Ethanol fuel that sits in outboard engines can separate and clog up engine components.

Ethanol, a corn-based fuel that is mixed with traditional gasoline, has been a boon to the fuel industry. It helps to keep costs down and provides a renewable, more eco-friendly option to fuel cars and trucks. While ethanol may perform fine in automobile engines, boat engines may not be so lucky, and some engine manufacturers have expressed concern over the use of ethanol in boat engines. Gasoline that contains 10 percent ethanol, commonly referred to as E10, is not recommended for outboard motors, particularly older engines. The only way to avoid ethanol is to purchase marine-grade fuel that has none of it. This fuel costs more, but many boaters prefer it because of the risks for costly engine repairs when ethanol is used.

Unlike car engines that burn through a tank of gas quickly and are used frequently, outboard boat motors are often left to sit until the boat is used again. When ethanol fuel is allowed to sit, the corn alcohol can separate from the gasoline and settle at the bottom of the fuel tank.

Ethanol is also hygroscopic. This means it absorbs water. In high moisture conditions such as those on a boat, this can further exacerbate the separation of ethanol and gasoline. The result is a sludge that forms in the fuel tank. Most outboard engines have their fuel outlet at the bottom of the tank. After being left to sit, the engine will draw in pure ethanol and water that has accumulated, which can muck up the engine and corrode internal parts. Ethanol also is a strong solvent, and it can dissolve substances in the engine, potentially clogging fuel ports. Many boat owners have noticed clogged carburetors due to ethanol fuels. Boat service centers have mentioned an increase of engines needing cleaning from oxygenated fuels like ethanol. Buildup in the engine can restrict gas flow and cause the engine to overheat — precipitating a breakdown.

Although many manufacturers and repair shops recommend avoiding ethanol fuel for boats, if they cannot be avoided it is best to heed these tips.

  • Buy as little as possible — just enough to fuel the trip so leftover fuel will not remain sitting in the tank.
  • Drain the carburetor on the engine after using ethanol.
  • Use a fuel stabilizer that will help prevent separation. Ethanol is not recommended for all engines, and it may cause costly engine problems on outboard motors when the fuel is allowed to sit in the tank for extended periods of time.