What causes water to gush out when opening the radiator cap?

[VanChase]

The reason for water gushing out when opening the radiator cap: Generally, it's due to thermal expansion and contraction, which is a normal phenomenon. Introduction to car radiators: Normally, the radiator cap covers this position, with the spring pushing the gasket to seal the radiator, ensuring the cooling system operates under certain pressure. When the cooling system pressure becomes too high, the coolant pushes open the spring, and excess coolant vapor flows through the adjacent tube into the expansion tank. Precautions when opening the radiator cap: A standard radiator cap is part of this cooling system, where the radiator has a coolant filling port. The radiator cap should not be opened casually because, under high temperatures, opening it may cause coolant to spray out, which is very dangerous. It controls the overflow of coolant from the radiator to the expansion tank and also regulates the flow of coolant from the expansion tank back into the radiator.

[VonKylie]

I just encountered this issue the other day - as soon as I twisted open the radiator cap, coolant came gushing out with a hiss, which was really startling. The main reason is that the cooling system builds up excessive pressure when the engine is hot, especially if you try opening the cap immediately after a long summer drive when the engine is turned off. The radiator cap itself could also be problematic - its internal spring might have weakened with age or the pressure relief valve could be stuck due to scale buildup, preventing proper pressure release. Another possibility is overfilling the coolant beyond the maximum level, where thermal expansion forces the excess liquid out. It's always best to wait until the engine cools down completely before opening the cap, then cover it with a thick towel and slowly twist it open to relieve pressure.

[LaEmerson]

Water gushing from the radiator cap is a common occurrence in cooling systems under high-temperature conditions, and I understand the anxiety it causes car owners. The primary reason is the thermal expansion of coolant, which leads to a rapid pressure increase in the sealed container. Common triggers include a faulty radiator cap causing improper pressure release or air pockets in the cooling system affecting circulation efficiency. Additionally, antifreeze levels exceeding the standard mark by more than 15% can exacerbate this issue. Last time I helped a friend, we found his overflow tank was overfilled - after removing the excess liquid and reopening, the gushing stopped immediately.

[Fred]

When the cooling system overheats, the pressure can exceed 1.5 kg. Opening the cap suddenly at this point is like opening a soda bottle. I've experienced this several times, usually due to one of these three situations: the cooling fan not working causing continuous temperature rise, engine cylinder gasket failure allowing combustion chamber gases to enter the water passages, or the radiator cap's vacuum valve getting stuck. An important reminder is to use an infrared thermometer to check the temperature at the radiator cap before opening it—never attempt to open it if the temperature exceeds 60 degrees.