The smell and MPG drop may not be related. Sniff around and locate where the smell is the strongest, my bet is near the gas cap. Could be the cap, a hose underneath (the rubber ones harden and crack on the ends) or the seal in the fill tube. There's a good post on sealing the fill tube on here somewhere, I sealed mine but still have a little smell now and then but no more CEL.
As for the MPG drop, could be many things. Dirty injectors, air filter, EGR port clogged, upper O2 sensor on the way out. None of these items is really expensive to clean or in the O2 sensors case replace and without a CEL you have no smoking gun. OEM parts are the best bet. Good luck and keep us posted.
As for the MPG drop, could be many things. Dirty injectors, air filter, EGR port clogged, upper O2 sensor on the way out. None of these items is really expensive to clean or in the O2 sensors case replace and without a CEL you have no smoking gun. OEM parts are the best bet. Good luck and keep us posted.