My old motor has its "aroma's" that are never going to be cured with my budget. When I have the vent air open and I am sitting still or going slow, the vent sucks hot smelly air from around the engine and brings it right to your face.
I dont know if anyone has found a cure for that, this is my method:
At $8, it is a inexpensive duct adapter from Lowes
It fit over the vent intake with a little bending and screwed in place with 3 sheet metal screws. I used some 6" aluminum flex duct and routed it to the grill where I made it oval shaped and we have fresh(er) fresh air.