What Causes Water to Spill Out from the Reservoir When the Temperature is Not High?

[DiMadison]

There are two main scenarios where the car is not overheating but the reservoir spills water: 1. Water spills from the radiator when the accelerator is pressed hard: This is mainly due to large gaps or cracks in the wet cylinder liner or cylinder head. When the engine is running, the pressure of the compressed air or combustion gases in the cylinder is greater than 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 water spilling from the reservoir. 2. No air in the coolant when the engine is running: Whether the water temperature is high or low, pressing the accelerator does not cause the radiator's filler neck to spray coolant, but releasing the accelerator causes coolant to spray out from the filler neck.