What Causes Pressure Build Up In Cooling System?

What causes pressure build up in cooling system? The answer is: heat. When you apply heat to a liquid or gas trapped in an enclosed space, it expands and creates pressure. The refrigerant in your car’s cooling system works in the same way.

Do you want to know how the pressure buildup happens? Keep reading.

What Causes Too Much Pressure In Cooling System?

It is hard to believe that a simple part like a radiator cap can actually push the coolant’s boiling point up by about 45°F (25°C). It works the way a pressure cooker raises water’s boiling temperature. The cap is set to 15 psi in most cars and operates like a pressure release valve.

cooling system pressure buildup
The radiator cap acts a pressure valve (Photo Source: liveabout)


The cap acts as a pressure and safety valve. When the cooling system’s pressure exceeds the safety limit, the cap’s safety valve opens to discharge the extra coolant. Vehicles these days have a recovery tank to store that discharged coolant.

How Does The Pressure Buildup Happen?

So, what causes pressure build up in cooling system? How does this work?

In modern vehicles, the cooling system does not leave any air space in the radiator. For this reason, the coolant does not get any space to expand even when it is hot. Open the radiator cap when the engine is cold and you will see that the coolant level is touching the top of the cap’s flange.

The rest of the cooling system comprises a small expansion tank and a hose connecting to the radiator. The refrigerant in the tank should be on the same level as ‘COLD’ when the engine is cold.

When the refrigerant expands, there’s no space in the radiator for it to take up. But it needs to go somewhere so the cooling system’s pressure builds to the maximum pressure limit of the radiator cap’s valve. Then, the cap opens and allows a restrained coolant flow into the hose attached to the expansion tank. The tank has enough space to contain the extra coolant even if it is a bad day with extremely hot weather.

What Happens When The Coolant Is Cold Again?

You know what causes pressure build up in cooling system and where the coolant goes after being expanded. But what happens when it cools down again?

coolant in radiator
Coolant will be back in the radiator after it cools down (Photo Source: depositphotos)

As like any other liquid substance, the refrigerant contracts when it cools down. When this happens, the coolant again flows back to the radiator. How is this done?

The radiator’s pressure valve offers a double-duty function. It allows refrigerant to flow into the expansion tank when pressure rises and then initiates reverse-flow when the pressure falls and the liquid is cold. Ideally, the cooling system creates a vacuum to pull open a spring-loaded valve that sucks the coolant back from the expansion tank to the radiator.