Engine Coolant Thermostat Thermostats are a simple device which regulates coolant flow through the engine. When cold it will stay closed, stopping coolant flow and helping the engine warm up faster. Once up to temperature, it will open and allow normal coolant flow and maintain proper engine temperature. Always install with the 'jiggle pin' bleed valve at 12 o'clock when possible. For many applications, multiple temperature ratings are available. These ratings denote when the thermostat opens, and in extreme climates different temperature ratings can be used to assist the vehicle in maintaining proper temperatures. In very hot desert climates for instance, a lower temperature rated thermostat may be preferred. There are two types of thermostat failures: Thermostat stuck open (most common): Coolant will flow through the engine constantly from cold, preventing the engine from reaching operating temperature. Usually this means a rich mixture resulting in poor fuel mileage and potential catalytic converter damage. Thermostat stuck closed: If the thermostat is stuck closed it will restrict flow into the engine resulting in an overheating condition, resulting in potential catastrophic engine damage. Engine Coolant Thermostat Failure Symptoms Overheating engine Engine not reaching optimal running temperature Radiator fan not engaging