Sorting the fuel delivery chain

A "dies at speed" or "loses power then recovers" complaint is fuel delivery until proven otherwise. Work from tank to engine — the pump is often blamed for a rotten hose's crime.

Quick field tests

1. Primer bulb test: squeeze while the problem happens (or have the customer do it underway, safely). If squeezing restores power, the lift pump is weak or an upstream restriction is starving it.
2. Bulb collapses flat: blockage upstream — clogged tank pickup, kinked line, blocked anti-siphon valve at the tank fitting (a top-three culprit on older boats), or a plugged tank vent (open the fill cap; if the problem stops, it's the vent).
3. Bulb never firms: air leak on the suction side, failed bulb check valves, or a split pickup tube in the tank.
4. Clear-hose test: tee a short clear section before the engine and watch for bubbles underway — bubbles mean a suction-side air leak (fittings, O-rings, primer bulb, water separator seal).

Mechanical lift pumps (diaphragm)

Driven by crankcase pulses on two-strokes and by cam/pulse on many four-strokes. Symptoms of a torn diaphragm: fuel starvation at high rpm, and on two-strokes sometimes fuel dripping from the pump or a cylinder flooding (pulse hole pulls fuel into the crankcase — check that cylinder's plug). Rebuild kits are cheap; follow the service manual's diagrams for check-valve orientation.

EFI: the VST and high-pressure pump

On EFI outboards, the lift pump feeds a vapor separator tank (VST) whose internal high-pressure pump feeds the rail. Symptoms of a failing HP pump: hot-start whine, power fade after long runs, lean codes. Connect a fuel pressure gauge to the rail and compare against the pressure in the service manual — don't guess. The VST float/needle and its filter clog with ethanol debris; cleaning the VST is a routine cure.

Common mistakes

• Replacing pumps when the anti-siphon valve or vent is the restriction
• Ignoring the water separator: drain it into a clear jar first — water and debris there redirect the whole diagnosis
• Testing at idle only; delivery problems show at load

When to walk away

Corroded aluminum tanks shedding oxide, or a foam-filled tank slowly collapsing its pickup, are boatyard jobs — quote accordingly.

Safety: pressurized rails hold pressure after shutdown — relieve per the manual. Fuel work means ventilation, no ignition sources, rags contained, and reconnecting every clamp before startup.