Symptoms
- Fuel lines look swollen, cracked, or soft
- Engine runs rough or stalls after the boat has sat with fuel in it for weeks
- Fuel smells sour or looks separated/cloudy in a clear filter or line
Likely causes
- CommonEthanol degrading older rubber fuel lines not rated for itOlder fuel lines can soften, crack, or swell from prolonged ethanol exposure if not made from ethanol-compatible material.
- CommonPhase separation from moisture absorption in a tank that sitsEthanol attracts water, and if enough accumulates, the fuel can separate into layers, with a water/ethanol layer at the bottom feeding the engine poorly.
- CommonVarnish or gum buildup from ethanol breaking down over timeThis clogs filters and small carburetor passages, especially after months of storage.
- PossibleOld fuel tank or lines not replaced when ethanol fuel became commonSome older boats have components not rated for ethanol blends at all.
What to do
- Inspect fuel lines for cracking, swelling, or a soft, mushy feel, and replace any that show ethanol-related degradation with ethanol-rated marine fuel line.
- Check a clear fuel/water separator filter (if equipped) for cloudiness or a visible water layer at the bottom, which indicates phase separation.
- If fuel has been sitting for months, especially over an off-season, drain and replace it with fresh fuel rather than risk running degraded fuel through the engine.
- Consider using ethanol-free marine fuel where available, since it's less prone to these specific issues.
- Add a marine fuel stabilizer/treatment designed to address ethanol issues if you can't get ethanol-free fuel, especially before storage periods.
- Replace the fuel/water separator filter regularly, and more often if you notice signs of contamination.
Tools and parts
- Basic hand tools
Ethanol-rated marine fuel line and fuel/water separator filterSearch: “ethanol rated marine fuel line and fuel water separator filter for [your outboard motor model]”
When to call a pro
If you suspect phase separation or heavy contamination, have a marine mechanic drain and clean the entire fuel system rather than just replacing a filter.
Safety
Use caution
Fuel system work carries a fire risk; work in a ventilated area away from ignition sources and dispose of old fuel properly rather than pouring it out.