Fuel Pump Checks - Is It the Pump or Something Starving It?

Draw a sample from the water-separating filter or the tank into a clean glass jar. Look for water at the bottom, cloudiness, debris or a dark, varnish smell. Mercury explains that once ethanol fuel phase-separates, no additive will mix it back; the water must be removed, the fuel disposed of properly and the tank started fresh. Boating Magazine printed the story of a master mechanic who rebuilt half his engine, including the fuel pump, before a tank of fresh gas cured the problem. Check the fuel before you buy parts.

A tank can't feed the engine if air can't get in to replace the fuel. Salt Water Sportsman notes that an engine that dies, runs sluggishly or slows down on its own may have a blocked vent; if you hear a rush of air when you open the fill cap, the vent is likely clogged. Boating Magazine adds that vent screens collect salt, grime and insects, that a sagging vent hose can trap fuel and block the vent, and that an in-line no-spill valve can stick with gummy residue (if you shake it and the ball inside doesn't rattle, replace it).

On portable tanks, make sure the vent screw on the cap is open. Test: run the engine with the fill cap loosened (on the water, or an outboard in a test tank or on external flush muffs, never on the built-in flush port; in a safe, ventilated place, never while refueling). If the problem disappears, it's the vent.

Look along the whole line from tank to engine. You're looking for cracks, kinks, chafed spots, hoses that feel soft or swollen, and loose clamps. Salt Water Sportsman points out that a loose fitting, a split line or a cracked primer bulb lets air in and interrupts fuel flow. Check the quick-connect fittings at the tank and engine; worn O-rings there are a common air leak. Older hoses may not be ethanol-resistant: Mercury suggests replacing fuel system rubber parts from the 1980s or earlier with ethanol-safe parts.

The primer bulb is a free diagnostic tool on boats that have one:

• It never gets firm when you squeeze it before starting: Yamaha's advice is to look for a fuel leak, and if there is none, replace the bulb. The check valves inside may be worn.
• It goes soft or gets sucked flat while the engine runs: Boating Magazine says a bulb should stay firm while the engine is running and drawing fuel. A bulb that flattens means the pump is pulling against a restriction between the bulb and the tank: a blocked vent, a sticking anti-siphon valve, a plugged tank pickup, a kinked hose or a collapsed inner hose liner.
• It stays firm but the engine still starves: the restriction or fault is more likely downstream (filters, the pump, or the carburetor or injection system).

Don't keep squeezing the bulb while the engine runs as a way to get home faster. Some owner's manuals warn that squeezing it with the engine running or tilted up can make fuel overflow, so check yours. If the engine runs only while you pump the bulb, slow down and head in, or call for a tow.

Most boats with a built-in tank have an anti-siphon valve where the fuel line leaves the tank. It stops fuel from siphoning out if a line breaks, so it is a safety part. A valve that sticks partly closed is a classic cause of starving at high RPM. Have it inspected or replaced with the correct part; never drill it out or remove it to "fix" the problem.

A partly clogged filter is the most common restriction of all. Yamaha recommends replacing its 10-micron external fuel/water separating filter every 50 hours or after a bad tank of gas, and warns not to dump a filter and reinstall it. Check the small on-engine filter too (often a clear cup on the powerhead), and replace it if you see water or debris. See the fuel-water-separator guide.

Don't add extra fine filters "for protection." Mercury's FAQ explains that another filter is another restriction that can starve the engine, and that the factory filtration is already designed for the engine.

Air left in the line can look like a pump problem. Mercury's quick tip says to squeeze the primer bulb until it is firm on smaller engines with a bulb, and on its FourStroke models of 75hp and up to turn the key to "run" and wait while the onboard electric pump primes the system. Salt Water Sportsman notes that many modern engines prime when you turn the key to the first position and wait about 30 seconds. Check your owner's manual for your engine's method.

With the boat in a quiet spot, have a helper turn the key to "on" without cranking. Mercury describes hearing a beep and then the electric fuel pump running for a few seconds. On many engines you'll hear a short hum from the engine.

• You hear it: the pump has power and is turning. That does not prove it makes enough pressure under load, but it rules out a dead pump or blown fuse.
• You hear nothing (and no key-on beep either): first make sure the kill-switch lanyard clip is in place and the switch itself isn't faulty, since on many engines the computer doesn't wake up without it. Then check the fuses and relays your manual lists for the fuel pump, the battery voltage, and the engine harness connectors. On many EFI outboards the pump gets its power through a relay and the engine computer switches it on, so a test light at the pump plug can read dead even when the pump is fine; your service manual shows where to test. If those are fine, the pump, its wiring or the engine computer needs a dealer diagnosis. Replace a blown fuse only with the same rating; if it blows again, stop and call a pro.
• Cheap relay check: many fuel-injected sterndrives and outboards have a small relay block with several identical relays. If your manual shows the fuel pump relay shares a part number with another one (horn, trim or a spare), swap them with the key off and try again. Owners regularly find a "dead" pump was a corroded relay or a loose relay socket. A relay that works again after swapping may just have had dirty contacts, so replace it rather than trust it.
• Twin engines: compare the two at key-on. One pump that sounds noticeably weaker or shorter than the other is worth mentioning to your mechanic, even if that engine still starts.

On fuel-injected outboards, BoatTest explains that the computer runs the lift pump to fill the VST and logs a "lift pump time out" type fault when the VST does not fill, which usually means no fuel is reaching the engine from the boat. That points you back to steps 1 to 6.

With the engine off and cool:

• Look for fuel wetness or stains around the pump body and fittings. Any leak means the pump or its gaskets need service.
• On pulse pumps (many two-strokes), look at the small pulse hose from the crankcase to the pump. A cracked, loose or oil-soaked pulse hose weakens the pump. Fuel inside that hose can mean a ruptured diaphragm.
• On four-stroke engines with a cam-driven pump, a torn diaphragm can let gasoline into the crankcase. A rising oil level that smells of fuel is a warning sign (see the oil-level-rising guide).

If everything upstream checks out and the problem persists, run the boat (or an outboard in a test tank) and note exactly when it happens: how many minutes, what RPM, and whether the primer bulb collapses. Ashore, an outboard needs a test tank or flush muffs, and the built-in flush port is for engine-off flushing on most models, so don't run on it unless your manual allows it. Most fuel-starvation problems only show up at higher RPM under load, so a run on muffs at idle may not reproduce it. That's useful information for your mechanic.

This is the point where testing moves beyond looks and listening:

• Vacuum (restriction) test: a vacuum gauge is placed in the suction side of the fuel system, between the tank and the pump. Power & Motoryacht explains the idea: compare the reading against a known clean baseline, and rising vacuum means a growing restriction such as a clogging filter. Your engine's service manual gives the maximum vacuum allowed and where to connect the gauge.
• Pressure test, mechanical pumps: a gauge on the pump outlet shows whether the pump makes the pressure the service manual specifies at a given RPM.
• Pressure test, EFI: the fuel rail operates at high pressure. Testing needs a gauge that fits the engine's test port, the factory pressure spec for key-on, idle and load, and the safety procedure for relieving pressure. On EFI engines this is a dealer job, often combined with a diagnostic scan.

Without the spec for your exact engine, a gauge reading doesn't tell you much. That's why this guide does not give pressure numbers.

• Fuel, vent, lines, bulb, anti-siphon valve and filters all good, but the bulb still collapses: look for a restriction you haven't found yet, such as the tank pickup screen or a fuel selector valve on boats with two tanks.
• Bulb stays firm, filters new, and the engine still starves at speed: the engine-side pump, the VST or the carburetor or injection system is the likely cause. Book a pressure test.
• External fuel leak at the pump or fittings: don't run the engine. Have it repaired.