A thermostat is a device in a car's engine that helps regulate engine temperature. The thermostat is essentially a small mechanical device that controls the flow of coolant from the radiator to the engine. If the engine temperature is still cool, water and coolant stay until it's hot enough, after which the thermostat opens its valve, and lets the hot water exit the engine into the radiator, and lets cooler water from the radiator flow into the engine. If you notice that your engine is getting hotter than usual, you might be encountering a faulty thermostat.
The first thing you have to do is to take note of your car's make, model and year, so you can purchase new parts and supplies from your local auto parts dealer. You will need a new thermostat to match your car's make and model. you will also need a new gasket and sealant. The gasket should fit the thermostat, but the sealant can be any generic brand. In warmer climates, mechanics would even suggest you remove the thermostat altogether, because your engine won't need to build up heat, anyway, and it's better to let water continually flow through the engine and radiator. But for purposes of those in cooler climates, here is a quick guide on replacing your car's thermostat.
First, focus on safety. It's best to work on your engine when it has already cooled. This is best done in the morning, before you use the car, or after you've let it rest for at least a couple of hours.
Finding the thermostat is easy. First look for the hose that lets water exit from the engine into the radiator. This would look like a thick black tube (of about two inches diameter) coming from the engine into the top of the radiator. The hose is usually clamped on to the radiator, and also to the engine block. The part where it clamps onto the engine is the thermostat cover, often affixed in place with clamps and bolts. If you cannot find the thermostat this way, consult your car's manual, as it might be located elsewhere.
Loosen the clamp by using a screwdriver to loosen the screws, or a small wrench if these are bolts. Carefully pull off the hose to dislodge it from the engine block. You can use lubricant around the cover if you find it hard to dislodge the hose. Upon removal, you will notice that a liquid will pour off the hose. This liquid is the coolant (or it may be antifreeze, depending on which you have used on your car).
Remove the bolts on both sides of the thermostat's cover using a ratchet or a wrench. When the bolts are off, take out the cover and then the thermostat. The thermostat should look like a big bottle cap with a spring and a small hole in the middle.
Remove the existing, hardened gasket from the cover and cover base using a knife or a scraper. Take away any portion gasket that has hardened and stuck to the engine's surface using sandpaper or emery cloth. The cover and cover base should be perfectly clean. Otherwise, there is a big chance that the cover will leak.
Get your brand new thermostat and place it inside the engine, with the spring portion facing inside. You will notice that there is a space where the top of the device should fit. You should now use the gasket to cover this area. Use sealant in between to ensure a tight fit. Then install some new rubber gasket on the cover base and make sure that the bolt holes are aligned. Replace the thermostat cover and secure the bolts in place. You can now replace the radiator hose.
Wait about 30 minutes for the gasket sealant to dry before adding any coolant or water, and before starting the engine. You can consult the sealant packaging to determine how long the gasket sealant needs to dry. As your coolant level will likely to have diminished, add the proper mixture of coolant and water to your radiator and the reserve while the engine is running warm. Be sure that the thermostat cover and hose have no leaks by checking them periodically.
The thermostat is a little device that can make or break a car. Moreover, this device is prone to wear and tear due to its continuous, almost non-stop operation. Replacing it when your engine is getting warmer than usual can be an inexpensive way to keep your car running well, and ensuring a longer life.