Why is the BMW 5 Series auxiliary water tank constantly boiling?
2 Answers
Overfilled coolant, cooling system blockage, gas-water channeling, etc. Overfilled coolant causing expansion and overflow: During the use of coolant, due to its significant thermal expansion, only 90% of the radiator's capacity should be filled, leaving expansion space at the top of the radiator. Otherwise, if overfilled, it is prone to overflow when the temperature is too high. Overheating causing overflow: Overheating overflow is a common phenomenon. When the engine temperature is too high, it causes the water in the radiator to boil, resulting in a large amount of overflow and steam, which can easily scald people if not careful. Causes of overheating include: insufficient water in the radiator, poor heat dissipation, malfunctioning shutters, damaged water pump, loose fan belt, or faulty thermostat.
I've driven quite a few BMW 5 Series models and often encountered the issue of the auxiliary water tank frequently boiling. This is usually caused by overheating problems within the cooling system. Specifically, it could be due to a damaged cylinder head gasket, allowing high-temperature gases from the engine to leak into the coolant, causing a sudden temperature spike and boiling—especially noticeable during acceleration. Alternatively, the thermostat might be stuck in the closed position, preventing smooth coolant circulation and causing heat to accumulate around the engine. Other common causes include an aging water pump, malfunctioning fan, or a clogged radiator. If left unresolved, the engine could suffer severe damage, such as cylinder head warping or cracking. Therefore, I recommend first checking the coolant level to see if it's too low or shows signs of oil contamination or air bubbles—if these are present, stop the car immediately and take it to a professional shop for diagnosis. A pressure test can accurately identify the leak point, and in most cases, replacing the gasket or cleaning the radiator will solve the problem.
