.


What Should I do to prevent issues with E10 Fuel

  • Run a stabilizer designed for E10 fuel every time you fill up your tank. Here at Mark's Marine we offer stabilizers by Yamaha, Mercury, and Stabill brand Fuel Stabilizers.
  • Make sure the stabilizer you used is designed for ethanol fuel to prevent adverse reactions.
  • Replace your fuel filter after every 25 hours of operation or at least once per year instead of the previous recomendation of every 50 hours or at least every other year.
  • Install a water seperating fuel filter. We like the 10 micron Yamaha water seperating filters.
  • Carry extra filters and change them at the first signs of deminished efficency.
  • Consider replacing the fuel tank and lines in older boats.
  • If at all possible buy your fuel from the same source.