What problems does ethanol (E10) fuel cause in outboards?

Ethanol (E10) fuel problems · Symptom: Gummy deposits, clogged filters soon after switching, hard starts.

Quick answer: Most often: e10 absorbs water → phase separation, e10 solvent action loosens tank debris into filters, incompatible old fiberglass tanks/hoses. Start here: Never use above E10 in outboards (Mercury, Honda). If that doesn't fix it, work through the checks below, and see a marine mechanic if you're unsure.

Likely causes (most to least common)

1. E10 absorbs water → phase separation
2. E10 solvent action loosens tank debris into filters
3. Incompatible old fiberglass tanks/hoses

Safe owner checks & fixes

1. Never use above E10 in outboards (Mercury, Honda).
2. Carry spare filters; replace as needed.
3. Add stabilizer; store tank nearly full if vented (or per tank type — Mercury notes low-emission tanks differ).
4. Use ethanol-free fuel where available.
5. Suspected tank incompatibility: contact tank manufacturer.

Parts / tools

Filters, stabilizer, separator.

Call a marine mechanic when

Old fiberglass tank may be E10-incompatible; repeated filter clogging; fuel hose softening or leaking. If you're on the water and unsafe, call TowBoatUS/Sea Tow or the Coast Guard (VHF 16).

Sources

• https://www.mercurymarine.com/us/en/service-and-support/owners-resources/maintenance-made-easy/fuel-basics
• https://cdn.powerequipment.honda.com/marine/pdf/maintenance/Avoiding-Carburetor-Fuel-Problems.pdf
• https://dlnr.hawaii.gov/dobor/boating/e10-gasoline/
• https://boattest.com/article/coping-wethanol-enhanced-gasoline

Values not stated here are model-specific; check your owner's manual.

Related problems

• How do I tell if there's water in my outboard fuel?
• How do I fix a carburetor gummed up after storage?
• How do I winterize an outboard motor?