A customer drives a vehicle into a shop and wants to know why the upper radiator hose collapses as the engine cools off. The most likely cause is a defective

• A. Thermostat
• B. Radiator cap
• C. Water pump
• D. Coolant

Answer: (A)

A defective thermostat is the most likely cause because it controls when coolant circulates and vents in the cooling system. If the thermostat sticks closed, coolant cannot flow to the radiator, so the system becomes effectively sealed. As the engine cools, the coolant contracts and a vacuum forms in the hoses. Without an open path for air to replace the shrinking coolant, that negative pressure pulls the upper radiator hose inward and causes it to collapse. A thermostat that frees up or opens at the proper temperature would allow coolant to circulate and the system to vent as it cools, preventing this collapse. (A bad radiator cap or other parts can cause related symptoms, but the vacuum collapse during cooldown points most directly to a stuck-closed thermostat.)