Diesels can pass emissions with all that black smoke, because that smoke is known as particulate matter, or soot, and it's completely different than hydrocarbon emissions. Soot is usually plain carbon (like the graphite in your pencil), whereas un-burned hydrocarbons lead to all kinds of nasty stuff, like acid rain, and holes in the ozone layer, and little kittens being killed, etc.....
Emissions requirements have always been ahead of technology. Since the Clean Air Act, fuel mileage regulations have been relaxed about once every two years, as the general public is simply not interested in purchasing vehicles that get 40 mpg. Too many Expedition-driving soccer moms.
In fact, the Expedition/Navigator can not be sold in California, because it's cold-start emissions are too high.
Cold start emissions come more from the excessive fuel being dumped into the cylinder to allow combustion to happen moreso than it does from the turbo acting as a heat-sync. The University of Texas at Austin is working on a distillation system that takes the highly-volatile factions out of gasoline to enable lower cold-start enrichment multipliers. This system has proven to reduce cold-start emissions by 70% in the Navigator/Expedition. Cool stuff.
Nick