If an upper radiator hose collapses after an engine cools, what is the likely cause?

The likely cause of an upper radiator hose collapsing after the engine cools is an overconcentrated anti-freeze-to-water solution. When the concentration of coolant is too high, it can lead to an imbalance in the cooling system, affecting the fluid's ability to expand and contract properly. As the engine cools, the reduction in temperature causes the coolant to contract. If the solution is overly concentrated, it might not have the appropriate properties to maintain pressure, which can lead to a vacuum forming in the upper radiator hose. This vacuum effect can pull the hose inward, resulting in its collapse.

A proper antifreeze-to-water mixture not only protects against freezing and boiling but also maintains the correct pressure in the cooling system to prevent such issues. Balancing the concentration according to manufacturer recommendations is crucial for system efficiency and component integrity.