What Causes the Coolant Reservoir to Overflow When the Car Is Not Overheating?

[MalachiAnn]

There are two scenarios where the coolant reservoir overflows without the car overheating: 1. Overflow occurs when aggressively pressing the accelerator: This is mainly due to significant 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 cylinders 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 the coolant reservoir overflowing. 2. No air in the coolant during engine operation: Whether the coolant temperature is high or low, pressing the accelerator does not cause the coolant to spray out from the radiator filler neck, but releasing the accelerator results in coolant spraying out from the filler neck.