What should you never do with a hot radiator cap?

Removing a hot radiator cap can be extremely dangerous. When the engine is running or has recently been turned off, the radiator can become pressurized due to the heat, which causes the coolant inside to expand. If the cap is removed while it's hot, the built-up pressure can cause hot coolant and steam to erupt violently from the radiator, potentially leading to severe burns or scalds. It is essential to let the engine cool down to a safe temperature before attempting to remove the cap, ensuring that the pressure has dissipated and preventing any hazardous situations.

Checking the coolant level or shaking the cap is also unsafe when hot, and directly adding coolant to a hot radiator can lead to thermal shock and damage to the radiator. Always ensure safety precautions are observed around hot engine components.