Under pressure: The importance of the radiator pressure cap.

NEVER OPEN WHEN HOT UNDER PRESSURE, is the unpunctuated warning we have all seen on the pressure cap of an engine’s cooling system. Regardless of the application, all engine cooling systems have been pressurized since the end of WWII. Yet, few of us have given any thought to the theory of operation.

Two types of pressure

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 the atmosphere, the pressure was created but to a lower level since it could 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.

Why pressure?

To understand why the cooling system is pressurized, look no farther than a pressure cooker in the kitchen of your home. That device allows food to be prepared quickly and with less water since it operates at a higher temperature, but how can it achieve that?

For every psi over atmospheric pressure, a liquid’s boiling point is raised three degrees Fahrenheit. Water will boil at around 212 degrees F at atmospheric pressure. If exposed to 15 psi (a typical radiator pressure cap), the boiling point will now be 45 degrees F higher (3 psi X 15 psi cap). Under this pressure, water will boil at 257 degrees F.

Due to this, altitude determines the boiling point. The higher elevation your farm is from sea level, the lower the boiling point of any liquid is, either in an engine or on the kitchen stove. This is due to less atmospheric mass being 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 more extensive spread between ambient temperature and the coolant in the radiator.

Open the hood and expose a hot engine on a 90-degree F day and then again on a 30-degree F day. In ten minutes, the engine will be much cooler on a cold day than the warmer one. 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 transferability 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 condenses and allows lower temperature liquid to meet the surface to cool it.

Signs of a weak/failed pressure cap.

If the pressure cap is failing and 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 standard.

If you have an older piece of equipment that was purchased used and you really do not know its history, check the shop manual to see what pressure cap the factory fitted. I have seen many engines with a low-pressure cap since the previous owner thought it was better or stressed the cooling system less.

The best way to check a pressure cap is to replace it with a new one of the proper rating for the engine. There are special tools to test a pressure cap. Still, in most instances, it is more cost-effective to buy a new one instead of spending a few hundred dollars on a tool you may only use once every five to ten years.