Hi

I work for the AA as a Technical Specialist and pressure cap failure is a relatively common cause of breakdowns and hence call outs by members.

The engine's cooling system needs to run fairly close to boiling point for maximum efficiency, but obviously we could do without the coolant boiling, overheating the engine and throwing coolant everywhere.

Raising the pressure in the system increases the boiling point of the coolant and this is the function of the radiator/expansion tank pressure cap. The cap seals the system so while the engine is running the coolant heats up, expands and raises the pressure in the cooling system. The cap is fitted with pressure relief valve in case the pressure rises abnormally (usually around 14 psi or 1 bar).

If you drive without the pressure cap on the expansion tank, along a fast running road there will be enough air passing through the radiator to keep the coolant well below danger point. When you're stuck in traffic, the air flow stops until the cooling fan cuts in. Some fans are designed so they don’t start working until the coolant is above 100ºC, making it much more vulnerable to boiling and the engine is likely overheat as a result.