By Sam Halberstadt · Reviewed by Marina Chen
Editor · USCG-licensed Master 50 GT · Updated May 6, 2026
What ethanol does to a fuel system
Ethanol is hygroscopic — it absorbs water from humid air, even through a sealed fuel tank vent. Over 30–90 days of storage, fuel can pick up enough water to saturate.
Once saturated (around 0.5% water by volume in E10), the ethanol-water mixture phase-separates: it drops to the bottom of the tank as a corrosive layer of water-laden alcohol, while the gasoline floats on top.
The bottom layer wrecks carburetor jets, injectors, and fuel-pump diaphragms. It also won't combust — the engine starves and dies.
Ethanol is also a solvent. It dissolves varnish and gum from older fuel system surfaces, then deposits the residue on jets and injectors. Cleans your tank, clogs your engine.
Prevention
Keep the tank 90%+ full to minimize air space and condensation. A full tank can't take on much water vapor.
Use a marine ethanol-treatment additive (Sta-Bil Marine 360, Star Tron Enzyme, ValvTect) every fill. These keep small amounts of water in suspension so the engine burns it harmlessly.
Buy ethanol-free (E0) fuel where available. pure-gas.org maintains a US-wide directory.
Burn through stored fuel within 30 days untreated, 90 days with stabilizer. Drain and dispose of older fuel — don't try to burn it.
Materials that ethanol attacks (and what to upgrade)
Older fuel hoses (pre-2009) are not ethanol-rated and will swell, harden, and crack from inside. Upgrade to USCG Type A1-15 or B1-15 hose, which is rated for E15 and below.
Fiberglass fuel tanks built before ~1990 used resins that ethanol dissolves. The dissolved resin then re-deposits in the engine as a black tar that destroys valves. If you have a vintage fiberglass tank, get it tested or replace it before running E10.
Brass and aluminum carb components corrode in the presence of phase-separated water-ethanol layer. Mercury, Yamaha, and Honda all redesigned carb internals around 2010 specifically for ethanol tolerance — older powerheads benefit from a carb rebuild with updated jets and gaskets.
How to recognize phase separation
Engine runs fine then suddenly dies. Won't restart. Carburetor bowl drains a watery, milky fluid instead of clean gasoline.
Fuel-water separator filter fills with clear water at the bottom (drain it routinely — Racor and similar separators are cheap insurance).
Injectors clog or run lean codes. Diagnostic: drain a sample from the lowest point in the fuel system into a clear glass jar. Phase-separated fuel shows a distinct water layer on the bottom.
What to do if it's happened
Drain the entire tank into approved containers. Dispose at a hazardous-waste facility.
Flush the fuel lines and replace the fuel filter and water separator.
Carburetors: drain the bowls, possibly remove and ultrasonic-clean the jets. Severe cases require carb rebuild.
Fuel-injected engines: pump fresh fuel through the lines, replace the inline filter, run injector cleaner through. Severe cases require injector replacement.
Frequently asked
30 days untreated, 90 days with marine stabilizer. Beyond that, drain or use only after testing for phase separation.
