General Cooling System Testing & Servicing: Testing: Thermostat
Visually inspect thermostat for corrosion and proper sealing. If this is satisfactory, suspend thermostat and a thermometer in a container of water. Do not allow either thermostat or thermometer to touch bottom of container as this concentration of heat could cause an incorrect reading. Heat water until thermostat just begins to open and read temperature on thermometer. This is the initial opening temperature and should be within specifications. Then continue heating water until thermostat is fully open and note temperature. This is the fully opened temperature. If either reading is outside of limits, thermostat should be replaced, as it is not adjustable.