The reason that there is a scale of radiator cap pressure ratings is that each cap has a maximum pressure that is supposed to hold. Above this point, the cap will release pressure through a pressure release valve. Having too much pressure in the system can result in damage. Not having enough pressure in the system can cause the vehicle to overheat. That's why it's important to always check the radiator cap to make sure it's helping to provide the right balance.
Using a Radiator Cap Pressure Chart
Each radiator cap has a specific amount of pressure that it will hold. The radiator cap pressure rating system shows a range of pressures that the individual cap has been tested for. Even with a cap that is properly rated, it may be necessary to do a physical radiator cap pressure check to determine the maximum pressure that the cap will hold. For the best chance of using the proper radiator cap and parts, always check your vehicle manual and resources from the manufacture to determine what type of radiator cap should be used with your vehicle. Select your caps and other parts accordingly.
Be sure to take all stated precautions in the manual when working around the radiator system. The danger in removing a radiator cap is that if the system is pressurized and hot, you can be injured. Always wait for the engine to cool down before removing the cap. Use protective eyewear and other appropriate safety gear. Stay clear of the fan belt system, and never work under a vehicle that is not properly supported.
How a Radiator Cap Works
The antifreeze and water mixture in your car's radiator is like any other type of liquid and has a boiling point. When any type of liquid is exposed to heat and reaches its boiling point, the liquid begins to evaporate. By placing a liquid inside of a pressurized container, you can considerably raise its boiling point. This is exactly what a radiator cap does for the antifreeze and water mixture in your car's cooling system—it raises the boiling point.
Most radiator caps have two seals on them. The first seal is usually located at the top neck of the cap and is responsible for sealing the other seal at the bottom of the filler neck. As the fluid temperature in the radiator begins to rise, the radiator fluid expands. With continued expansion, pressure increases to the point to where it is greater than the pressure limit for the spring seal in the radiator cap. As the pressure continues to increase, the bottom spring seal is broken and the radiator cap rises. This allows the heated coolant to pass into the overflow reservoir tank, until the pressure is sufficiently relieved to allow the radiator spring to force the lower seal back into the filler neck of the radiator.
If you look at the bottom center of the radiator cap, you will find a brass or stainless steel plate. This circular plate acts as a check valve that operates in only one direction. As the temperature in your engine starts to cool down, the coolant shrinks in volume. As the volume of the coolant contracts, the pressure in the radiator creates a vacuum that forces the circular plate to be pulled down and opens the check valve.
Once the check valve on the radiator cap opens, the coolant is sucked through the tube that leads to the radiator reservoir tank and refills the car's radiator. This is a very effective system in that it pushes and pulls the fluid at each temperature cycle. It pushes fluid into the reservoir tank when the radiator is too hot and pulls fluid from the radiator reservoir tank back into the radiator as the engine cools.
If the radiator cap becomes defective and no longer works the way it should, it allows the radiator fluid to boil. You'll experience low levels of radiator fluid or notice that it evaporates very quickly. If your radiator cap is damaged, you should replace it immediately.
What Is a Radiator Cap Tester?
People who are working on vehicles use a radiator cap checker or pressure tester to see if the radiator cap can handle the pressure that is being applied to it.