What causes a vacuum draw through the oil fill cap?

A vacuum draw through the oil fill cap can be caused by several factors, primarily related to engine design and condition. It often indicates that the engine's positive crankcase ventilation (PCV) system is malfunctioning, leading to inadequate ventilation and creating a negative pressure in the crankcase. Additionally, excessive blow-by gases from worn piston rings can contribute to this condition, resulting in a vacuum effect as the engine operates. Lastly, a clogged or restricted ventilation system can also lead to a vacuum draw through the oil fill cap.