What do carbon-deposit caked valves, decreased fuel economy, oily intake tracts and inefficient combustion all have in common? The resulting effects of engine oil blow-by. During combustion, the top side of the piston forces combusted gasses, with remnants of fuel and oil past the piston rings, and into the crankcase. This would force the crankcase to become pressurized without some sort of relief, so the content of what enters the crankcase exits the system through a positive crankcase ventilation (PCV) system and gets recycled through the engine via the intake tract.

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You may have heard of a device called an air oil separator or AOS. These devices tap into your PCV routing system