What Are the Reasons for Coolant Overflowing from the Reservoir?

[Cayden]

Here is a detailed explanation of the three main reasons for coolant overflowing from the reservoir: 1. A damaged cap can cause the coolant to leak out. 2. Overfilling the coolant—if the coolant level exceeds the marked line on the reservoir, it may overflow while driving. If the coolant level is not excessive, overflow will not occur. 3. Thermostat failure—a faulty thermostat can lead to excessively high engine temperatures. When the engine temperature is too high, the coolant cannot enter the engine's water passages for a full circulation, causing the coolant to circulate only within the radiator (small circulation), which may result in overflow. When the engine temperature rises, the coolant expands, and some of it may flow into the overflow reservoir through the radiator cap. Once the temperature drops, the cooling system's pressure will draw the coolant back from the overflow reservoir into the radiator. The overflow reservoir has marked lines to indicate the proper coolant level. If the coolant is overfilled, it may cause overflow from the overflow reservoir. Additionally, these marked lines help determine whether the current coolant level is sufficient.