Hurshi,

Is your car an automatic transmission ? Is your "economy"meter working ? On most MB's with auto trannies there is a vac or boost connection from the manifold to a switch over valve. And as yours is not a Diesel, there should be a line from the manifold to the economy meter as this works on reading the manifold "press" or better, the manifold vacuum for non turbo's.
Let us know your findings...

Danny