How can I stop having to prime the engine daily on my Monsun 31?
The diesel on my Hallberg-Rassy Monsun 31 needs priming before the first start each day. Once primed it starts and runs, but the problem returns after standing overnight. Where should I start looking?
Answer
A Monsun 31 diesel should not need daily priming: if bleeding restores easy starting, suspect air entering the fuel supply or fuel draining back while the engine stands. Check the primary fuel filter seals and suction pipework from the tank first, then test the lift pump before fitting extra priming equipment.
Confirm loss of prime
Identify the engine from its plate. A Monsun may still have an older Volvo Penta or a replacement engine. Bleed points, filter arrangements and starting procedures depend on the engine fitted.
Distinguish between working the hand lever and bleeding air from the system. If the correct low-pressure bleed screw releases bubbles before clean diesel appears, that points to air ingress. If no air appears and pumping makes no consistent difference, investigate the engine's cold-start procedure, cranking speed and compression instead.
Record whether the engine starts normally after an hour, whether a low tank level makes the fault worse, and whether it runs cleanly under load. These checks help distinguish an overnight leak from restricted fuel delivery.
Stop the engine before working, catch spilled diesel and keep ignition sources away. Never loosen common-rail injector pipes.
Check filters and suction connections
On boats with the original keel tank, much of the supply system may operate under suction. A joint can admit air without leaking diesel, so dry fittings do not prove the system is airtight.
Follow the supply route from the tank outlet to the engine lift pump:
- Primary filter: inspect the lid seal, bowl seal, centre bolt seal, drain and unused ports. Delphi/CAV-type assemblies have several sealing points. An old seal left in its groove or a misplaced new seal can cause persistent trouble.
- Hoses: check for hardening, cracks, chafe and loose connections, particularly where a hose bends over a tail. Replace suspect sections with marine diesel hose of the correct internal diameter.
- Unions and valves: inspect compression fittings, banjo sealing washers and the tank shut-off valve. Support fittings as you tighten them to avoid twisting the connected pipe.
- Filter history: if the problem began immediately after servicing, check that assembly again before buying a pump.
Use the specified seals and follow the tightening instructions. Overtightening can distort a filter housing or damage a seal. There is no universal torque setting for these different assemblies.
Test the lift pump
A mechanical lift pump normally contains valves that control fuel flow. Worn valves, a damaged diaphragm or a leaking pump seal can cause loss of prime or inadequate delivery.