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

[DiSavannah]

There are two scenarios where the coolant reservoir overflows without the engine overheating: Overflow occurs when the throttle is pressed hard: 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 the compressed air or combustion gases inside 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. No air in the coolant during engine operation: Whether the coolant temperature is high or low, pressing the throttle does not cause the coolant to spray out from the filler neck, but releasing the throttle does result in coolant spraying out from the filler neck.