What would be some tells that the previous owner changed the program in the ECM? I have black smoke come out the exhaust when I accelerate, it's enough that it leaves residue on the rear bumper. I've had the truck for about 6 weeks. Since I put a new thermostat in it seems as though the mpgs have been declining. Prior to the change of the thermostat, it would take forever to warm up, now it works like it should. Also I got consistant 18 MPG, hand calculated, now 16 seems to be the best. I would think that since the engine is warming up completely, the burn would be better therfor providing better mpgs.Thoughts?

Best idea is to head to a dealer and let them re-flash the ECM with up to date software then you know your good.

I had considered that, but then I thought that I was being paranoid, and decided to ask a few questions before I go to that trouble and expense.Anyone know what it costs to get that done?

Usually charge 1 shop hour for ECM flash... But you might have the injector test for bleed down to see if the black smoke is from a possible failed injector. Fairly common on CR engines.