I have noticed that when the AC is running and the cabin is closed, the smell usually comes from the outside, by smell i'm referring to the truck exhaust smell and generally. What could be the reason
cars are not and cannot be air tight even when the cabin is closed.maybe some especially designed cars for that can be, but normal cars are not air tight so it is quite normal for every car.
check condition of AC Cabin Filter and make sure to set the air circulation to Indoor only.
Check this knob. While using AC, keep the knob on left side (Inside Air circulation only)
I had a gap between the blower housing and the part it connects to in the firewall. I sealled it up with more with packing foam, problem solved - no direct fumes from the front.
If you are surrounded by smell overall, it will still come in side.
