PCV Filter
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Slightly dirty air filters have additional rough surface area to trap particles. They work even better when slightly soiled. Throw one rock in a stream. Throw a lot more, leaves and grass flowing by begin to get trapped. Do too many, and you have a pretend beaver dam. But if PCV vent is blocked up and your blow by and evaporation of oil can make a pressure to cause some seals to leak that otherwise would not.