If you've noticed that your 1999 Subaru Legacy Outback has not been running at the right temperature and you feel that it's time to address that aging thermostat, you're probably right. Thermostats fail in a manner that make the engine run cooler or hotter than it should; one stuck open will not allow the engine to heat up properly, whereas one stuck closed will cause overheating. The good thing about changing the thermostat in an Outback is that it is easy to locate and inexpensive to replace.
The thermostat housing is where the lower radiator hose attaches to the engine block. Remove the thermostat housing bolts with a 10 mm socket and ratchet and pull the thermostat housing off of the Outback's intake manifold to expose the thermostat. You should be able to leave the hose attached.
Pull the old thermostat gasket off of the intake manifold and throw it away. It will not be used. Pull up on the thermostat to remove it from the intake manifold. It can also be thrown away, after comparing it with the new one to be sure you have the right replacement part. You may have to use a scraper to get all of the gasket material off the machined surfaces.
Place the new thermostat into the intake manifold. Most thermostats come with a new gasket. Replace the old one with the new one. If you don't have a new gasket, place a bead of RTV gasket maker on the flat machined surface of the intake that surrounds the thermostat. Allow the RTV gasket maker 15 minutes to firm up before moving on.
Replace the thermostat cover and bolt the cover into place using the old bolts that were previously removed.
Start the Outback and check for antifreeze leaks between the thermostat cover and the intake manifold. If leaks occur, tighten down the bolts that hold the cover onto the intake.
