[83GMCK2500]

The black smoke is normal when you roll into it to pass. Particularly since you've removed the cat, they have a tendency to trap some of it.

The white smoke when getting off of it is cause for concern. What kind of RPMs were you turning to pass the person? The LBZ Duramaxes (3rd gen, 06 & 07 Classic) are hands down the best Duramax to date. They've got a lot of improvements over previous generations without most of the newer emissions devices. My roommate is from Poulsbo, he says take it to Jet Chevrolet, not Grey (they are hit & miss). Both of us actually have 2007 Classic Duramax equipped trucks.