What Causes Too Much Pressure In Cooling System?

What causes too much pressure 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 car component 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.

pressurized cooling system
The radiator cap acts as a pressure valve. (Photo: 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?

You now know what causes pressure build up in cooling system. So how does this occur within your system?

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.

pressure in the cooling system
The rest of the cooling system comprises a small expansion tank and a hose connecting to the radiator. (Photo: Vehicle Freak)

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 radiator 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?

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.

pressure in cooling system
Coolant will be back in the radiator after it cools down. (Photo: Deposit Photos)

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.

How Can Pressure Be Released in the Cooling System?

An easy way to release pressure in your cooling system and lessen the heat produced by the engine is to turn on the heater. 

Due to the heater core’s proximity to your radiator, when you turn on the heater, it will aid in removing some of the heat from your engine.

In order to assist lower the temperature, you can also activate the fans at their highest speed. Additionally, this will assist in circulating cold air and lessen the amount of heat produced by your engine.

If these steps don’t appear to relieve the increasing pressure in your cooling system, stop immediately and turn off the engine. You should have a chance for a little reprieve from the engine overheating if you allow it some time to cool down.

FAQs on Cooling System Pressure

  1. How long should a cooling system be able to withstand pressure buildup?

At least two minutes, however it can occasionally take longer depending on the type of coolant used, how quickly the lid is removed, and the technician’s ability. whether not, see whether the coolant boils or leaks out of the reservoir.

  1. Can faulty or stuck thermostats cause too much pressure in coolant system?

Indeed, in order to prevent excessive pressure in cooling system, we advise checking for leaks in the damaged thermostat housing.

  1. Why does the pressure in the car radiator remain overnight?

If your cooling system holds pressure overnight or there is still pressure in your radiator after it has cooled, we would be worried that you have a blown head gasket that is pouring exhaust gas into your water manifold.

pressurizing cooling system
Be aware of a pressurized cooling system through the night. (Photo: DSPORT Magazine)
  1. What happens if the radiator keeps pushing water out?

A poor cap, air buildup in the cooling system, coolant leak, malfunctioning thermostat, and damaged seals or o-rings can all cause a radiator to continuously pump water out. 

You could also have worn-out hoses, a leaking heater core, a cracked head gasket, and other malfunctioning radiator components.

  1. How long to wait to open coolant cap?

It usually has to cool for 1-2 hours before it is cold enough to open the coolant cap securely. 

If you’re trying to check the coolant level, you should probably wait for 3-5 hours (because the coolant may expand if it’s still warm, giving a falsely high result).

Final Words

So you now have grasped some notices about what causes too much pressure in cooling system and ways to manage the extra pressure and make sure the engine tank is no longer bothered by it.

The cooling system is always subject to excessive heat caused by sunshine, filth, grime, and other external causes, unless the weather in your area is consistently very cold.

Check out this video from EricTheCarGuy to learn more about Pressure Testing Cooling Systems properly!

After each ride, be sure to thoroughly inspect everything, and have experts check them as part of their regular maintenance!