Why can't the water in the car's auxiliary tank return to the main tank?
2 Answers
When the main radiator is short of water, the auxiliary tank will supply water to it. If the main radiator is short of water but the water in the auxiliary tank doesn't flow into it, it indicates that the radiator cap is faulty. Only when a negative pressure is created in the main radiator due to water shortage will the water in the auxiliary tank flow into the main radiator. Additional information: A poorly sealed radiator cap will cause the engine cooling system's internal pressure to increase when the temperature rises, allowing water to flow into the auxiliary tank through the radiator cap. Air in the cooling system can lead to poor water circulation, and the increased internal pressure may force open the valve in the radiator cap, causing water to flow into the auxiliary tank. However, this situation is usually accompanied by overheating symptoms.
When I used to drive my old car, I encountered a situation where the water from the auxiliary tank couldn't return to the main radiator, mainly due to a malfunction in the cooling system's water circulation. The most common issue was a poorly sealed radiator cap, which couldn't maintain system pressure, resulting in insufficient negative pressure during cooling to draw the water back. Another possibility was clogged dirt or air bubbles at the hose connections, obstructing fluid return. Once during a long drive, the water temperature suddenly rose. After stopping, I found the auxiliary tank full but the main radiator empty. Upon inspection, I discovered that the connecting hose had aged and accumulated grime. After cleaning, everything returned to normal—otherwise, it could have led to engine overheating or even a blown gasket. During regular maintenance, I always recommend checking the radiator cap's condition to ensure the spring and sealing ring are intact, the hoses are clear and free of cracks, and the coolant is added according to the manual to avoid introducing air bubbles. These small precautions prevent major troubles.
