How to Inspect Your Boat's Fuel Tank, Fill, Vent and Hoses

Open every hatch and the engine compartment and sniff before you do anything else. A gas smell in a closed bilge is a stop sign (see the safety box). A faint smell right after fueling that quickly clears may be a spill at the fill; a smell that comes back on its own is a leak until proven otherwise. Run the bilge blower for at least four minutes before starting the engine any time the boat has been closed up (see the bilge blower guide).

• Look for cracks, swelling, and soft or bulging sides, especially near the fittings.
• Check the cap seal and the vent: the vent must be open when the engine runs and closed for storage.
• Newer tanks may have no vent screw. Federal evaporative rules (40 CFR 1045.112) have applied to new portable marine tanks since 2010–2011, so many current tanks use a self-sealing cap or automatic vent valve. These tanks can look slightly swollen on a hot day or pulled in on a cold one. Open the cap slowly, away from the engine, to let pressure out. Never drill a vent hole or replace the cap with an old-style one; follow the tank maker's instructions. A tank that stays badly deformed, cracks or won't feed fuel should be replaced.
• Check the fuel line connector at the tank and the O-ring inside it. A worn O-ring can suck air (engine stumbles) or leak. Quick-disconnect fittings are brand-specific (Mercury, Yamaha, OMC-style and others), so buy replacements that match your engine and tank, and keep the dust caps on when the line is unplugged.
• Store portable tanks out of the boat, in a dry, ventilated place out of the sun, when you are not using it. BoatUS notes rain often finds its way in through the cap if the tank stays aboard.
• Never fill a portable tank while it is in the boat: take it out and set it on the ground.

Mercury suggests draining or siphoning a few ounces into a clean glass jar and letting it sit for about 30 minutes. Water shows as a clear bubble or layer on the bottom; phase-separated ethanol fuel shows as a cloudy, jelly-like layer. BoatUS calls a fuel sample an important first step in judging a tank: any water means you need to find out how it got in (steps 6 to 9). Do this outside the boat, and pour the sample into an approved container for disposal.

Built-in tanks are usually under a deck hatch or floor plate. Open what you can reach without cutting anything. Many boats only show the top of the tank, with the sending unit, fill, vent and fuel pickup fittings. Use the flashlight, mirror and your phone camera to see down the sides.

The sending unit is the round plate with a wire that measures the fuel level for your gauge. Look for:

• Fuel stains, wetness or a gas smell around the sender gasket or any fitting.
• Water sitting on top of the tank. BoatUS warns that water pooling on the tank top finds its way in through the sender gasket. Fix leaky hatches and deck plates above it.
• A dished (concave) top on plastic tanks that holds water and keeps the sender from sealing.
• Loose sender screws. If you ever remove the sender, mark its position first; most have an uneven hole pattern.

Gauge reading empty with fuel in the tank? Owners often chase wiring first, but a common cause is a sender float stuck at the bottom, sometimes after the tank was drained for storage; it can free itself once the boat is moving again. A replacement sender that doesn't match the gauge also reads wrong, because US and European senders use different resistance ranges. Checking the float means taking the sender out of the tank, which opens the tank to gasoline vapor: leave that to a technician unless you know how to do it with the area ventilated and nothing that can spark nearby.

Aluminum tanks usually fail from the outside, where water stays in contact with the metal. On many boats, foam was poured around the tank during building; moisture trapped between the foam and the tank can corrode right through it over time, as BoatUS explains. Look for:

• White, powdery corrosion, blisters or pitting, especially along the bottom edges and under straps or pads.
• Wet or waterlogged foam around the tank, or a bilge that never dries.
• Fuel stains or a sheen on bilge water.

You often cannot see the bottom of a foamed-in tank. If you find corrosion on what you can see, assume the hidden part is worse. Aluminum tanks are rarely repairable: a corroded, leaking tank is replaced.

Mercury warns that fiberglass fuel tanks built before 1991 may not be compatible with gasoline containing ethanol: some resins can be drawn out of the fiberglass and carried into the engine, where they can cause severe damage. If your boat has an old fiberglass tank, check with the boat or tank maker whether ethanol fuel is safe in it. Signs of trouble include a soft or blistered tank, dark sticky deposits in filters, and an engine with gummed-up intake valves. Mercury also advises checking all fuel system parts on engines from 1990 or earlier often for softening, swelling or leaks.

• Open the deck fill cap and check its O-ring. A dry, cracked O-ring lets rain and washdown water in. It is a cheap, easy replacement.
• Look at where the fill sits: water should not pool around it.
• Follow the fill hose to the tank. It should be firm, not cracked or mushy, and secured at each end with two metal hose clamps (the federal rule for fill hoses). The fill fitting should also be bonded with a ground wire on metal fills, to prevent static sparks while fueling.

The vent lets the tank breathe. On older boats it is a hose from the tank to a small fitting on the side of the hull, with a flame screen inside. Federal rules require that screen (a flame arrestor) to be cleanable.

• Make sure the vent fitting is not blocked by paint, wax, mud daubers or bugs. A blocked vent can starve the engine or let pressure build.
• The vent hose should have a gentle loop or rise with no low sag where water collects, and no cracks.
• If heavy seas push water into a side vent, BoatUS suggests a cover that faces aft.
• Plugging the vent for winter? Makers disagree, so follow your own boat's manual. Yamaha says never plug or cap the tank vent during storage, because the tank must breathe as temperatures change, and suggests storing the tank about 7/8 full. MasterCraft allows sealing the vent over winter on its boats, but only with the tank no more than 90 to 95% full, and the seal must come off before the first fill-up in spring.

Newer boats with EPA low-emission fuel systems may have a carbon canister and a different vent setup; do not alter it.

Follow the fuel line from the tank to the engine, through the anti-siphon valve and the water-separating filter.

• Federal rules call for hose marked USCG Type A1 (or A2/B in certain locations) on built-in gasoline systems. Read the printing on the hose.
• Wipe each hose with a dry rag and sniff it: BoatUS notes a fuel smell on the rag means the hose is breaking down. Ethanol fuel tends to make old hose soft.
• Squeeze the hoses: they should be firm and flexible, not hard, cracked, swollen or sticky.
• Clamps should be stainless, tight and placed over the fitting's barbs, not on bare hose. Clamps and straps must not cut into the hose.
• Check every joint for stains or wetness.

Replacing hoses on a built-in system is best left to a qualified technician, who will use the right hose and fittings.

A built-in tank must not move. Push on it with your hand: it should be firmly strapped or chocked, with non-absorbent pads between the tank and its supports. Straps sitting directly on aluminum, or wet carpet or wood pads, hold moisture against the metal and start corrosion.

• Small fixes you can do: fill cap O-ring, cleaning the vent screen, storing portable tanks properly, fixing leaky hatches above the tank.
• For a technician: any fuel leak, corroded aluminum, a swollen or damaged plastic tank, an old fiberglass tank with ethanol questions, and hose replacement. A technician can drain the tank and do a low-pressure air test to confirm a leak.
• Tank replacement is a pro job. Many tanks cannot come out without cutting the deck or floor. BoatUS notes the repair on an older boat can sometimes cost more than the boat is worth, so get a quote before you buy a used boat with a suspected tank problem.