Now that is colder than crap here, the shortbox was running a little funny. Turns out that the flap on the air cleaner inlet was pulled shut all the time. GM set the intake temp so ridiculously high so they could lean out the mixture. Not good when its so cold that it never gets hot enough to open the flap. My engine had to breath through the little stove stack pipe until I caught it. The fix is easy enough. Pop off the air cleaner and look for the thermal vac switch. It will in the back left with a plastic cover on it. Pop the switch out (there's a clip underneath) and take off the plastic cover. There is a set screw that is now visible after doing this. Turning the set screw counter clockwise will allow you to lower the intake temp. I'd also take off the snorkel tube going to the grill when its extremely cold out. It just going to cool things off again and shut the flap. No more pathetic mileage and better power after the adjustment.