All cars that have coolant circulating through their engines have a thermostat. The purpose of the thermostat is to restrict the circulation of the coolant when the engine is below the needed operating temperature. The thermostat opens to give as much as necessary to prevent the engine from overheating. As long as the thermostat in your car functions correctly, the engine should run efficiently between an upper and lower prescribed operating temperature. With 5 years as a mechanic and many years of doing my own repairs and helping friends, I have found replacing a thermostat is a relatively simple job on most cars that a backyard mechanic can complete in about an hour.
Locate the upper radiator hose.
Cars have two radiator hoses. One comes from the top of the radiator and runs to the top of the engine block. This is the upper radiator hose. The lower radiator hose connects the bottom of the radiator to the lower part of the engine. Almost all thermostats are located beneath the end of the upper radiator hose where it connects to the engine block. On a few models, the thermostat will be at the engine end of the lower radiator hose. The method to change the thermostat is the same except that the lower positioning is a little harder to work with because of needing to be under the vehicle.
Follow the hose to where it connects to the engine.
Examine the area where the upper radiator hose connects to the engine block. Usually there will be a small domed shaped cover with a connector for the hose to attached to the housing. The thermostat sits under this domed housing. It will be held to the engine block with 2 to 4 bolts.
Find the bolts that attach the thermostat housing to the engine.
Once you have located the bolts, use the appropriate sized wrench or ratchet and socket to remove them. You may need to remove the radiator hose to access these bolts. This is done by either unscrewing a clamp or compressing it with pliers to allow you slide it up the hose. Work the hose back and forth until it slips off of the fitting. If this is not necessary for accessing the bolts, leave the hose and clamps in place.
The bolts generally loosen by turning them counterclockwise.
Lay each bolt aside when it has been extracted because you will need them later to attach the housing back onto the engine block. Lift the housing up and away from the thermostat and engine. This may require a little force to break it loose.
Notice the orientation of the old thermostat.
Once you have made a note of the correct way for the thermostat to sit on its seat, lift it up and out of the engine. Normally, this does not require any force. If your engine is still warm, be careful to avoid being burned.
Thoroughly clean the residue from the old gasket from the thermostat housing and seat.
This cleaning may require some scraping. Take care not to damage the area on the engine or the housing where the gasket rests. Insert the new thermostat. Use gasket sealant and lightly coat the flat area on the engine where the gasket sits and the underside of the housing that will make contact with the gasket. Set the gasket in place and make sure that it is oriented with the holes for the bolts.
Replace the housing.
Gently ease the housing down over the gasket and make sure that the gasket does not slip to one side or the other. Slide the bolts into their respective holes and finger tighten them to hold everything in place. Use your wrench or ratchet and socket to tighten the bolts until the housing is firmly in place.
You will have lost a little coolant from your system while replacing the thermostat. Add enough coolant mixture back into your radiator and/or reservoir to replace what was lost. Bleed system of trapped air if possible. Cars that need to have their cooling systems bled usually have a fitting on the housing covering the thermostat. Start the car. Turn the fitting with a wrench just enough to let the air escape. When coolant starts to come out, tighten the fitting back up. If the car overheats unexpectedly, you may need to repeat this process. Check for leaks. Clean up any spilled coolant.