This type of thermostat operates mechanically. It makes use of a wax pellet which is inside a sealed chamber. The wax is solid at low temperatures but when the engines heats up the wax will then melt and expand. The sealed chamber has an expansion provision that operates the rod that will open a valve when the operating temperature is exceeded. However, the operating temperature is fixed, but it is also determined by the specific composition of the wax. Therefore thermostats of this type can maintain different temperatures, often in the range of 70 degC to 90 degC, which is 160 to 200 degF.
Modern engines are run at over 80 degC or 180 degF, which is quite hot, but it is in order to run more efficiently and to reduce the emission of pollutants. A lot of thermostats contain a small hole in which they vent any gas that might get into the system. In other words, if air is introduced during the coolant replacement. Modern cooling systems contain a relief valve in the form of a spring-loaded radiator pressure cap, that has a tube leading to a partially filled expansion reservoir. Because of the high temperature, the cooling system becomes pressurized to a maximum that is set by the relief valve. The additional pressure will then increase the boiling point of the coolant and that would be above that of the atmospheric pressure.