The Subaru WRX is a rally-inspired, all-wheel-drive sedan or hatchback. Because the WRX is also turbocharged, a healthy cooling system is of paramount importance. This includes the periodic replacement of the thermostat, the small but important device that keeps track of the coolant temperature. Replacement of a WRX thermostat is relatively easy. It's also a good time to upgrade to a cooler thermostat.
Lift the front of the car with a floor jack and lower it onto jack stands. You can also drive it up on ramps to raise the front end. Make sure the car is thoroughly cooled down if it has recently been driven.
Remove the plastic skid plate that protects the bottom of the engine. Use a socket and ratchet to remove the 12mm bolts that hold it on. There are also some small plastic clips holding on the skid plate; pry them off with a screwdriver.
Remove the radiator caps (some WRX models have only one cap). Place a large container under the thermostat and the radiator hose, which is at the bottom of the radiator and connects to the thermostat housing. Loosen the hose clamp that holds the hose to the thermostat housing and slowly pull the hose off. The coolant will start to pour out and into the container. Let all the coolant drain out.
Remove the two 12mm bolts that hold the thermostat housing cover to the motor with a socket. Remove the cover. Pry the old thermostat out of the housing. Some more coolant may pour out.
Wipe the area where the thermostat goes with a rag. Install the new thermostat in the orientation that the old one was removed. Reinstall the bolts for the housing cover and tighten them to 10 foot-lbs. with a torque wrench. Reinstall the lower radiator hose to the housing and tighten the hose clamp.
Fill the radiator with new coolant up to the bleeder hole in the radiator. Reinstall the radiator cap or caps. Fill the coolant reservoir to the full mark. Start the motor and run it for 5 to 10 minutes until it is at full operating temperature. Shut the motor off and check the coolant level in the radiator once the motor cools. Top it off if necessary. Check the level in the reservoir as well and top it off if necessary.
