Most cars, trucks and SUVs manufactured within the past 20 years have a pressurized cooling system. Consisting of a radiator, radiator cap, hoses, fans and a water pump, it's responsible for transferring heat away from the engine and out through the front of the radiator via the fans. Modern automotive cooling systems contain coolant that becomes pressurized as the engine warms up. Without pressurization, the system won't work as intended.
Cooling System PSI
Automotive cooling systems are designed to support a specific amount of pressure. Depending on the specific make and model, a typical vehicle's cooling system may support anywhere from 10 to 15 pounds per square inch (PSI) of pressure. Once the pressure has exceeded this limit, any additional pressure created is released to the coolant reservoir.
How Pressure Is Created in a Cooling System
So, how is pressure created in a cooling system in the first place? Pressure is created when the engine warms up and, subsequently, heats up the coolant. Coolant, like most liquids, expands when hot. Because automotive cooling systems are closed -- they don't allow air to escape -- coolant can't expand. Instead, it becomes pressurized.
The coolant will continue to expand and pressurize as it heats up until it reaches the PSI for which the radiator cap is rated. And as previously stated, any additional pressure is released to the coolant reservoir.
Pressurized Coolant Has a Lower Boiling Point
The reason that automotive cooling systems are pressurized is because pressurized coolant has a lower boiling point than non-pressurized coolant. Coolant already has a lower boiling point than plain water. To further lower its boiling point, automakers design cooling systems so that they'll hold pressure. When pressurized to 15 PSI, the boiling point of coolant is about 45 degrees Fahrenheit lower than non-pressurized coolant. As a result, it's able to accommodate the engine's hot temperatures without turning to steam and evaporating.
The bottom line is that your vehicle's cooling system needs to be pressurized. Otherwise, the coolant will boil over, which could lead to more serious automotive problems like overheating, a blown head gasket, a cracked engine or warped cylinder head.
You can see if your vehicle's cooling system is holding pressure by testing it. There are radiator pressure testing kits that are designed specifically for this purpose. You connect a fitted component on the neck of your radiator, followed by pumping it with pressurized air to see if it holds. For more information on how to pressure test your vehicle's cooling system, check out this blog post.