1987 560sel.Has a rough idle but runs great.Also has a slight starting problem.When I shut off the engine and crank it up within a few seconds it starts fine but still idles rough.If I leave it off for a little more than a minute or so, I crank it and it starts but dies right away.I then crank it again and it starts fine but still idles rough.Is there a check valve in the fuel system that keeps it pressurized after shut down and would the rough idle and starting problem be related.