You are certainly running very rich -- you should smell sulfury smells, too.
The connection for the O2 sensor is just in front of the front seat, passenger side, under the carpet, I think (this is the location in the 300TE). Three wires, two together and one separate, unplug the two.
There are other tests, but I'm not sure on the 91 as it has LH Jetronic injection, I think.
I suspect the rich running problem has nothing to do with switching between sport and normal mode.
I would suspect the O2 sensor, though. You may not get a check engine light until it is completely non-functional.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!