Coolant flow out from radiator when open cap continuous

Removing the radiator cap is not proper procedure. It needs to be on tightly because pressure has to build up in the cooling system. The cap has a spring-loaded pressure relief valve that opens at near fifteen pounds.

The water in the coolant boils at 212 degrees Fahrenheit. Under normal operation, many GM engines run as high as 226 degrees or more. That means the water will start to boil and turn to a vapor. Steam does not carry heat to the radiator as well as liquid does. For every pound of pressure on the coolant, the boiling point is raised by three degrees. That means a fifteen pound radiator cap increases the water's boiling point by 45 degrees to 257 degrees. With the cap installed, and no other leaks, the coolant can get to 226 degrees and not boil.

When you remove the radiator cap, you can expect the coolant to expand and overflow as the engine warms up. If you are having a problem that needs to be solved, the symptoms are only accurate if the radiator cap is in place.