Learning to understand your marine diesel engine cooling system is a necessary part of basic marine diesel engine maintenance.

Raw Water Cooling

A flexible impeller pump provides an efficient solution to most raw water pumping needs. The primary advantage of flexible impeller pumps is that they are self-priming, which means that when the vanes of the impeller are depressed and rebound, they create their own vacuum, drawing fluid into the pump. A dry pump can lift water up to as much as three meters. Thus a flexible impeller pump being used for engine cooling does not need to be manually primed or located below the water line. An added feature of a flexible impeller pump is that it can pass fairly large solids without clogging or damaging the pump. This reduces the need for filtration of incoming fluids.
For general or fresh water applications, a standard long lasting neoprene rubber impeller is used.

A general feature of all flexible impeller pumps is that they cannot be permitted to run dry for more than 30 seconds. Both the impeller and the seals require water for lubrication and will soon burn out if run dry. Parts that start to show wear are easily replaceable and service kits are readily available for most models.

Fresh Water Cooling

For circulation of the internal, closed, fresh water circuit of the cooling system it is common to use a flexible rubber pump if it is located on the cold side of the system (max. 55°C). Other types of belt-driven centrifugal pumps are also used. The closed circuit normally transfers heat from the engine to the heat exchanger. The liquid used is water and anti-freeze.

Cooling Capacity

The required output of the cooling pump is related to engine type and size, not to the size of the heat exchanger and exhaust system. This is true for both raw water as well as fresh water handling systems.

Temperature Regulators (Thermostats)

Thermostats are usually placed in the outlet at the top of the cylinder head to prevent the coolant from moving to the header tank until the marine engine has nearly reached operating temperature.

There are different types of thermostats, the most common being the wax pellet type. The capsule on the lower part of the thermostat has a mixture of wax and copper (to increase the thermal conductivity) sealed in it. As the coolant temperature increases, the wax expands and forces a rod to open the poppet valve at the top of the thermostat, which allows the coolant to circulate.

Cooling system checks

  • To test your thermostat, boil a pot of water and drop in the thermostat. (The water must be 100°C–the thermostat usually opens at 85°C.) If the thermostat opens it is okay. If it doesn’t open, replace or clean carefully as they can become sticky with deposits. Yanmar thermostats can and should be regularly serviced. Some thermostats cannot be serviced.
  • If the thermostat doesn’t work, do not remove it and run the engine without it, as the engine will run cold and tight. You can drill a series of 1/4″ holes to give equivalent flow to an open thermostat. This will get you home, but you must then replace it. Be careful not to fit thermostat upside down.
  • Thermostat housings often corrode and need to be replaced. Some can be fabricated.
  • The cooling system should be checked after 100 hours running, or at least once each season, for leakage, deposits, etc.
  • The thermostat can be taken out of the housing on the front of the engine.
  • The heat exchanger core should be removed bi-annually for cleaning and inspection.
  • Many heat exchangers are fitted with anodes to protect the expensive core. Check regularly.
  • Check all hoses and clamps regularly.

Replacing the sea-water pump impeller

The pump impeller is made of neoprene rubber and this can be damaged in the case of water deficiency if, for example, the sea-water intake should be blocked. The pump impeller is changed as follows:

  1. Remove the cover from the sea-water pump. Note that there is the risk of water getting into the boat. With the help of two screwdrivers pull the shaft with the pump impeller out of the housing as far as necessary to reach the bolt retaining the impeller. Place some kind of protection under the screwdrivers in order not to damage the impeller housing. Alternatively, using channel-lock pliers, slide jaws between blades of impeller, rotate and withdraw.
  2. Pull the impeller off the shaft. Clean the inside of the pump housing and fit the new impeller. Always have a spare impeller on board.
  3. Check that the pump coupling is not damaged, by trying to turn the pump impeller. Fit the cover with the original gasket, which has the right thickness.

Check out the impeller trouble shooting guide to discover how to repair and replace a sea water pump impeller.

Did you know that if you understand the basics of marine diesel engine maintenance you can service your own marine engine and save on maintenance costs?

Every boat owner should have at least a minimum understanding of their marine engine. Keep your family safe on the water, find out how to service your marine diesel engine yourself and know that it is properly maintained. Learn which spare parts you absolutely must carry on board your boat and discover how to make emergency boat repairs on the water.

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