Idle on 1987 560sel

[Fishead]

I have been trying to trace the awful idle on my 560sel.Once past 1500RPM it is as smooth as silk.My fuel economy guage seems to be off towards the red a little at idle.Never sits pegged to the left unless I am at highway speed and easy on the gas.Is this fuel economy guage reading a possible indication of a vaccum leak and were on this vehicle does it attatch to the manifold.I have searched everywhere for a vaccum leak and cannot find one.Also my milage is lousy!