I'm confused. My '93 940 Turbo has a strong, clean engine. The mileage is right where it should be - 17 in town and 22 on the highway. It idles rock steady at 700 rpm. The plugs are nice and clean yet the tailpipe is black and sooty. If it was running rich I'd see it in the mileage and plugs, but it's not. So what's happening? Any clues?