Yamaha Marine Outboard Thermostats

A thermostat is a valve in the outboard's cooling system that opens or closes depending on temperature. When the thermostat is closed, it prevents the flow of cooling water passing through the outboard, thus increasing the motor's operating temperature.

A thermostat regulates the temperature of the water for engine cooling and keeps pressure in the cooling system to facilitate heat transfer from the heads and the cylinder walls. Today’s emission regulations make the thermostat even more important for correct engine operation and depending how the laws are interpreted, may put you in violation by removing it or substituting one with a different rating. Today's low-emission computer-controlled marine powerplants are dependent on thorough combustion for reduced exhaust emissions. That is made possible by having the proper engine temperatures and fuel mixtures. As the temperature rises, the thermostat opens and allows coolant to flow through the outboard, thus decreasing its operating temperature. By constantly opening and closing, the thermostat keeps the motor at its ideal operating temperature. Boat thermostats are an integral part of your marine engine and should not be overlooked when completing your routine maintenance. These devices regulate the temperature of the water used in cooling your boat’s engine. They also help to maintain the correct pressure in the cooling system so that heat will transfer from the engine heads to the cylinder walls. When the thermostat is closed, air can not be passed through the engine, causing the temperature to rise. Once the temperature rises, the thermostat opens to allow the coolant to flow to the outboard. This is a continual cycle and often boat thermostats will need to be replaced. Not maintaining both of these components can cause costly damage and repair. Make sure to routinely check and replace your marine thermostat. One of the least appreciated components of an outboard motor is the lowly thermostat. ... That is made possible by having the proper engine temperatures and fuel mixtures. The brain of a thermostat is a component called a vernatherm that is fastened inside the thermostat housing. Overheating can also be caused by a worn or damaged water pump impeller. ... If the impeller is damaged and is missing a vane, make sure any displaced pieces are accounted for and removed. Any debris left in the water passages may reduce flow, which leads to persistent overheating. The brain of a thermostat is a component called a vernatherm that is fastened inside the thermostat housing. Shown on left side of the illustration, this device has a piston that moves upward to open the thermostat valve at a pre-determined temperature. The vernatherm consists of brass housing with an internal chamber containing a mixture formulated to expand at a pre-determined temperature. A piston fits into the chamber sealed with a rubber diaphragm. As the waxy mixture expands from water heating up, it pushes upward on the piston attached to the bottom of the thermostat valve. This motion (about 1/8”) lifts the valve off its seat allowing hot water to flow through the open valve to exit the engine block.