What is a Radiator Cap and What does it Do?

Radiator caps are designed to seal the top of the radiator filler neck to prevent the loss of coolant, pressurize the cooling system and raise the coolant boiling point. They are manufactured with a metal or plastic design, depending on the specifications of the vehicle manufacturer.

The car radiator acts as a heat exchanger, transferring excess heat from the coolant fluid of the engine to the air. The radiator consists of tubes that carry the coolant fluid, a protective cap that is actually a pressure valve, and a tank on each side to hold the coolant fluid overflow. In addition, the coolant fluid-carrying tubes usually contain a turbulator that agitates the fluid inside. This way, the coolant fluid is mixed together, cooling all fluids evenly, and not just cooling the fluid that touches the sides of the tubes. The fluid can be used more effectively by creating turbulence inside the tubes.

When the coolant fluid overheats, it expands and causes the fluid to become highly pressurized. When it enters the radiator, the pressure increases even more because it is in the enclosed space. The radiator cap acts as a release valve set to be opened at the maximum pressure point. Usually this is set to a density of 15 pounds per square inch (psi). When the fluid pressure inside the radiator exceeds 15 psi, it forces the valve to open, allowing heat to escape and excess coolant fluid to overflow into the tanks on either side of the radiator. Once the radiator cools down, the coolant fluid in the overflow tanks is sucked back into the pump, continuing through the cooling system.

Radiator caps are designed to perform a number of specific functions, the first of which is to increase the coolant boiling point. This is done by creating a pressurized system. Radiator caps are rates for a certain amount of pressure (in pounds). Once attached, it creates a pressurization system and then raises the boiling point of the coolant. Without a pressurized system, the coolant would simply boil and evaporate.

In addition, the radiator cap allows the expanded coolant to be transferred from the radiator to the coolant reservoir. When the coolant heats up, it expands — and this additional coolant has to go somewhere, so it's transferred to the reservoir. The radiator caps contain a pressure spring that compresses automatically once the system has reached the specified pressure rating of the cap.

Finally, the radiator cap pulls the cooled coolant from the reservoir back to the radiator. After the engine has cooled and the coolant has contracted, the radiator cap's vacuum valve opens; thus, allowing the coolant to flow back into the radiator.

How will I Know if My Radiator Cap Needs Replacement?

Just because it creates a "snug" fit, it doesn't necessarily mean that it does its job. The only real way to test it is to mimic the effects of a closed and functioning cooling system. You can do this by performing a compression test on the cap of the radiator. The radiator cap is attached to a device that applies a specific amount of pressure, after which you can monitor the device to see if the pressure drops. The radiator cap should be able to hold the amount of pressure to which it is rated.

Dress up your vehicle with a Radiator Cap to give your engine bay a cleaner look. Vivid Racing carries Radiator Caps from Buddy Club, Greddy, HKS, Cusco, Power Enterprises and more.

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