Yanmar 2GM20 Cooling water checks
I've had the 2GM 20 since 1986, here's some options to look at starting with easiest & least cost to the more involved: Basically, start at the water intake thru hull and valve. Close the valve, back off the hose clamps and pull off the intake hose. With a straight piece of coat hanger wire at hand, open the valve and push the wire thru to clear any debris. Close the valve, re-attach the hose and clamps and move to the intake strainer. Open the strainer and clear the basket. With the lid off the strainer, open and close the thru hull intake valve to check the water flow to the strainer. If all was clear to this point, follow the intake hose to the front of the engine and the belt driven seawater pump. Check for slippage at the belt to pump drive pulley, and proper belt tension. Run the engine and check for the overheat condition. If it still persists, remove and open up the seawater pump, look for damaged impeller or missing blades & replace the impeller and gasket; re-install & adjust belt tension. If impeller was damaged and blades missing they may be down stream in the heat exchanger and it would need to be opened and cleared. However if no damage to impeller, and overheat condition still persists, skip to removal, inspection and cleaning of the water injection elbow; start with removal of a small aprox. 5/8" injection hose to a brass fitting into one side of an upside down "U" fitting. If this is clogged or partially clogged, remove and clean; then remove the exhaust hose from the back of the "U" (a couple small flat head screw drivers and "PB Blaster" might be necessary). Four bolts hold the exhaust elbow to the back of the engine. Un-bolt & remove the exhaust elbow, injection fitting & "U". At this point it may be easier to take the entire assembly to a muffler shop for disassembly as the exhaust elbow and "U" fitting are connected by a right & left hand threaded stainless steel coupling. With the three pieces apart you can determine if the two parallel chambers of the "U" are clogged or partially clogged and if it can be cleaned and salvaged.
At this point you have inspected and cleaned or re-built the entire seawater side of the cooling system. The antifreeze side, being a closed system typically only needs replacing of the thermostat and occasional draining and replacing of the antifreeze coolant.
