What happens when the thermostat is stuck open or closed?
How does it work?
Basically, the thermostat regulates the temperature of water/coolant in an engine. When you first start the car it should be closed, essentially blocking the water/coolant from traversing the radiator thus allowing the engine to reach its optimum temperature as quickly as possible. Of course, everybody knows that an overheating engine is bad news but most people probably don't realise that engines HATE the cold.
This is mainly to do with lubrication. If you think about it, there's a massive amount of friction involved in an internal combustion engine. Without a lubricant, an engine quickly becomes a system of fast moving metal components grinding and seizing up against each other that will actually destroy itself quite rapidly from the inside out.
Anyway; back to the thermostat. The faster the engine heats up, the faster the oil thins out and gets distributed throughout the block etc. and so on to protect all the moving parts. Now everything is running smoothly but the temperature continues to rise. As the engine begins to exceed its ideal operating temperature, a substance inside the thermostat expands in such a way that the device (and thus a pathway) is forced open, unblocking the radiator and allowing the water to flow & circulate, ultimately bringing the temperature back down to that sweet spot.
The thermostat continues to expand and contract dynamically in such a fashion, relative to the fluctuating changes in temperature, so as to maintain balance or equilibrium.
Like all things, thermostats have the potential for failure, particularly with age they can seize up. If it seizes while it's too hot, it stays jammed open. If this is the case, it's pretty much the equivalent of removing the thermostat completely. It will be safe from overheating, but it will take longer to warm up etc. as we covered earlier. Of course, if it seizes whilst cold, it will stay closed and the cooling system will remain obstructed, causing the engine to overheat quite quickly. Either way, it is best to replace the thermostat ASAP in such cases, or even better; in advance as a preventative measure.
I had no idea it would be called this, but the article in Wikipedia describes the "low level" detail of how the device works.
There is an optimum temperature for the engine coolant (not too cold, not too hot) and a simple mechanical thermostat helps make that happen. When it's closed, the water pump circulates the coolant through the engine and heater, so the water in the engine heats up. Coolant in the radiator itself will not be circulating and will be cooling down. Once the water in the engine reaches the desired temperature, the thermostat opens, allowing circulation from the engine through the radiator.
With the basics out of the way, what happens when it gets stuck should be obvious.
Stuck Open: Water will always circulate. On a cool day, the radiator will keep the coolant at a fairly low temperature. People normally figure this out in the winter when their heater "stops working". This isn't really noticeable other than the temperature sensor in the car never getting to where it should be.
Stuck Closed: Water never circulates through the radiator. Water in the engine will heat past it's boiling point causing burst hoses, and the engine will over heat. This is much more obvious.