Engine Crankcase Breather hoses are part of a vehicle’s Positive Crankcase Ventilation (PCV) system. The job of the PCV system is to allow pressure to escape from the crankcase in a controlled manner. It is necessary to vent the crankcase of positive pressure caused by engine blow-by (gas from the combustion event that leaks past the pistons.) This blow-by gas is routed back into the intake air stream thereby reducing the emission of unburned hydrocarbons into the atmosphere. Damaged crankcase breather hoses are a common source of engine vacuum leaks and often a source of check engine lights in modern vehicles.