Later models of the Toyota Celica had a 1.8L four-cylinder engine. While this engine isn't that different from other Toyota four-cylinders, it does require extra work when replacing the thermostat. The Toyota Celica's oil filter must be removed prior to accessing the thermostat. If your cooling system isn't properly cooling your engine, you may have a sticking or faulty thermostat, which will require replacement. You can do it at home, provided you are fairly experienced in auto repair and have the necessary tools.
Shut the engine off and let it cool down for 30 to 45 minutes. Engine components and coolant get extremely hot (around 93.3 degrees C) and can cause severe burns.
Unhook the negative battery cable and position it out of the way, so it doesn't make contact with the post.
Remove the plastic engine shroud from underneath the Celica. Position the drain pan underneath the radiator drain plug. Remove the drain plug and allow the coolant to completely drain out.
Unhook the engine temperature switch from the water inlet housing near the thermostat. Remove the oil filter to access the thermostat. Use a proper oil filter wrench.
Loosen the clamp on the lower radiator hose and pull the hose away from the water inlet housing. The hose will likely have a small amount of coolant in it; let it drain into the container.
Remove the two bolts from the thermostat housing. Pull the housing away and set it aside. Pay attention to the position of the thermostat valve. The new thermostat needs to be in the same position (facing the bolt stud).
Pull the old thermostat off and remove the gasket from the mount. Discard the gasket. Clean the mount with a dry rag.
Place the new gasket on the new thermostat. Insert the thermostat into the mount. Make sure the valve is facing the stud.
Replace the housing and tighten the bolts. Hook up the lower radiator hose and tighten the clamp.
Reattach the engine temperature switch and replace the oil filter. Tighten the oil filter by hand; do not use the wrench.
Reconnect the negative battery cable. Replace the radiator drain plug. Replenish the cooling system.
Crank the engine and check for leaks. Closely monitor the engine operating temperature to make sure its within the acceptable range.
If your coolant is dirty, go ahead and replace it, if possible. Make sure you properly dispose of your used engine coolant (automotive waste facility).
Replacing a thermostat is safe if proper caution is taken; if not, it can be very dangerous.