Same exact problem with a brand new 2018 Silverado 2500 LT.
I thought the recirc was broken. I got behind a garbage truck spewing diesel fumes. I saw it 1/4 mile away and popped on recirc. As I got closer the fumes kept increasing. It was so bad my eyes and throat were burning. I couldn't exactly roll down the window for fresh air.
Just today I saw a restaurant's septic being pumped. Put on the recirc and sure enough when I parked I still got some of the odor into the cabin. That is what prompted me to find this conversation. I guess I won't pursue it with the dealer as no one has made any headway.
This is my first GM vehicle in 30 years. I love my truck except for this. If other truck makers can eliminate outside outdoors without killing their truck owners, why can't GM?
