the invention concerns a device for controlling the vehicles fuel - air mixture by regulating the air in the ventilation passage leading to the engine air intake from the crankcase . in a vehicle provided with a pcv valve , the device is located in the ventilation passage leading from the crankcase to the engine air intake and the device is downstream of the pcv valve . the device admits outside air to the ventilation passage to lean the gas mixture when the engine is operating at normal highway speeds or under load conditions and shuts off this flow of outside air as when the engine is running at low speeds or idling when the gas mixture need not be leaned .