The Radiator Pressure Cap

To further retard the boiling point of the liquid in the cooling system, the entire system is placed under pressure. This pressure generally runs between 7 to 16 pounds per square inch (psi). As the pressure increases, the boiling point rises well. This combination of pressure plus coolant gives the liquid in your cooling system the capability to resist boiling at temperatures that can rise as high as 250 degrees Fahrenheit or more in some new vehicles.

To keep the lid on the pressure in the system, and to provide a convenient place to add water and coolant, each radiator has a removable radiator pressure cap on either its radiator filler hole or its coolant recovery system.

The pressure cap has two valves: a pressure valve and a vacuum valve. The pressure valve maintains a precise amount of pressure on the liquid in the system. the vacuum valve allows the liquid to travel back into the radiator from the recovery reservoir when the engine cools down.

Pressure caps are relatively inexpensive, but if you have a cap that isn’t working properly, or if you have the wrong type of cap, you will be amazed at the amount of trouble it can cause. For example, if the gasket (rubber ring) inside the cap isn’t working, the pressure in the system will escape, allowing the liquid to boil at a lower temperature. A modern cooling system that’s been designed to operate normally at temperatures over 212 degrees Fahrenheit, and that is filled with liquid and in perfect condition, will still continually boil over if the radiator cap isn’t operating efficiently. That boiling liquid will be forced into the overflow system, and your vehicle’s engine will overheat. This can cause an inconvenient highway breakdown and possible danger to your engine.

Although most vehicles come equipped with a safety pressure cap, which features a little lever that releases the pressure before you remove the cap, some older vehicles have a cap that lacks this feature. If you have one of these caps, replace it with a safety pressure cap immediately. Doing so can keep you from burning yourself if you have to remove the cap while there’s still some pressure in the system

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