How to Test a Marine Diesel Lift Pump (Feed Pump): Flow Test, Vacuum Gauge and Warning Signs

With the engine off and cool, follow the fuel hose from the primary filter to the engine. On a mechanical engine the lift pump sits on the side of the block with a hose in and a hose or pipe out to the secondary filter; look for the priming lever. On an electric system, find the pump in the line and its two wires. Mark the inlet and outlet and take photos before you open anything.

• Fuel in the tank (don't trust a fuel gauge on its own).
• Fuel valve at the tank fully open.
• Primary filter bowl free of water and sludge (see the fuel water separator guide).
• Filter elements not overdue.
• Hose clamps tight on the suction side. Air can only get in where there's suction, between the tank and the lift pump, as D'Antonio points out, so a loose clamp there lets air in without dripping fuel.

Work the hand lever on a mechanical pump. A good pump usually gives a firm stroke with some resistance as it moves fuel. If the lever moves only a little or feels dead, the cam inside may be holding it up. Cruising World and BoatUS both describe the fix: turn the engine over a little, by hand or with a very brief bump of the starter (seacock closed), and try again. A lever that moves with no resistance at all, every time, can mean an empty system, an air leak upstream, or a failed diaphragm.

In a Practical Sailor owner's account, the clues to a worn-out lift pump on a 29-year-old engine were a sluggish priming lever with very little resistance that took ages to fill the secondary filter, and old filters that came out clean, which told him the filters weren't the cause. Fresh filters are still a sensible first step, but if the old ones look clean, look harder at the pump.

Loosen the bleed screw on the secondary (engine) filter about a turn, with a pad underneath, and work the priming lever (or turn the key on for an electric pump). After some froth, you should get a steady, bubble-free stream of fuel, as Cruising World describes for bleeding. Tighten the screw while fuel is still flowing.

• Steady fuel, no bubbles: the pump can at least prime, and the suction side is reasonably tight.
• Endless bubbles: air is getting in on the suction side. Recheck clamps, the primary filter lid and seal, and fittings.
• Nothing at all: blockage, closed valve, empty tank, or a dead pump. Go to the flow test.

A flow test shows how much fuel the pump actually delivers, separate from the rest of the system. With the engine off:

• Close the raw-water seacock and hang a tag on the key (you may crank the engine).
• Put pads under the pump and secondary filter.
• Disconnect the pump's outlet line (or the inlet to the secondary filter) and fit a short clear hose that runs into your clear container. Keep the container steady and below the outlet.

• Mechanical pump, hand lever: pump the lever with steady strokes and watch the hose.
• Mechanical pump, engine-driven: crank the engine in short bursts as your manual allows; it can't start because fuel is going into the jar, not the injection pump.
• Electric pump: turn the key on (or use the priming switch if your engine has one).

You're looking for a firm, steady flow with no air. Some engine manuals or service manuals give a delivery rate (an amount in a set time); if yours does, time it. If not, compare with what you saw when the engine ran well, or with a mechanic's judgment. A weak dribble, a flow full of bubbles, or flow that fades out points to a restriction on the suction side or a worn pump. Reconnect the line and tighten it, clean up, and reopen the seacock before the engine is run.

If flow is weak, find out whether the pump or the supply is to blame. D'Antonio describes feeding the pump from a separate fuel supply as one way around a clogged pump screen. With a short hose from the pump's inlet into a container of clean diesel, repeat the test. Strong flow now means the problem is upstream: tank pickup, fuel valve, hose, primary filter or an air leak. Still weak means the pump (or its own filter screen, which D'Antonio notes some lift pumps have) is the problem. Keep the container stable and below anything hot.

Many boats have a vacuum gauge on the outlet of the primary filter, measuring how hard the lift pump has to pull. Power & Motoryacht describes how to use one: note the reading with a new filter element at cruising speed as your baseline, then watch for it to rise as the filter clogs. D'Antonio adds that if the vacuum climbs while you crank, there's a restriction on the suction side, so check and change the element first; if it stays low (he uses roughly 3 inches of mercury as the line) the restriction isn't between the tank and the pump. A low reading together with a weak flow test points at the pump itself or an air leak. Your filter or engine maker may give its own change-out reading.

If an electric pump is silent or weak, check its power before replacing it. With the key on, measure the voltage at the pump's terminals with a multimeter and compare it with the battery voltage; a big difference means a bad connection, ground, fuse or relay rather than a bad pump. Listen for the pump ticking or humming. Clean and tighten corroded connections (see the wiring and corrosion guides). Don't hot-wire it permanently; Cruising World mentions temporary hot-wiring for bleeding on engines whose pump only runs with the engine, and a proper priming switch is the safer long-term answer.

A mechanical lift pump has a diaphragm with the fuel on one side and the engine's crankcase on the other. In Good Old Boat, a Yanmar owner describes a leaking lift pump letting diesel into the oil, with three signs: the oil level rising on the dipstick, lower oil pressure (especially at idle when warm), and an oil analysis showing fuel. His Yanmar dealer called the lift pump the most likely source, ahead of the injection pump and worn rings. If your oil level is rising or the oil smells of diesel, see the rising oil level guide, send an oil sample, and don't run the engine more than you have to until it's fixed.

Lift pumps are usually a bolt-on part. Use the pump your engine maker specifies, new gaskets or washers, and the orientation of the old one (take photos). On cam-driven pumps, the lever must sit correctly on the cam; follow the manual. Bleed the system afterward, run the engine, and check every fitting for drips and the oil level after the first few hours.

Record the flow test, vacuum readings and filter changes in BoatMend. A baseline from a healthy engine makes the next test much easier to read.