Oil Filler Cap Gasket
A rubber seal on the underside of the oil filler cap that prevents oil vapors and crankcase pressure from leaking.
An oil filler cap gasket is a rubber or silicone ring or flat seal located on the underside of the oil filler cap. The filler cap is positioned on top of the valve cover, providing the primary access port for adding new motor oil to the engine. The gasket's job is to create an airtight seal when the cap is twisted or screwed into the valve cover opening.
Because the crankcase is a sealed system under slight vacuum or positive ventilation (regulated by the PCV valve), the oil filler cap must be fully sealed. A cracked, compressed, or missing gasket will allow engine oil vapors to escape, creating a messy buildup of oil grime on the valve cover. More importantly, it creates a vacuum leak by letting unmetered air enter the crankcase, which can disrupt the PCV system and cause rough idling or trigger a Check Engine light.
During oil changes, the filler cap gasket should be inspected. If the rubber has become hard, cracked, or compressed flat, the gasket or the cap itself should be replaced. Applying a light smear of fresh engine oil to the gasket before installing the cap ensures a smooth seal and prevents it from sticking during the next service.