Coolant boils when its temperature exceeds its boiling point, which is significantly influenced by the pressure within the cooling system.
Automotive cooling systems are specifically designed to prevent coolant from boiling under normal operating conditions. This is primarily achieved by maintaining the system under pressure.
The Role of Pressure
The core principle is that cooling systems are pressurised to raise the boiling point of the fluid, with the pressure cap and the overflow helping to raise the temperature at which the coolant/antifreeze boils. This means coolant can reach temperatures well above 100°C (212°F), the boiling point of water at sea level, without turning into vapor.
- Normal Operation: Under pressure, coolant remains a liquid even at high temperatures, efficiently transferring heat away from the engine.
What Causes Coolant to Boil?
Coolant typically boils when the pressure in the system drops below its normal operating level. This can happen for various reasons:
- Leaks: When there's a leak in the cooling system, the pressure of the liquid drops and so does the boiling point of the coolant/antifreeze. Even if the engine temperature hasn't significantly increased, the reduced pressure causes the coolant to reach its lower boiling point, resulting in boiling (often seen as steam or rapid bubbling).
- System Malfunction: Issues with the pressure cap or other components that maintain pressure can also lead to a drop in pressure and subsequent boiling.
Pressure vs. Boiling Point
Here's a simple illustration of how pressure affects the boiling point:
| System Condition | Pressure | Coolant Boiling Point (Example) |
|---|---|---|
| Atmospheric (Leak) | Low (Ambient) | ~100°C (212°F) |
| Pressurised System | High (System) | ~120-130°C (248-266°F) |
(Note: Specific boiling points vary based on the coolant mixture and system pressure.)
In essence, coolant boils when the temperature it reaches surpasses its current boiling point, which is significantly lowered if the protective pressure of the cooling system is lost, often due to a leak.
