What specific symptom would indicate a failure of the cooling system pressure cap?

A primary symptom of a failed cooling system pressure cap is coolant overflow or loss from the coolant reservoir or radiator. The pressure cap maintains a specific pressure within the cooling system. This pressure raises the boiling point of the coolant, preventing it from boiling over at normal operating temperatures. If the pressure cap fails to hold the designed pressure, the coolant will boil at a lower temperature than intended. When the coolant boils, it expands and overflows, leading to coolant loss. This coolant loss can be observed as coolant leaking from the overflow tube of the coolant reservoir or directly from the radiator cap if the system doesn't have a reservoir. Another symptom is overheating. A faulty pressure cap reduces the cooling system's ability to dissipate heat efficiently. Because the coolant boils prematurely, it's less effective at transferring heat away from the engine, which can lead to the engine overheating. Hoses collapsing can also indicate a faulty pressure cap. During engine cool-down, the pressure cap is designed to allow coolant to flow back into the engine from the coolant reservoir, preventing a vacuum from forming. If the cap malfunctions, it may not allow this coolant return, creating a vacuum that can cause hoses to collapse. A visual inspection might reveal a damaged or deteriorated rubber seal on the cap, further indicating its failure. Therefore, coolant overflow combined with overheating are strong indicators of a failing cooling system pressure cap.