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Solved problems · Engine · Crew answer

Why is my sailboat's diesel engine overheating?

Most marine diesel overheating starts in the raw-water circuit: a blocked strainer, a worn impeller, or a clogged exhaust elbow. Check the flow first, then work from the seacock to the exhaust.

Symptom

The temperature gauge climbs or the high-temperature alarm sounds, often under load or after a long motor. There is less water than usual coming out with the exhaust, or more steam.

Likely cause

Loss of raw-water flow is the most common cause: a blocked seacock or strainer, a worn or broken pump impeller, a slipping belt, a collapsing hose, a scaled heat exchanger, or a corroded exhaust mixing elbow. Low coolant on the closed freshwater side or a stuck thermostat can do the same.

Stop now if

The alarm sounds or you see steam. Throttle back and shut down as soon as it is safe (sea room, sails or an anchor ready). Never open a hot coolant cap; the system is pressurized and can scald. Close the raw-water seacock before opening the strainer or pump, and reopen it before restarting.

The fix, in short

  1. Confirm raw-water flow at the exhaust. With the engine idling, check for a steady spurt of water at the exhaust outlet and compare it with what you normally see. Weak, irregular or steamy discharge points to the raw-water side.

  2. Check the seacock, strainer and intake. Engine off and seacock closed: clean the strainer basket and check the lid seal, because an air leak there starves the pump. Then, with the lid off, crack the seacock briefly, then close it. Water should rush in freely; if it doesn't, weed or barnacles may be blocking the through-hull. Refit the lid securely before reopening the seacock to run the engine.

  3. Inspect the raw-water pump impeller. Engine off and seacock closed: remove the pump cover and look for cracked, bent-over or missing vanes. Replace a damaged or old impeller (many engine makers suggest yearly) and check the cam, wear plate and cover for grooves. If vanes are missing, find the pieces; they usually lodge at the heat exchanger inlet.

  4. Check belts, hoses and coolant. Engine off, hands clear of belts and pulleys: if the pump is belt-driven, check belt tension and wear. Check hoses for soft spots or cracks (a soft suction hose can collapse at higher rpm), and with the engine cold, top up coolant to the correct level.

  5. Look for restrictions downstream. If the pump is healthy, suspect a scaled heat exchanger or a corroded exhaust mixing elbow. A rough check: exhaust water should be no more than about 20 °F (11 °C) warmer than the seawater. A bigger difference suggests a restriction; cleaning or replacing these parts is often a job for a marine mechanic.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Raw-water cooling: Seacock, strainer, flexible-impeller pump, heat exchanger, exhaust mixing elbow
  • Engine: Freshwater-cooled marine diesel (any make)

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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