The thermostat is part of the heat management system in a vehicle. No coolant goes to the cold engine when it begins operating. The thermostat's main function is to block the coolant flow to the engine. This allows heat to rise quickly and help bring the engine sooner to its operating temperature. The thermostat does so to reduce emissions, gunk deposit and engine wear. The thermostat opens to let coolant into the engine once the engine reaches about 200 degrees Fahrenheit (95 degrees Celsius), the general operating temperature. Replace your Volvo S40's defective or worn thermostat as soon as possible to avoid engine problems and polluting the environment.
Wear safety glasses and gloves. Look for the upper radiator hose and follow it to the S40's engine block. The hose is connected to a metal device called the "water inlet." Loosen the band clamp using a flat-head screwdriver that is securing the hose to the water inlet.
Find and remove the two bolts that holds the water inlet to the engine block using a 12mm socket and ratchet. Add pressure to the sides of the water inlet by squeezing it between your thumb and the index finger. Firmly pull the water inlet off the engine block. Remove the water inlet gasket, being careful not to break the inlet's bottom seal.
Wipe clean with a cotton cloth the inlet base and the area where it meets the engine to remove any rust and debris that may be present.
Lift the old thermostat straight up and out of the engine block from the water inlet. With the spring end facing the engine, insert the new thermostat into the exact location where you pulled the old one out.
Put a bead of RTV gasket sealant around the base of the water inlet and allow 15 minutes for it to dry. Replace the water inlet gasket.
Reinstall the water inlet over the thermostat and secure it into place by replacing the two bolts.
Reconnect the radiator hose to the water inlet by tightening the band clamp. Make sure the radiator hose is secured.
Start the S40's engine. Place the transmission in "Park" and set the emergency hand brake. Push the accelerator until the engine speed reaches 2,500 rpm. Step on the accelerator two more times, each until the engine speed reaches 2,500 rpm.
Allow the engine to run at idle for five minutes. With a correctly installed thermostat, should hear the radiator fan operating in less than one minute. The temperature gauge on the S40's instrument cluster be near or at the "normal" level.
