* Crankcase ventilation: The crankcase isn't airtight. As the pistons move, they create pressure changes within the crankcase. The PCV (Positive Crankcase Ventilation) system is designed to control this pressure. It vents blow-by gases (gases that escape past the piston rings) and maintains a slightly negative pressure within the crankcase. This negative pressure helps to prevent oil leaks and keeps the crankcase clean.
* Disrupting the vacuum: Removing the oil cap removes a key component of the PCV system, creating a large opening in the crankcase. This immediately disrupts the carefully balanced negative pressure. The sudden influx of air into the crankcase can significantly affect the engine's ability to breathe properly. The air-fuel mixture may become unbalanced, leading to a lean condition.
* Lean mixture and stall: A lean air-fuel mixture means there isn't enough fuel to combust properly with the suddenly increased amount of air. The engine may sputter, misfire, and eventually stall due to insufficient combustion.
* Oil splashing: While the disruption of the air-fuel mixture is the primary cause, the sudden change in pressure can also cause oil to splash excessively. This can further disrupt combustion and potentially damage engine components.
In short, removing the oil cap essentially introduces a massive air leak into the engine's breathing system, throwing off the carefully balanced air-fuel mixture and leading to a stall. It's crucial to never remove the oil cap while the engine is running.
