R56/N12 engine. Engine idles rough when cold and throws a P2789 error. When warm, still idles rough - tries to stall. This may be unrelated, but after warmed up makes a rapid, muted, thumping sound for about 10 seconds after engine shutdown. Smoke test done, MAP tested, Vac Hoses checked. Been suggested it might be the PCV valve in the valve cover. Does anyone know a good way to verify that before replacing a $400 valve cover? Also, has anyone tried one of the aftermarket PCV valve replacement kits? Not sure how the kits works, since it is supposed to be a non-serviceable part.