Most modern outboards are designed to run on gasoline with up to 10 percent ethanol (E10), but ethanol brings problems in boats. It absorbs water from humid marine air, and when enough water collects, the ethanol and water separate from the gasoline and sink to the bottom of the tank, where the engine can draw them in. Ethanol fuel also goes stale faster, leaving varnish in carburetors and injectors, and it can damage older fuel lines and some older fiberglass tanks. Higher blends such as E15 are not approved for marine engines.
Ethanol problems in boats
| Problem | What happens |
|---|---|
| Phase separation | Water and ethanol settle out; the engine stalls, runs rough or is damaged if it draws them |
| Stale fuel and varnish | Gums up carburetor jets and injectors; see outboard won’t idle |
| Hose and seal damage | Older lines and seals not rated for ethanol can soften or crack |
| Older fiberglass tanks | Some older tank resins can be attacked by ethanol |
Can I run ethanol gas in my boat?
Usually yes, up to E10, if your outboard manual allows it. Check the manual for the maximum ethanol content and octane. Do not use E15 or higher blends in a boat. Ethanol-free gasoline, often sold as recreational fuel at marinas and some stations, avoids most of these problems and is preferred by many boaters when available.
Fuel stabilizer for boats
A marine fuel stabilizer slows fuel breakdown and is worth adding when fuel will sit for more than a few weeks, and always before winter storage. Add it to fresh fuel and run the engine so treated fuel reaches the fuel system; see how to winterize an outboard. Stabilizer does not reverse phase separation once it has happened.
How to protect your outboard
- Use fresh fuel and buy only what you will use within a few weeks.
- Fit a water-separating fuel filter and check it regularly; see outboard fuel filter.
- Keep vents clear and tanks mostly full in storage to limit condensation, leaving room for expansion.
- Replace old fuel lines and primer bulbs with ethanol-rated parts; see outboard primer bulb.
Signs of bad fuel
Hard starting, rough running, loss of power at full throttle and stalling can all come from bad fuel; see outboard loses power at full throttle and outboard won’t start. If you suspect phase separation, drain the fuel and refill with fresh fuel.
Related
See outboard motor maintenance and our outboard motors.
More guides: EFI vs carbureted outboards · outboard fuel lines.
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