The oil seperator on the valve cover is vented to the intake system just upstream of the throttle body. As there are crankcase gases released inside the engine they are sucked out of the valve cover and into the inake and burned to reduce emissions. Sometimes oil residue will build up in the throttle body, usually leaving a black carbon like deposit. In some cars this can cause a stuble at idle or when coming off idle. You can remove the intake tube, use the throttle cable to open the butterfly and use carb cleaner and a toothbrush to clean it. Any carb cleaner that gets inside will smoke on startup, it's normal.