Why is my marine diesel water pump not working, and how can I troubleshoot it?
My diesel normally spits cooling water out with the exhaust, but now there is very little water and the temperature starts climbing. How do I check whether the seawater pump has failed or something is blocked?
Answer
A marine diesel that stops discharging cooling water usually has a blocked seawater intake, a suction-side air leak or a damaged raw-water pump impeller. Check the seacock and strainer first, then the impeller and pump drive; if exhaust water looks normal but the engine overheats, check the separate freshwater cooling circuit.
Identify the failing cooling circuit
Most heat-exchanger-cooled diesels have two pumps. A flexible-impeller pump moves seawater through the heat exchanger and into the exhaust; a circulating pump moves antifreeze coolant around the engine. Older directly seawater-cooled engines use a different arrangement, so identify your engine model before ordering parts.
Little or no exhaust water points to the seawater circuit. Normal exhaust water with rising engine temperature suggests low coolant, a thermostat problem, restricted heat transfer or a circulating-pump fault. A partial seawater restriction is still possible.
Stop the engine if cooling water disappears or the temperature alarm sounds. Let it cool before opening the coolant cap, and close the seacock before disconnecting seawater hoses. Repeated test runs can destroy a dry impeller and overheat the rubber exhaust hose.
Check the intake and suction hose
Work from the hull towards the pump. These checks often reveal the fault without removing the pump.
- Seacock: confirm it is fully open for operation. A moving handle does not prove that a damaged valve opens internally.
- Intake: look for weed, plastic bags, mud or marine growth. With the seacock closed, inspect accessible pipework. Arrange an external inspection if the hull inlet appears obstructed.
- Strainer: clean the basket. Inspect the lid seal and sealing face for damage, and check the housing for cracks. Seat the gasket correctly and tighten the lid as the manufacturer specifies.
- Suction hose: check for loose clips, splits, kinks and internal delamination. A softened hose can collapse under suction even when it looks sound with the engine stopped.
An above-waterline strainer can admit air without leaking water into the bilge. If trouble starts immediately after cleaning the strainer, suspect its lid seal. Trouble after launching can indicate a dry pump or trapped air. Prime through the strainer only if its installation instructions permit it, then refit the lid securely.
Inspect the impeller and drive
Stop the engine, disable starting and close the seacock before removing the seawater pump cover. Photograph the arrangement and retain any spacer or wear plate. Check the rubber impeller for missing blades, cracks, permanently folded blades or separation between the hub and rubber.