Where is the thermostat located on a 1990 Honda Accord LX?

Follow the LOWER radiator hose to a housing behind the distributor and the motor. There is no need to remove the radiator hose or to even drain the radiator. Wait for the car to cool down completely. Place one or more large collection pans, preferably square, under the car to catch the coolant overflow. Using a 10mm combination wrench, remove two 10mm thermostat cover mounting bolts; and a third 10mm bolt, on the bottom of the cover, that holds a bracket onto the thermostat cover. Tug the thermostat cover away from the housing. As coolant overflows, double-check the placement of your collection pans. Note precise orientation of old thermostat Remove old thermostat with some pliers, put new gasket on new thermostat EXACTLY as old one is. Install new thermostat in EXACTLY the same orientation as the old one was (there are three slots that must line up with two of the gasket dimples, and the tiny bypass valve that is on the thermostat.) Replenish radiator with a 50/50 mixture of water/quality anti-freeze (will take about 1.5 liters). With a garden hose, dilute and flush away any spilled ant-freeze, to prevent your pets and other animals from drinking it and getting poisoned.