Which component is primarily responsible for preventing cooling system boil over?

The component that is primarily responsible for preventing cooling system boil over is the pressure cap. The pressure cap is designed to maintain a specific pressure within the cooling system, typically around 15 psi. By maintaining this pressure, the boiling point of the coolant is raised, which helps to prevent it from vaporizing while the engine is operating at high temperatures.

When the engine heats up, the coolant absorbs heat and expands, increasing the pressure in the cooling system. The pressure cap allows the system to contain this increased pressure, preventing the coolant from boiling over. If the pressure exceeds the cap's rated pressure, it will open and allow coolant to escape to a reservoir, which helps to prevent damage from excessive pressure and maintains the integrity of the cooling system.

Other components like the water pump, thermostat, and radiator play critical roles in the cooling system's overall function, such as circulating the coolant, regulating engine temperature, and dissipating heat, respectively. However, they do not directly prevent boil over in the same manner that the pressure cap does.