The cooling system hoses on an engine collapse when the unit is left parked overnight. What is the likely cause?

The situation described, where cooling system hoses collapse when left parked overnight, is likely caused by a defective radiator cap. The radiator cap plays a crucial role in maintaining the proper pressure within the cooling system. When the engine cools down, if the radiator cap is faulty, it may not maintain the correct vacuum pressure. As the engine cools, the pressure inside the cooling system decreases, resulting in a vacuum that can cause the hoses to collapse.

If the radiator cap fails to provide an adequate seal or regulate pressure, it allows air to enter the cooling system, which can exacerbate the collapse of hoses due to this vacuum effect. Properly functioning caps create a pressurized environment during operation and allow the system to manage changes in temperature effectively when the engine cools.

The other choices do not lead to hose collapse in the same consistent manner. A defective thermostat would typically lead to overheating or underheating, while improper coolant might affect the system's efficiency but wouldn't cause hoses to collapse. Normal behavior would not entail hose collapse; thus, the condition described is not typical and suggests an issue specifically with the radiator cap.