Tell me something. Do cooling system thermostats really stick or do they just go bad? And when they go bad - why?
Okay - time for some truth telling here. Thermostats can stick. Thermostats do go bad. But is it the same thing?
In order to understand thermostat function - and failure - you have to understand how they work. Cooling system thermostats haven't changed much in the past 100 years. Their function is simple and based on simple high school physics - thermal expansion against spring pressure. Thermostats consist of a spring-loaded poppet valve and a wax capsule. As wax expands with coolant temperature, it overcomes spring pressure and opens the valve.
As coolant from the radiator enters the block and contacts the thermostat, the wax capsule cools and the spring overcomes capsule pressure, which closes the thermostat. When thermostats fail to open, it is due to a leaking or ruptured wax capsule. Wax expands and leaks from the capsule rendering the capsule useless. Without wax expansion inside the capsule, the thermostat cannot open.
Thermostat operating temperature is based on spring pressure. A 160-degree F thermostat yields less spring pressure than a 180 or 195-degree F thermostat.
On the rare occasion a thermostat sticks, it's normally in the open position, which can also cause an overheat when you get into heavy traffic.