Never remove the radiator cap when the engine is hot! The boiling coolant can shoot upwards all over as the pressure is released in the atmosphere. Pressure because of the steam. The boiling coolant and steam can burn you. If you read the owners manual you will find information about not removing a radiator cap when the engine is hot. This can be done when the engine is cooler and when you are flushing it out from the system. To summarize, the cap should not be removed just to check the coolant level or to add coolant. There are other arrangements for the same. In fact, the coolant level is checked visually by looking at the calibration provided on the expansion tank. If coolant is to be added, it is added through this expansion tank.