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The Real Dangers of Ethanol Fuel in Your Outboard Motor
You pull the starter cord on a beautiful Saturday morning and get nothing. Or worse, the motor fires, runs rough for a few minutes, and dies on the way out of the cove. You check the obvious stuff: spark plugs look fine, the primer bulb is firm, there's plenty of fuel in the tank. What you might not be thinking about is what's in that fuel. If your motor sat for a few weeks between trips with a tank of regular pump gas, ethanol might have already done its work. It's one of the most common causes of outboard problems that marine technicians see, and most boaters don't realize what's happening until the repair bill shows up.
What Is Ethanol and Why Is It in Your Gas?
Ethanol is an alcohol made from corn that fuel companies are mandated to add to gasoline. By law, companies are required to add a minimum percentage, which varies depending on where you live. The fuel most of us pump at the gas station, labeled E10, contains 10% ethanol and 90% gasoline. That blend works reasonably well for car engines, which run regularly, burn through fuel quickly, and are designed with ethanol in mind. Outboard motors are a different situation entirely.
The Three Main Ways Ethanol Damages Your Outboard
Phase Separation: The Hidden Killer
Ethanol is hygroscopic, meaning it attracts and absorbs moisture. It pulls water out of the air right through your gas tank vent. In the humid environment that most boats live in, this happens faster than you'd expect.
Here's what happens next: when the concentration of water molecules in your fuel tank reaches just half of one percent, the water molecules bond with the alcohol and sink to the bottom, right where your fuel pickup is. The upper gasoline layer is left depleted of ethanol with a reduced octane level. The lower phase-separation layer is a corrosive mix of water and ethanol.
If that mixture reaches your engine, it can clog filters, carburetor jets, and fuel injectors. In more serious cases, it causes internal corrosion and engine damage that no amount of cleaning will fix.
Corrosion and Material Damage
Ethanol combined with water creates a mildly acidic solution that attacks aluminum, zinc, and other metals commonly used in carburetors, causing visible corrosion, white or green powdery deposits on aluminum parts, pitting on metal surfaces, and blocked passages from corrosion debris.
Ethanol is also a powerful solvent. At the levels found in E10 gas, it acts as a solvent that cleans sludge while dissolving polymers. That sounds useful until you realize it can attack rubber fuel lines, gaskets, and seals that weren't designed for ethanol exposure, particularly on older motors.
Varnish and Gum Buildup
Ethanol fuel degrades fast, often in 30 to 60 days. When ethanol-blended fuel sits in a carburetor bowl or fuel line and begins to evaporate, it leaves behind gum and varnish deposits. A buildup of this residue gradually restricts the size of the carburetor's main jet, which means the motor won't receive all the fuel it needs. The classic symptom is a motor that misfires and runs roughly with a loss of power, even when warm.
Carbureted vs. EFI: Does It Matter?
Yes, but maybe not in the way you'd expect. Carbureted engines are generally considered more vulnerable because fuel sits exposed in the carburetor bowl between uses. Varnish builds up in the jets, the needle and seat corrode, and deposits restrict fuel flow. The symptoms are hard starts, rough idle, and poor performance under throttle.
EFI engines aren't immune. The Vapor Separator Tank filter on fuel-injected outboards clogs with the same ethanol varnish and sediment that kills traditional fuel filters. It's hidden inside the VST canister, and the symptoms are identical to a clogged external filter: runs fine at idle, but starves under throttle. EFI systems also tend to be more expensive to service when ethanol-related damage does occur.
What You Can Do About It
Use Ethanol-Free Fuel When You Can
Using recreational fuel, commonly labeled REC-90, is the most reliable way to avoid ethanol-related problems because it doesn't contain ethanol. It costs more, but it eliminates phase separation and the corrosion that comes with it. If your marina carries it or there's a station nearby, it's worth the extra cost, especially if your boat sits between uses.
Treat Every Tank With a Marine Fuel Stabilizer
If ethanol-free fuel isn't an option, a quality marine fuel stabilizer is your next best defense. Fuel stabilizers are specifically formulated for marine use and help slow fuel degradation, reduce the risk of phase separation, and inhibit corrosion inside the fuel system. Fuel treatment isn't just for extended storage. Today's ethanol-blended gasoline can begin to break down in a matter of weeks, not months, so treating every tank is the right habit, not just a pre-storage routine.
Don't Let Fuel Sit
Never use ethanol-blended gasoline that’s more than 90 days old. If your boat sits between trips, try to run it regularly enough to cycle through your fuel supply. A motor that runs every week is far less vulnerable to ethanol damage than one that sits with a half-tank for two months.
Run the Carb Dry Before Storage
If you're laying up the motor for any significant stretch of time, running the carburetor dry after your last trip removes the fuel that would otherwise sit, degrade, and varnish up the jets. Shut the fuel valve and let the motor run until it dies on its own. This significantly reduces the risk of a gummed carburetor at your next launch.
Buy Fresh Fuel From a High-Volume Station
The goal is to keep your fuel fresh and potent. Buy your gas where they sell a lot of it. A busy station turns over its underground tanks regularly. Fuel that's been sitting in a low-volume station's tank for weeks is already partially degraded before it ever goes into yours.
The Bottom Line
Ethanol in pump gas is a fact of life for most boaters right now. A few simple habits go a long way toward keeping it from causing real damage: use ethanol-free fuel when you can find it, treat every tank with a quality marine stabilizer when you can't, don't let fuel sit untreated for extended periods, and run the carb dry before any significant storage. The boaters who run into trouble are almost always the ones who didn't know what was happening in their tank until something stopped working.
If you're in the market for a new motor built to handle today's fuel conditions, browse our full outboard lineup or check out our accessories collection for maintenance supplies to keep what you have running right.
