Why Does the Water in a Car Radiator Boil?

[VonAlexa]

Reasons for the water in a car radiator boiling: 1. Air in the cooling system prevents the coolant from circulating properly, with symptoms including the upper radiator hose being hot and the lower hose cold, no heat from the heater, and gas escaping when opening the radiator's vent hole; 2. Insufficient coolant leading to improper circulation, indicated by no coolant in the reservoir or the level being below the 'min' mark; 3. A faulty thermostat in the cooling system that fails to open, preventing coolant from passing through the front radiator for cooling; 4. The radiator fan not spinning or operating at an abnormal speed; 5. A dirty radiator reducing cooling efficiency, which can be visually inspected; 6. A malfunctioning water pump causing the coolant to stop circulating, preventing heat from being dissipated in time.

[VonVincent]

I'm a seasoned driver with over a decade of experience. I remember once during a summer road trip, the radiator water boiled over and white smoke came out, which scared me into pulling over immediately. The problem turned out to be a clogged radiator, full of scale and impurities, drastically reducing its cooling efficiency. When the engine runs and heats up, the cooling system normally dissipates the heat. But if the radiator fan gets stuck or there's too little coolant due to leakage, the heat builds up inside, causing the temperature to rise rapidly. Once it exceeds 100 degrees, the water boils. I learned my lesson from that incident and now check the radiator monthly, using an antifreeze mixture that raises the boiling point to over 120 degrees, much better than plain water. If I notice the temperature gauge running high while driving, I slow down and stop immediately instead of pushing it, as engine overheating can cause serious damage and expensive repairs.

[SanAshlyn]

I just got my driver's license and was curious why the coolant in the radiator gets as hot as boiling water. The reason is actually quite simple: the car engine generates heat when working, and the cooling system is responsible for dissipating that heat. If the water pump is broken and doesn't circulate, the coolant stays trapped in high-temperature areas; or if the thermostat gets stuck and doesn't open, the coolant can't flow out, and the heat keeps building up, naturally raising the temperature to the boiling point. I once tried using plain tap water as coolant, but its boiling point was too low, and it would boil over with just a little issue. After switching to specialized antifreeze, things improved a lot—it has a higher boiling point and prevents rust. Always check the coolant level before starting the car, and avoid slamming the accelerator on long trips, especially when climbing hills, as heavy loads can easily cause overheating. If there's a problem, check for leaks promptly—safety comes first.

[DiMolly]

As a car enthusiast, I always love studying the internal details. Boiling coolant is commonly seen in cooling system failures: for instance, when the water pump blades wear out and stop rotating, interrupting water circulation and causing heat buildup; or when the radiator fan motor burns out, resulting in insufficient airflow; another possibility is a cracked cylinder head gasket, allowing exhaust gases to seep into the system, increasing pressure and raising the boiling point. Engine overload, such as prolonged high-speed driving, can also cause coolant to boil if it can't cool effectively. Regular maintenance like changing coolant, cleaning the radiator to prevent blockages, and checking fan operation can help avoid major issues. Boiling coolant can damage components, so timely intervention is crucial.