I suspect you have a pretty large vacuum leak somewhere. The economy gauge should be sitting on the peg (or just off of it) at idle on the black end.

Have you checked spark? I only ask because I had a similar issue a few years ago and it turned out to be a loopy EZL, only found when it finally went belly up.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)