What can happen if you fill a radiator while the engine is very hot?

Filling a radiator while the engine is very hot can lead to a dangerous situation where the coolant may boil and blow back. When the engine operates, it heats the coolant to high temperatures, creating pressure within the cooling system. If you add cold water or coolant to a hot radiator, the sudden change in temperature can cause the coolant already in the system to rapidly boil. This boiling can create steam and increase pressure even further, potentially forcing hot coolant to escape from the radiator or overflow reservoir, leading to burns or engine damage.

This scenario emphasizes the importance of allowing the engine to cool down before attempting to add coolant. Waiting ensures that the temperature and pressure are reduced, minimizing the risk of injury or damage during the process.