Fuel Smell on Board: What to Do and How to Find the Source

• Engine running? If the smell is strong, shut down the engine and any generator.
• Do not touch the ignition, bilge pump switch, lights, radio or battery switch if they are in the area that smells. Only switch off the main battery if the switch is well away from the fumes.
• Everyone puts on a life jacket. No smoking, no flames.
• Open hatches, doors and ports to let air flow.
• At the dock, get everyone off. On the water, if the smell is strong or you see fuel, prepare to abandon the boat and call for help on VHF 16 or 911.

Only look for the source if the smell is faint, there is no liquid fuel in the bilge, and the boat has aired out. If the smell stays strong after airing out, or keeps coming back each time you open a hatch, assume there is an active leak. Stop here and call a marine mechanic or have the boat towed.

The smell's timing is your best clue:

• Right after fueling: overfill, a spill on deck, fuel burping out of the vent, or a leak in the fill or vent hose.
• After a hot day: fuel expands. BoatUS says fuel pumped from a cool underground tank can expand by as much as 10 percent as it warms, pushing fuel out of a full tank's vent.
• Only with the engine running: a leak on the engine side (pump, filter, carburetor, fuel rail, hoses).
• Every time you open the boat: a slow, constant leak or old hose letting vapor through.
• Engine oil smells like gas: see Oil Level Rising.

With a flashlight switched on before you lean in (or a light rated for hazardous areas), look into the bilge, under the tank and around the engine. Look for:

• A rainbow sheen on bilge water
• Wet or stained spots, drips at fittings, or white crusty residue
• Wet fuel on the deck near the fill

Do not run the bilge pump if there is fuel in the bilge. It pumps fuel overboard and the pump or switch can spark. Fuel that leaves a sheen on the water must be reported to the National Response Center at 1-800-424-8802. See Bilge Oil and Fuel Spill Cleanup.

On outboard boats, portable tanks cause many fuel smells. Check:

• The cap seal and vent. A tank left in the sun with a closed vent builds pressure and can seep at the cap and fittings. On older tanks with a manual vent screw, open it before running and close it for storage. BoatUS notes newer EPA-compliant portable tanks have a self-sealing vent that opens and closes on its own, so there is no screw to forget.
• The quick connectors at both ends; a torn O-ring leaks.
• The primer bulb and hose for cracks, soft spots or wetness. Yamaha's check: if the bulb never gets firm when you squeeze it before starting, look for a leak in the fuel system; if there is none, replace the bulb. BoatUS adds that a bulb that has gone hard or stiff is aging inside too: its one-way valves wear out, and old bulbs and hoses can leak vapor even when you see no liquid. Replace it with a quality bulb (or a full fuel-line kit) and fit it with the arrow pointing toward the engine. Never start a siphon by mouth.
• Where the tank sits. Never stow a portable tank in a closed cabin or compartment.

See Portable Fuel Storage.

These are out of sight and often forgotten. BoatUS surveyors report finding fill hoses so cracked they pulled apart by hand, with hose clamps rusted to paste. Open the access panels and check:

• The fill hose from the deck fill to the tank: cracks, softness, loose or rusted clamps
• The vent hose from the tank to the hull vent: kinks, cracks, loose ends
• The deck fill cap O-ring

A cracked fill hose often leaks only when the tank is full or the boat heels.

Wipe each fuel hose with a clean, dry rag, then smell the rag outside the boat. BoatUS says a rag that smells of gas means the hose is breaking down and needs replacing. Hoses also stain, swell or go hard with age. BoatUS also notes that the general rule is to replace rubber fuel hoses older than about 10 years, no matter how they look. See Fuel Line Replacement.

Look at everything on top of the tank: the fill and vent fittings, the fuel pickup and anti-siphon valve, and the fuel gauge sender. The sender gasket is a common slow leak. BoatUS notes the same gasket lets water into the tank when rain or washdown water pools on top of it, and that the top of an old plastic (poly) tank can sag into a dish that keeps the sender from sealing. Use a mirror to see the back and sides. If a cheap anti-siphon valve at the tank outlet seems to starve the engine, BoatUS warns not to pull the check valve out of it: that valve keeps fuel from siphoning into the bilge if a line fails. Replace it with a quality valve instead. On aluminum tanks, look for white powder, pitting or blistered paint where the tank touches wet foam or chocks. A weeping tank must be replaced, not patched. See Fuel Tank Inspection and Fuel Gauge Sender.

With the engine off and cool:

• Fuel filter or water separator: a weep at the gasket, a loose filter, or an old gasket stuck under the new one
• Carburetor: fuel dripping from the carb or a strong raw-fuel smell after shutting down can mean a stuck float or needle. See Carburetor Cleaning.
• Fuel pump and hoses on the engine. On outboards, take the cowl off and look for shiny, wet fuel lines and sniff for raw gas. BoatUS reports that about one in ten outboard fires starts with a fuel leak, most often a hose clamp that vibration has loosened. Tighten loose clamps and replace the small spring-clip type if it is damaged.
• EFI fuel rail, injectors and vapor separator: these run at high pressure. Look only; do not loosen anything. See EFI Vapor Separator.

Generators have their own fuel lines, filters and pumps, often tucked away in a corner of the engine room. Check them the same way.

Replacement fuel hose must be the right USCG-rated type for its location, and clamps and fittings must meet the boat's original standards. Never use automotive or garden hose, tape or sealant on a fuel leak. Anything involving the tank, EFI system, or work inside a closed engine compartment is a job for a qualified marine technician.

After any repair, air the boat out, run an ignition-protected marine blower for at least 4 minutes on inboards and sterndrives (BoatUS and Discover Boating both use this guideline), then do a sniff test in the bilge and engine compartment before turning the key. If you still smell gas, do not start. See Bilge Blower and Ventilation.

BoatUS recommends a gasoline vapor detector for boats with enclosed spaces, mounted low in the bilge but above normal bilge water, because vapor settles low. It is an extra layer, not a replacement for your nose and regular inspections.