Glad the diagnosing is revealing clues!
Next step is to check out the vent flaps. I don't remember exactly where they are off the top of my head- but if you carefully pull back the trim on the sides near the bumper you will likely see them right along the bottom / side area of the fender walls.
You'll need a flashlight- you're looking for something like this:
Don't spend too much time sniffing BTW- carbon monoxide will sneak up on you quickly--- AMHIK!
It's not what you buy, it's what you build- Unknown
EN90283 - Stabled for HG repair
DH06247 - Running like a champ