Why Does Water Spray Out When Opening the Radiator Cap?

[OJesse]

Generally, this is due to thermal expansion and contraction, which is a normal phenomenon. Below is relevant information about diesel radiators: 1. Function: To cool the engine's water. In summer, it's necessary to switch to summer antifreeze coolant for cooling purposes. The boiling point of antifreeze is around 108 degrees Celsius, higher than that of water, preventing overheating. In winter, winter-specific antifreeze should be used to ensure it doesn't freeze outdoors while still providing cooling. Higher-grade antifreeze should withstand temperatures as low as -60 degrees Celsius. 2. Cleaning Method: First, prepare a cleaning solution with 30 kg of water, 2.5 kg of caustic soda, and 0.6 kg of kerosene, then pour it into the radiator. Start the diesel engine and run it at medium speed for about 10 minutes before shutting it off. After 10 to 12 hours, restart the diesel engine and run it at medium speed for another 10 minutes before shutting it off again to drain the cleaning solution. Add clean cooling water, start the diesel engine, and let it run at medium speed for self-cleaning. Repeat this process 2 to 3 times to completely remove scale buildup.

[KylieLynn]

A few days ago, I personally encountered this situation while checking my car. When I opened the radiator cap, water sprayed out violently, feeling like being hit by a high-pressure water gun, which startled me and made me jump back. The reason is simple: when the engine is running or has just been turned off, the internal temperature is very high. The coolant heats up, boils, and turns into steam, creating significant pressure inside the sealed radiator system—similar to a boiling kettle releasing steam. Once the cap is opened, this pressure is released instantly, and the hot liquid can easily scald someone. From my experience, it’s crucial not to rush. You must wait until the car has completely cooled down before opening the cap. For example, after turning off the engine, wait at least half an hour and ensure the radiator’s exterior is cool to the touch. For routine coolant checks, it’s best to do so when the car is cold to prevent such spraying accidents. After years of driving, developing this habit has significantly reduced risks.

[DiAustin]

I've seen many novice drivers directly open the radiator cap on a hot engine, resulting in scalding injuries requiring medical attention. The reason water sprays out when opened is because the high engine temperature creates pressurized air bubbles in the cooling system, which accumulate until they erupt violently. The solution is straightforward: after stopping and turning off the engine, wait patiently for the temperature to naturally decrease. As an experienced driver, I believe safety comes first—don't risk saving time. Be especially careful in hot weather when temperatures are higher and spraying is more likely. Remember, protecting yourself is more important than checking fluid levels. I recommend checking the cooling system during each car maintenance to ensure proper operation.

[GiaAnn]

Water spraying from the radiator originates from the pressurized cooling system. When the car runs and heats up, the coolant boils and expands, creating pressure that is released when the cap is opened. This is very dangerous, and scalding injuries are common. The correct approach is to turn off the engine and wait for the car to cool down before opening the cap to inspect.