Ethanol (E10) Fuel Care: Stabilizer, Phase Separation, Fuel Lines and Tank Vents

Buy from busy stations or fuel docks that sell a lot of gas. Add marine fuel stabilizer at the rate on the bottle when you fill up, not weeks later. Yamaha recommends treating every tank, not just before storage. For portable gas cans, add stabilizer before you fill them so it mixes, and store them out of the sun.

Read the pump label before you squeeze the handle. Many stations now sell E15 from the same pump as E10, often as a cheaper "88" or "Unleaded 88" grade.

Drain or siphon a few ounces of fuel from the tank (or the bottom of the fuel filter) into a clear glass jar. Let it sit for about 30 minutes. Mercury describes what to look for:

• Clear bubble or layer at the bottom: water.
• Cloudy, jelly-like layer below the gas: phase separation.

Another option is to coat a dowel with water-finding paste and lower it to the bottom of the tank. The paste changes color where it touches water. If you find water or separated fuel, see step 12.

• Older boats with vented tanks: fill with fresh, stabilized fuel before storage to leave less room for damp air. Mercury suggests stopping at about 95% full so the fuel has room to expand and doesn't push out the vent.
• Newer boats with low-emission, non-vented tanks: Mercury says these can usually be stored with a low fuel level, but any fuel left in the tank should be treated with stabilizer. Ask your dealer if you're unsure which you have.
• Portable tanks: run the engine on fresh, stabilized fuel, then use any leftover gas in your car or lawn equipment.

After adding stabilizer, run the engine about 10 minutes so treated fuel reaches the whole fuel system. Run it in the water, in a test tank, or (outboards and sterndrives) on external flush muffs with the water on first; inboards and PWCs use only the flush method in their manual. Never run an outboard on its built-in flush port; that port is only for flushing with the engine off.

Ethanol is a strong solvent and can break down older hoses from the inside. Mercury notes that a line can look fine outside but delaminate inside, shedding bits that clog the engine. Check every fuel hose you can reach:

• Hard, cracked or swollen hose
• Soft, spongy or sticky spots
• Fuel weeping or a gas smell at fittings (run a finger under each joint and look for stains below it)
• Rusty or loose hose clamps
• On built-in tanks, the fill and vent hoses too: BoatUS insurance claims show they can leak fuel and fumes just like the supply line. Hoses should be supported by clips or straps without sharp edges.

Replace damaged hose only with Coast Guard-rated marine fuel hose that is also rated for ethanol, never automotive or garden hose, and use stainless marine clamps. If you don't know how old your fuel lines are, replacing them is cheap insurance.

On outboards with a primer bulb, squeeze it before starting. It should become firm. Yamaha says that if it doesn't firm up, check for a leak, and if there is none, replace the bulb.

Also replace the bulb if it's hard and stiff, sticky, cracked or swollen. BoatUS explains that a stiff bulb usually means the small one-way valves inside are failing, and old rubber can shed bits into the fuel. Don't try to soften an old bulb.

Check the arrow on the bulb: it must point toward the engine. A bulb installed backward stops fuel flow.

A primer bulb that collapses (goes flat) while the engine is running means the engine is pulling against a restriction between the tank and the engine. Common causes, cheapest first:

• Tank vent closed or blocked (step 8)
• Kinked or pinched fuel hose
• Clogged fuel filter
• Blocked or sticking anti-siphon valve (step 7)
• Clogged pickup screen inside the tank

BoatUS uses a similar rule: too much pressure points to a fuel-line connection problem, a vacuum points to a venting problem, and no pressure at all usually means an air leak in the line.

On many boats with built-in tanks, a small valve sits where the fuel line leaves the tank. It stops fuel from siphoning out of the tank and into the bilge if a fuel line breaks, so it's an important safety part. Over time it can stick or partly clog, which starves the engine at higher RPM. Symptoms are a collapsing primer bulb and an engine that loses power or dies at speed but idles fine.

Never remove the anti-siphon valve or drill it out to "fix" fuel flow. If you suspect it, have a mechanic test it and install the correct replacement for your fuel system.

Find the fuel vent fitting on the side of the hull (it usually has a small screen). Boating Magazine notes that salt, corrosion, insects and spiders often clog the screen. Clean it gently with a small brush, or replace the vent if the screen is eaten away.

Signs of a blocked vent: the engine dies after running for a while, you hear a "whoosh" of air when you open the fuel cap, or the tank is hard to fill and fuel spits back. If you can see the vent hose, make sure it has no sag or low loop where fuel can collect and block it.

The vent can also let in water. When washing the boat, point the hose spray aft along the hull so you don't shoot water into the vent.

Modern EPA-compliant portable tanks don't vent freely, so they build pressure in the sun. BoatUS explains that this pressure can push fuel through the outboard and out into the water. Open the cap slowly, away from the engine, to release pressure before connecting the line. A fuel-demand valve installed between the tank and the primer bulb holds that pressure back and only lets fuel through when the engine is running. Older tanks with a manual vent screw must have the vent opened before you run the engine and closed for transport.

A 10-micron water-separating filter between the tank and the engine is your last line of defense. Yamaha recommends changing its external 10-micron filter every 50 hours and Mercury says many filters are changed every 100 hours or yearly. Change it sooner after a bad tank of fuel. Typical; confirm in your manual. Carry a spare. See the fuel-water separator guide.

Use a marine stabilizer for every fill and for storage. Some makers also sell fuel-system cleaners for periodic use during the season. Don't pour in "water remover" additives expecting them to fix a tank of separated fuel; they can't. Use the dose on the bottle: Westerbeke reminds owners that a bigger dose doesn't mean better protection, which is a good rule for every engine. Don't stack several different additives in one tank.

Don't try to burn it off or dilute it with fresh gas; adding fresh fuel to a half-tank does not reverse phase separation. The contaminated fuel needs to be pumped out and disposed of properly, the filters replaced, and the system checked. On built-in tanks this is a job for a dealer or mobile fuel service. For a portable tank, empty it into an approved container, take it to a hazardous waste collection site, rinse the tank with a little fresh gas, and refill.

Mercury also advises discarding gas more than a year old, even if it was stabilized.