Why is there no pressure when I take my radiator cap off?

Radiator caps have a spring loaded seal that keeps pressure in the cooling system. If you remove that seal from an old radiator cap, you have a cap that will keep the coolant in, but will not retain any pressure. A small leak leaks much more slowly when there is no pressure in the cooling system. via

What is the typical radiator cap pressure?

The radiator cap maintains system pressure to 16 pounds per square inch (psi) on most vehicles. This allows the coolant to reach much higher temperatures without boiling away. via

Will a blown head gasket hold pressure? (video)

https://www.youtube.com/watch?v=SFEBI2c1-Lg

What builds pressure in a 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. via

How do you get air out of radiator pipes?

Use a radiator key, 1/4-in. 12-point socket, or a flat screwdriver (depending on your valve type) and slowly turn the valve counterclockwise until water starts dripping out. This will release trapped air and let hot water into the cold fins. While you're at it, you should repeat the process with your other radiators. via

What would a bad radiator cap do?

A faulty radiator cap can result in engine overheating, loss of coolant or major engine damage. So, Pressurized cooling systems help prevent overheating and coolant loss. Also, It raises the coolant boiling point about 3-degrees F for each additional psi above atmospheric pressure. via

How do you vacuum test a radiator? (video)

https://www.youtube.com/watch?v=oHPQZUr_kf4

How do you pressurize a radiator? (video)

https://www.youtube.com/watch?v=2C4R96icPpI

How can you tell if the radiator cap is ready to be replace?

Check to see if the radiator has holes in it or if the cap looks worn or damaged. If so, then replace the cap. When coolant leaks out of the radiator filler neck and dries, it often leaves white streaks behind. While you may not notice coolant leaking from under the radiator cap, look out for these white streaks. via

Will a pressure test reveal a blown head gasket?

Pressure Test

You can attempt this test with the engine cold which is much safer, but if the leak is small it won't show up without when the engine is cold. You will first need the proper fittings to attach an air compressor to your spark plug threads. via

Should the coolant reservoir be pressurized?

The bottom line is that your vehicle's cooling system needs to be pressurized. Otherwise, the coolant will steam and evoporate, which could lead to more serious automotive problems like overheating, a blown head gasket, a cracked engine or warped cylinder head. via

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