Is your Yanmar 3GMF diesel engine overheating? As with many fresh water loop cooling systems, a restriction of the seawater or freshwater loop can cause the engine to overheat. Possible failures in the freshwater loop include a bad thermostat, bad water pump, blown head gasket or a buildup of minerals in the cooling system. The thermostat and water pump are relatively easy to access and replace. Replacing the head gasket is a major task. Cleaning the freshwater cooling loop can be accomplished in a few hours with a diesel-grade cleaning solution. It can also be cleaned with an on-line cleaner such as Penray 2001, which is added to the antifreeze for 30 days, flushed out, then replaced with a 50-50 mix of antifreeze and distilled water. For what it's worth, the experts say that the antifreeze mixture begins to loose its heat transfer effectiveness after two years of operation. Considering the cost of a Yanmar diesel and the risk that engine failure can create, regular replacement of the antifreeze is relatively easy and inexpensive. Be sure to use antifreeze that is approved for use with aluminum diesels.

The seawater loop is relatively simple and easy to troubleshoot. The loop is made up of a seawater strainer, a water pump, heat exchanger, mixing elbow and muffler, which is all connected with hose.  A properly operating cooling system will exhaust approximately 4 GPM at 1,000 RPM and 8 GPM at 2,000 RPM.  A bucket at the engine exhaust port is a good way to measure the output.  If the output is significantly less than the volumes mentioned above, try this 3 step process to isolate the area that has the problem.

1. To determine if the failure is in the first part of the seawater loop, connect a length of heater hose to the output of the water pump and measure the output by discharging the water into the bucket while running the engine briefly.  If the output is significantly less than the volumes mentioned above, inspect the intake seacock, the seawater strainer, position of the seawater strainer (keep the handle out of the water flow path), seawater pump/impeller, belt tension, hose and hose clamps.  Apply air pressure back through the seawater pump input hose to clear the passage between the pump and seawater intake.  Make sure that none of the hose clamps have been applied too far back on the hoses which restricted the inside diameter of the hose.  A minor reduction in size will significantly reduce the volume of water flow.

2. To determine if the seawater side of the heat exchanger is restricted, connect a hose to the output of the heat exchanger and measure the output by discharging the water into the bucket while running the engine briefly.   If the volume is significantly less than the amount measured in step 1, the heat exchanger is probably restricted.  Debris or a buildup of minerals most commonly causes the restriction.