This is a very good observation. I'm from the Philippines and we drive with AC on all the time. We also use AC under RECIRC since air outside is not so pleasant unlike in the US. That exhaust fume smell comes in most noteworthy when all windows are up and AC on RECIRC. Before the TSB 12-12-04, you can get a whiff of that smell right away when you suddenly accelerate from 2500RPM to 3500RPM. My dad has a 2017 Chevy Suburban and that SUV does not get any SMELL whatsoever even under WOT. So yeah, MR. GUBING, it's right of you to say you have no idea about their design but you do on the Explorer. Another Explorer owner here has a CO meter in his vehicle. It registered 30 ppm. I googled 30 ppm and here is what it showed: "30 ppm - Earliest onset of exercise-induced angina (World Health Organization)". If you have that exhaust fume smell, then it would be best to register it with the NHTSA. We don't have that kind of agency here in the Philippines, but if a considerable number of complaints have been filed there then it would reach here. All the Explorer units we have here are completely built units (CBU) from the US. Any recall there would also be applied there. For the record, I have not modified my Explorer at all and yes my unit has that smell.