It would really be helpful if you can post the emission test report. Various values give a good clue as to what the problem(s) may be. Also search for emission research posts started by me, which include easy ways to lower M103 emissions like shorting the R16/1 resistor socket, disablinlg the vacuum advance, and verifing that the installed spark plugs are non-resistor type.

My 190E 2.6 has a five-speed manual and normal idle in Neutral is 700 @ 15". An automatic idling is drive will show lower speed and vacuum, but I don't have any data as to what is correct. These engines have a pretty aggressive camshaft with a fair amount of overlap compared to most of that era, so idle may not be perfectly smooth and may contribute to relatively high HC emisssions.

It's actually likely that the problem(s)may be minor, so don't start tearing everything apart until you research M103 emission test issues as recommended above.

Duke