What symptom indicates a faulty vacuum valve in the radiator cap?

A faulty vacuum valve in the radiator cap can lead to a collapsed radiator hose, which occurs when the hose is unable to maintain its shape due to loss of vacuum pressure within the cooling system. The vacuum valve's primary function is to allow air to enter the cooling system when it cools down, preventing it from collapsing under negative pressure. If this valve fails, the necessary air may not be allowed to enter the system, leading to the creation of a vacuum that can cause hoses to collapse during engine operation.

In contrast, symptoms like engine overheating are often related to coolant circulation or other cooling system malfunctions, while extended warm-up time typically points to issues with the thermostat or engine components. An overfilled coolant reservoir is frequently caused by improper filling or thermal expansion of coolant and does not directly relate to the function of the vacuum valve. Therefore, a collapsed radiator hose specifically points to the failure of the vacuum valve, making it the most indicative symptom of its malfunction.