Where is the thermostat located and how do you replace it located on a 1990 Toyota Camry 3S-FE 4 Cylinder?

General procedure: Make sure the engine is cool. Loosen hose clamp where lower radiator hose attaches to engine - pull forward. Place clean pan under hose to catch coolant that might spill out. Pull hose off. Catch coolant in pan. Remove bolts on the fitting that the hose was attached to. Remove gasket. Remove thermostat (note how it was positioned and which side was facing out before removing -- there may be an alignment notch). Install the new thermostat (the way the original was sitting,with the little "barbell" at 12:00) and gasket. Re-attach hose fitting w/bolts. Re-attach hose and clamp. Put coolant that spilled out back in radiator. Start engine and allow to warm up and check for leaks. Note - If you need to remove your alternator so as to access the nuts holding the thermostat cover remember to disconnect Battery before removing the alternator leads to prevent shorting. Good post on parts location and repair procedure. Only thing I'd like to add is antifreeze has a limited life before it loses it's Ph balance and will turn acidic or base, either one will cause corrosion in the cooling system. Loss of additives can also cause the cooling system to start acting as a ground conductor which will damage the cooling system. There are many different ideas on when to change, I try to change mine every 3 years or 24K miles. If you want there are test strips to check Ph balance, protection temp and Phosphate levels (prevent oxygen molecules from collecting around cylinder liners and etching them - usually more problematic on diesels). HTH, Dave