The thermostat in an engine is responsible for regulating the temperature of the cooling system. In order for your engine to run efficiently, it needs to operate within a specific heat range. Heating up is easy, since the combustion process gives off a good deal of heat. To prevent overheating, engine coolant (antifreeze) is circulated through the engine block to pick up some of that heat and send it to the radiator where it dissipates into the atmosphere. The coolant returns to the engine at a lower temperature to pick up more heat and do it all over again in a near-constant cycle.
If the coolant, however, is allowed to circulate continuously, too much engine heat can be lost. Heat that is necessary for proper engine function. Therefore, the thermostat serves as a valve to close off the passage of coolant. It controls the amount of coolant that cycles through your engine by opening and closing. It opens when the temperature falls to a certain level, and it opens back up when the temperature rises to another designated level. Most engines prefer to operate somewhere between 195 and 220 degrees Fahrenheit.
The thermostat housing is an enclosure mounted to the engine where the thermostat is located. The housing is sealed against the engine with a gasket to prevent coolant from escaping.
The thermostat housing is typically mounted to the top of the engine block and is connected to the radiator by a rubber hose. The most common sign that a thermostat housing has failed is a coolant leak at that location. This is usually due to either a cracked or warped housing. A bad seal can also be the culprit.
On some engines, the thermostat itself is integrated into the housing. Therefore, if the thermostat fails, the housing will need to be replaced along with it. Overheating is a common symptom of a thermostat that is stuck closed, whereas insufficient heat coming from the heater vents might signal one that is stuck open.
For a technician to replace your thermostat housing, your engine must first cool down completely to prevent injury from steaming hot coolant. That usually takes about a half an hour. It is common to install a new thermostat when the thermostat housing is replaced.
Once the engine is cool, the following stems are generally taken: