What is a likely cause of the upper radiator hose collapsing when the engine cools?

When the engine cools down, a vacuum can form within the cooling system as the coolant contracts. If the radiator pressure cap is defective, it may fail to maintain the necessary pressure, allowing the system to create a vacuum. This vacuum can be strong enough to collapse the upper radiator hose, as there is insufficient pressure to keep it expanded.

A properly functioning radiator cap is essential for maintaining the right pressure in the cooling system and preventing issues such as hose collapse. The other options do not directly lead to the collapsing of the upper radiator hose in the same manner. Blocked coolant passages would hinder flow but wouldn't create the vacuum effect associated with hose collapse. A worn-out water pump can lead to inadequate circulation of coolant but doesn't typically affect the pressure in the hoses themselves. Leaking coolant would lead to a loss of coolant and potential overheating, but it wouldn’t generate the vacuum situation that causes the upper hose to collapse.