The crankcase ventilation leads the gas that forms in the crankcase back to the intake and separates the oil out of it on the way. If the system clogs or the membrane tears, the pressure cannot get away: it finds a way out past the seals, or it pushes the oil through the piston rings. This is where the oil consumption comes from, and the oil that appears around the valve cover gasket. In the other direction a faulty breather takes unmetered air into the intake, and that is what makes the idle rough, the throttle response hesitant and the running coarse. Starting also gets harder, and in an extreme case the engine stalls at idle. Two characteristic signs help: weak or absent suction at the oil filler with the engine running, and noticeably rough running around 3,000 rpm. The hose at the rear of the valve cover can also let oil out. The sound insulation then soaks itself with oil, and oil gets everywhere in the engine bay. On the six-cylinder engines the breather is built into the valve cover, so the repair is replacing the cover or the whole oil separator together with the hoses, between 80 and 900 euro.