How does a pressure cap function in a cooling system?

A pressure cap plays a crucial role in a vehicle's cooling system by regulating the pressure within the system, which in turn allows the coolant to flow effectively. The primary function of the pressure cap is to maintain a certain level of pressure within the cooling system. When the engine heats up, the coolant expands and increases the pressure. The pressure cap allows some of this pressure to build up while preventing it from exceeding safe limits.

When the pressure within the system reaches a certain point, the cap opens, allowing excess pressure to escape, which prevents damage to the cooling system. At the same time, a properly functioning pressure cap ensures that coolant can flow back into the cooling system from the overflow reservoir as the engine cools down. This circular movement of coolant helps maintain optimal temperatures and prevents overheating.

The other choices address functions that are related but do not capture the primary or unique functions of the pressure cap. For example, while the cap might prevent steam from escaping, its more critical role is pressure regulation. Similarly, it does not circulate the coolant or directly elevate boiling points, although maintaining proper pressure does indirectly support both of these processes.