Why does the oil cap is being sucked in?

If the oil cap is being sucked in, it could indicate a problem with the engine's internal pressure, often due to excessive vacuum in the crankcase. This can occur from a malfunctioning Positive Crankcase Ventilation (PCV) system, which is responsible for regulating pressure and gases in the engine. Additionally, it may signify a more serious issue, such as excessive blow-by from worn piston rings or other internal engine problems. It's important to address this issue promptly to avoid potential engine damage.