An integral part of an engine's cooling system is the radiator valve, oftentimes referred to as a thermostat. The thermostat is positioned between the engine and the top radiator hose. Although the valve is part of the cooling system, the thermostat's job is actually to warm the engine. The valve assists this process by preventing the radiator fluid from reaching the engine until the engine is warm. At the specified temperature, the valve then opens. It is for this reason that a defective valve usually results in overheating. You can test your valve, but must remove it to do so.
Allow the engine to cool then twist the radiator cap in a counterclockwise direction to remove it. Note that some caps feature a lever that must first be raised before the cap can be twisted.
Loosen the petcock plug on the bottom of the radiator with a wrench and allow the radiator fluid to drain into a bucket until the fluid level falls below the top radiator hose, as viewed through the radiator cap opening.
Follow the top radiator hose from the radiator to the engine. Note that the hose attaches to a hose fitting. That fitting is also called the thermostat housing, as it conceals the thermostat.
Remove the fasteners that secure the thermostat housing to the engine with a wrench, then pull the housing away from the engine to expose the thermostat.
Lift the thermostat out of the engine.