Yanmar diesel engine cooling problems can be isolated with little difficulty. The engine's cooling system is made up of a freshwater loop and a raw-water circuit. Clogs can form in the cooling hoses, heater exchangers or in the raw-water strainer, mixing elbow or muffler. A bad water pump or thermostat will reduce water flow as well. Any restriction at any of these points will cause overheating.
Open the seacock. Remove the hose clamp and hose from the output nipple of the raw-water pump. Connect the heater hose to the nipple and tighten the hose clamp.
Start the engine and run it for 30 seconds with the heater hose led into the bucket. Turn the engine off and gauge the amount of water in the bucket. The pump should be putting out 4 gallons per minute (gpm) at 1,000rpm (revolutions per minute) and eight gpm at 2,000rpm. Failure to develop proper flow at this point indicates a clogged raw-water strainer, clogged seacock, loose water pump drive belt or a bad water pump impeller. Reconnect the cooling hose to the output nipple of the raw-water pump and tighten the hose clamp with a screwdriver.
Remove the hose clamp and hose from the raw-water discharge nipple on the heat exchanger. Install the heater hose on the nipple and tighten the hose clamp with a screwdriver.
Start the engine and run it for 30 seconds with the heater hose led to the bucket. The discharge rate should be about the same as it was for the pump/strainer test. A significantly lower discharge rate indicates a restriction in the heat exchanger or the associated hoses. Reconnect the raw-water discharge hose to the nipple and tighten the hose clamp with a screwdriver.
Place the bucket under the engine exhaust port on the exterior of the boat. Start the engine and run it for 30 seconds. Shut the engine off and gauge the amount of water in the bucket. The amount should be about the same as with the previous tests. A significantly reduced amount indicates a restriction in the mixing elbow or the muffler.
Remove the thermostat housing with a wrench. Remove the thermostat and inspect the area for debris or mineral deposits that may be causing a restriction. Excessive deposits indicate a need for a freshwater system flush according to factory recommendations.
Test the thermostat. Heat a pot of water to 90 degrees Celsius, then place the thermostat in the water. Soak the thermostat for one minute and scoop it out of the water with a metal spoon. Verify that the thermostat is fully open as soon as it comes out of the hot water. The thermostat valve should lift 4mm (0.3149 inch) when fully open. Less than this indicates a bad thermostat.
Inspect the freshwater pump belt tension. A loose belt casues slippage that results in reduced circulation of the fresh water and overheating.
Remove the freshwater pump from the engine with a wrench. Remove the pump cover with a wrench. Inspect the impeller inside the pump. Breaks, tears and erosion causes the impeller to lose effectiveness and reduce freshwater circulation.
Inspect the contents of the freshwater sub tank for traces of engine oil. Pull the engine oil dipstick and inspect it for traces of water in the oil. Water in the oil will result in oil that looks milky or has the colour and consistency of peanut butter. Either condition will indicate an internally blown head gasket that will require a top-end rebuild to rectify.