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A malfunctioning radiator cap pressure valve that fails to seal can lead to coolant boil-off. The primary function of the radiator cap is to maintain pressure in the cooling system. When the system is under pressure, the boiling point of the coolant is raised, preventing it from vaporizing and helping to maintain a proper cooling temperature.

If the pressure valve does not seal properly, the pressure can drop, allowing the coolant to boil at a lower temperature. This boiling can result in vapor pockets forming within the cooling system, which reduces the efficiency of heat transfer and can lead to overheating. Ultimately, this can cause the coolant to evaporate, leading to coolant boil-off. This condition can seriously affect engine temperature regulation and potentially result in engine damage due to overheating.

The other outcomes presented do not directly correlate with a failure of the radiator cap pressure valve. For example, cooler operating temperatures would imply that the engine is maintaining a more efficient cooling process, which contradicts what happens with a failed cap. Higher HC emissions are typically associated with fuel combustion issues rather than cooling system pressure. Finally, cavitated cylinder liners occur due to cavitation from improper coolant flow, which is not directly related to the radiator cap's ability to seal.