Why does my marine diesel's water pump keep running?
I can hear a water pump running near the engine compartment after I stop the diesel, even with all the taps shut. Sometimes it runs briefly and stops, then starts again later. How do I tell whether this is an engine cooling problem or a freshwater system fault?
Answer
Your diesel's mechanically driven cooling pumps run whenever the engine turns and stop when it stops; any pump still running afterwards is electrically powered. Identify it first: a domestic freshwater pressure pump usually runs continuously or keeps restarting because it cannot build or hold pressure, while an electric cooling pump must be checked against its installation.
Identify the pump
Most small yacht diesels, including typical Yanmar YM installations, have two engine cooling circuits. A seawater pump draws water through the seacock and strainer; a coolant circulation pump moves antifreeze mixture through the engine and heat exchanger. Older directly cooled engines differ, but their mechanically driven pumps still stop with the engine.
The domestic freshwater pump is separate, though it may sit beside the engine or calorifier. Follow its hoses. One comes from the drinking-water tank; the outlet supplies taps, showers and often the calorifier. Common makes include Jabsco, Shurflo and Whale.
- Runs only with the engine: continuous operation is normal for a mechanically driven cooling pump.
- Runs with the engine stopped and responds to a tap: probably the domestic pressure pump.
- Has electrical wires and hoses connected to the cooling circuit: identify its model and purpose before changing anything.
Use the labelled domestic pump switch to confirm the source of the noise. A pump heard near the diesel does not necessarily belong to it.
Brief bursts with taps shut
A one-second burst every half-hour with every tap shut usually means the domestic system is slowly losing pressure. The pressure switch starts the pump. Pressure recovers and the pump stops again.
Check accessible plumbing first: even a small external leak can eventually empty the tank into the bilge. Dry each connection, then use kitchen paper to find fresh moisture around:
- Tap tails, hose joints and the pump's inlet strainer bowl.
- The transom shower, including its trigger head and mixer.
- The calorifier connections and pressure-relief valve discharge.
- Pipework hidden beneath lockers and berth cushions.
If everything stays dry, water may be leaking backwards through the pump's internal non-return valves into the tank. This causes cycling without leaving water in the bilge. Debris or worn valve parts can be responsible. Inspect or replace the valve assembly following the instructions for that pump model.
Water also contracts as it cools in a hot calorifier, which can cause an occasional pump start. Compare behaviour with the calorifier cold. Repeated cycling throughout the day needs investigation; do not simply put it down to temperature.