Thermostats control the water flow from block to radiator. They work by a temperature controlled spring which when heated to a preset temp open a valve letting water flow through the passage back in to the radiator for cooling. Why is it needed? well in certain climates like here in Florida we really don't, but in northern climates where it get cold or freezing it prevents engine damage. In older cars thermostats were often removed to increase cooling but in newer cars this is not a great idea. Though it can be done, most newer cars require the engine to be at a certain temperature to go into a normal driving mode. Until that temp is met the car will maintain a higher fuel percentage also known as "cold start" mode to prevent any internal damage being done.
Unfortunately factories have not adopted a fail-safe model thermostat into cars yet, all though most cars now have fail-safe replacements available at your local auto part stores. How do you know if your thermostat has gone bad? Well there is an easy way to tell. With the car at normal operating temperature or starting to over heat if diagnosing a problem. Carefully feel the top radiator hose for approximate heat. Then feel the lower hose they should be at the same temperature. If the lower line is colder then most likely your thermostat is not function properly and should be replaced.
The steps to replacing are pretty simply and can be done with little effort on most vehicles. You'll need a drain pan, couple hand tools and some more antifreeze to replace any that leaks out during the change.
First locate the drain plug on the bottom of the radiator and empty the fluid in the pan. You don't have to do this but will save you a ton of mess and allow you to reuse all your fluids.
Next locate your thermostat housing. Each model car has one, but they are NOT located in the same areas. Once located there are normally 2-3 bolts holding the housing to the engine.
|Gasket on thermostat|
|Gasket on housing|
When filling the fluids back in, leave the radiator cap off and start the car after returning all fluids. This will allow any and all trapped air to escape preventing any air from trapping the thermostat valve shut.