Automotive radiator caps perform a very important function in the regulation of coolant throughout the engine. They serve as a safety valve to bleed off excess radiator pressure, while maintaining a consistent level of engine coolant for both hot and cold operation. Simple in design and function, the radiator cap is frequently overlooked as a major cooling system component. The theory of radiator cap operation can be understood by learning its parts and how they function to keep the engine running at normal and safe operating temperatures.
Radiator Cap Construction
A typical radiator cap consists of a circular metal shell, with ear-like locking tabs designed to screw down over the radiator fill neck. The top of the cap consists of a spring disc diaphragm and gasket that forms a seal against the upper lip of the filler neck. The spring keeps a pressure valve and lower gasket closed, to retain water in the radiator. The spring is pressure-rated to open when the system pressure exceeds the psi (pounds per square inch) rating on the radiator cap. Overflow coolant is sent to an overflow reservoir. A vacuum valve, located at the bottom of the cap, allows coolant to be drawn back into the radiator when the system cools.
Radiator Cap Filling Function
Most radiator caps are designed to be removed, except on sealed systems that might have bolt-on plugs that need to be removed only when cooling system maintenance is performed. Radiators receive coolant through the cap neck, for the purpose of adding or changing coolant. The cap must be removed for pressure testing, as well as gasket and seal inspection of the cap. Sealed radiator systems have overflow or recovery reservoirs to which coolant is added if the level appears low.
Boiling Point and Pressure
The radiator cap raises the coolant boiling point by maintaining pressure in the system. A 50-50 mixture of glycol antifreeze and water boils at 227 degrees Fahrenheit, equipped with a 4-pound-rated cap. A 7-pound-rated radiator cap will raise the boiling point to 248 degrees. The higher the pressure cap rating, the higher the boiling point. If the pressure inside the cooling system exceeds the rated cap pressure, the cap pressure valve opens to vent the pressure and bleed off a small amount of coolant to the overflow reservoir. Pressure venting keeps vulnerable cooling components like the heater core, radiator core and hoses from being damaged.
The Relief Valve
The relief valve function of the radiator cap siphons stored reservoir water back into the radiator. After the engine has run at normal operating temperature and then is shut down, the engine temperate rises due to a heat soak process. The pressure valve opens to relieve excess pressure, transporting coolant to the reservoir. As the engine cools, pockets of air and vapor that have expanded in the cooling system begin to contract. This creates a partial vacuum. The relief valve opens and draws any residual coolant from the reservoir back into the radiator.
Constant Pressure Caps
Constant pressure caps use a relief valve that stays closed with the use of a light spring. The system is totally sealed, allowing the pressure to instantly rise upon engine starting. Pressure can not escape once the coolant begins to expand.
Pressure Vent Caps
Pressure vent caps, also known as open vacuum valve caps, do not have a spring to keep the vacuum valve shut. They use a small calibrated weight that opens upon engine starting and low rpm operation. By opening in this manner, the cooling system operates at minimum or atmospheric pressure. The result reduces the strain and pressure on hoses and clamp connections, water pump seals and the heater core and heater control valve. When the engine heats up, steam pressure is created, which pushes the valve up and closed. When the engine begins to cool, radiator vacuum pulls on the weight, opening it.
