What should you never do with a radiator cap on a pressurized system until it has cooled?

Removing the radiator cap on a pressurized system before it has cooled can be extremely hazardous. When a vehicle's engine runs, the coolant heats up and creates pressure within the cooling system. This pressure raises the boiling point of the coolant, which helps prevent overheating. If the cap is removed while the system is still under pressure, hot coolant could erupt from the radiator, potentially causing burns or injury. Therefore, it is crucial to allow the system to cool down before attempting to remove the cap to ensure safety and prevent injury from the scalding fluid or steam.

Other actions like checking the coolant level, inspecting for leaks, or adding more coolant should also be approached with caution, but they can often be done without directly engaging with the pressurized system. However, directly removing the cap is a clear safety hazard that should always be avoided until the engine has cooled sufficiently.