NEVER OPEN WHEN HOT UNDER PRESSURE. This is the unpunctuated warning you have seen on the pressure cap of an engine’s cooling system. Regardless of the application, all cooling systems have been pressurized since the end of World War II.

Without getting mired in physics, the liquid in a cooling system is raised in pressure by expansion through heating (vapor pressure) and via resistance in flow through the engine cooling jackets, radiator, and water pump. These two different events cumulatively result in the pressure in a cooling system. 

Years back when the radiator was vented to atmosphere, pressure was created but to a lower level since it was allowed to escape. For this discussion, we will be concerned with pressure from thermal expansion of the cooling medium. The movement of the coolant through the different engine passages is deemed hydraulic pressure.

To understand why the cooling system is pressurized, consider that for every 1 psi over atmospheric pressure, a liquid’s boiling point is raised 3°F. Whereas water boils at around 212°F. at atmospheric pressure, if exposed to 15 psi (a common radiator pressure cap), the boiling point will now be 45°F. higher (3 psi times 15 psi cap). Under this pressure, water boils at 257°F. Due to this, altitude determines boiling point. The higher the elevation your farm is from sea level, the lower the boiling point of any liquid since less atmospheric mass is present. Heat transfer from an engine to the liquid coolant is based on a temperature differential. A higher boiling point allows more heat to be removed from the engine due to the larger spread between ambient temperature and the coolant in the radiator. 

Open the hood and expose a hot engine on a 90°F. day and then again on a 30°F. day. In 10 minutes, the engine will be much cooler on the cold day than the warmer day. Hot goes to cold, and as thermal equilibrium is approached, heat transfer diminishes. 

The other purpose of a pressurized cooling system is to move the coolant through the cylinder head. Under high engine load, the coolant will boil at the contact point around the combustion chamber and become a vapor. When the liquid coolant becomes vapor, 97% of its thermal transfer ability is lost. The pressure created by the radiator cap forces the liquid and vapor to release from the cylinder head casting water jacket wall. It then recondenses and allows lower-temperature liquid to come in contact with the surface to cool it.

If the pressure cap is failing or failed, the following issues can occur.

Most modern engines employ the minimum of a 15-psi radiator cap, while designs up to 21 psi are now common. If you purchased an older piece of equipment and don’t know its history, check the shop manual to see what pressure cap was factory-fitted. Many engines have low-pressure caps because the previous owner thought it was better or it put less stress on the cooling system.

The best way to check a pressure cap is to replace it with a new one with the proper rating for the engine. There are special tools to check a pressure cap. In most instances, it costs far less for a new cap than a new tool.

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