What precautions should be taken when filling a radiator while it is hot?

Filling a radiator with cool water while the engine is idling is a practice that can help prevent sudden temperature changes and pressure build-up within the cooling system. When an engine is hot, the coolant within the radiator is also hot and under pressure. Introducing cool water while the engine is running can create a gradual temperature adjustment, reducing the risk of thermal shock, which can lead to warping or cracking of engine components.

This approach also allows for a more even flow of coolant throughout the system, ensuring that the water mixes appropriately with the existing fluid and helping to maintain an even operating temperature. It's essential to monitor engine temperatures and gauge levels during this process to avoid overheating or further problems.

Other methods, such as allowing the engine to cool down completely before filling, are also effective but do not provide the immediate benefit of maintaining a stable temperature within the system during operation. Filling with hot water can exacerbate overheating risks, and opening the radiator cap immediately can lead to scalding injuries due to steam and boiling coolant escaping under pressure.