Why does the radiator backflow when the engine is not overheating?

[DelHannah]

There are two scenarios when the car is not overheating but the coolant reservoir backflows: 1. Radiator backflow when pressing the accelerator pedal hard: This is mainly caused by large gaps or cracks in the wet cylinder liners or cylinder head. When the engine is running, the pressure of compressed air or combustion gases in the cylinder exceeds the pressure of the coolant in the cooling passages, allowing these combustible gases to enter the engine's cooling passages through the gaps, resulting in coolant reservoir backflow. 2. No air in the coolant during engine operation: Whether the water temperature is high or low, the radiator filler neck does not spray coolant when accelerating, but sprays coolant from the filler neck when decelerating.