The Radiator Cap keeps the cooling system under pressure. It is also the system's safety valve. It allows the pressure to escape if it gets too high.
Why is it so important?
As the coolant warms up, it expands. This creates pressure in the system. Without a “safety valve” the pressure will eventually blow past gaskets and seals. It could also burst hoses and lines.
However, some pressure is good. It raises the boiling point of the coolant. This allows for a higher operating temperature range.
Consider the following:
- Water boils at 212° F.
- 50/50 coolant boils at 223° F.
- The boiling point goes up 3° F for every pound of pressure (psi).
- A 15-psi radiator cap will raise the boiling point by 45° F.
With the proper Coolant Mix and Radiator Cap, the maximum temperature is raised from 212° F to 268° F. Hopefully, your engine never gets that hot. But under some extreme conditions, you may need the additional wiggle room.
For Example
If your engine normally operates at 180° F - 190° F, boiling is not an issue. However, on a hot day, the coolant can get up to 200° F - 210° F. If you ran just water, you are pushing the boiling point. Any additional strain on the system (like getting stuck in traffic, passing a truck, or going up a hill) and the fluid will boil.
When the coolant boils, it makes bubbles and vapor (steam). This creates air pockets in the system. Air does not transfer heat the way fluid does. In this case, your engine would overheat.