Technical Tidbits: The Technology of a Thermostat

Current EPA regulations make the thermostat an important component for efficient engine operation and to meet low-emission requirements.  A gasoline-powered engine needs the proper temperature to help vaporize the fuel, to provide for the proper piston to cylinder fit, and to promote complete combustion with low emission output. Thermostats regulate the temperature of the water in the cylinder block and help pressurize the system allowing rapid heat transfer from the cylinders to the cooling water.


Most outboard motors use a similar system for cooling, regulating pressures, and maintaining correct engine temperatures. Water is drawn in through the gearcase inlet screens where it is then pumped up to the powerhead. It enters the bottom of the cylinder block and travels through passageways around the exhaust areas. This not only cools the hot gases but also pre-heats the water to prevent uneven temperatures as it surrounds the cylinder walls and combustion chambers. The water continues to fill the block until it encounters the closed thermostat. As the water temperature increases to a pre-determined point (about145°F), the thermostat opens and hot water exits the motor. At the same time, additional cool water enters the cylinder block and the cycle repeats itself as the incoming water temperature is colder and causes the thermostat to close.



To allow the water to flow into the engine, air has to be expelled or it would compress and “air-lock” the cooling system. Some thermostats have a relief slot in the valve to bleed this air and to allow a small stream of water over the temperature-sensing thermal element. Other air-bleed designs include a small slot or hole in the cylinder block. If an air-bleed opening is plugged with debris, the engine can overheat within a few minutes after starting. The trapped water cannot reach the thermostat and continues to get hotter until the overheat alarm sounds. This was a common problem with the older Johnson and Evinrude cross-flow V-4 outboards.


At the heart of a thermostat is the thermal element, technically called a vernatherm. It provides the mechanical action to open the thermostat valve at the designated temperature. It is a brass housing with an internal chamber containing a waxy mixture formulated to expand at a predetermined temperature. A piston fits into the chamber and as the metallic wax expands from heat, it pushes upward against the thermostat valve. This motion opens the valve about 1/8″ allowing water to flow through and out of the engine. At the same time the hot water is exiting, cooler water enters the engine causing a temperature decrease. When the thermal element senses this cooling effect, the  metallic wax contracts and the spring pushes the piston inwards to close the valve until the engine temperature increases again.


Outboard thermostats are small in comparison to their automotive counterparts. They cannot flow enough cooling water through them that larger-horsepower engines require at high rpms. Many outboards have a pressure-relief system to allow more water through the motor to help with cooling as more power (heat) is generated. At advanced throttle settings, power output increases the combustion-chamber temperatures significantly and the outboard is less dependent on thermostatic temperature regulation.


There is no way to adjust carburetors correctly, or to troubleshoot poor idle quality, or to determine plug fouling causes without testing the motor’s operating temperature first.  Dash mounted gauges are not known for accuracy and only indicate the metal surface temperature where the sending unit is located. Because V-4 and V-6 outboard motors have two cylinder heads and two thermostats, the temperature gauge may only display a reading on one side of the motor, the warmest one. The other bank may be running ice cold and not operating efficiently unbeknownst to the operator.


Thermostats do wear out, corrode, and may sustain damage by an engine overheat condition. It is best to change them about every 3 years in saltwater, more often if the motor is not flushed regularly. Engine idle temperature should be checked at every scheduled service interval and thermostats replaced if needed. Never run an outboard without a thermostat as that can affect the engine’s efficiency plus possibly cause hot-spots in the cylinder heads and block due to disrupted coolant flow and reduced water pressure.


If you perform your own engine servicing, be aware that an accurate temperature reading cannot be obtained while running the outboard on a flushing device. It has to be in the water for the pump, relief valves, and thermostats to operate as they were designed. Be sure to use common sense and observe all safety precautions when around a running motor. Always refer to the factory service manual and the latest service literature before attempting any tests or repairs on your outboard motor.