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A 4.3-liter MerCruiser motor is an internal combustion engine and, as with all such engines, some sort of cooling system must be working to prevent it from excessive heat. There may be several possible causes for this particular engine to overheat.
Most boats have a heat gauge on the dashboard to monitor the engine temperature. The normal temperature for a 4.3 MerCruiser is 160 degrees, but under a heavy load the gauge may register slightly higher readings. When the engine temperature exceeds 200 degrees, a warning alarm will sound.
MerCruiser 4.3 liter engines are cooled by water pumped out of the lake or river in which the boat is operating, through the engine block, and then pumped back into the lake or river. If your engine is overheating, chances are the cooling water isn't pumping through the motor.
Mercruiser 4.3 liter engines have 2 water pumps in the cooling system. One is located in the lower unit and raises water up to the engine water pump. The engine water pump circulates the water through the motor and back into the lake. If either pump fails, the motor will overheat.
The cooling system can't work if it's plugged up. It's possible the water intake on the lower unit may be plugged with trash or weeds. In below freezing weather, ice can form in intake hoses, forming a blockage. These motors have a thermostat to regulate the engine temperature, which can stick shut and clog the cooling system.
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