Fumes/stale air in passenger compartment

Get a CO detector in the car to verify it's exhaust fumes that are giving you the headache. If the exhaust is 100% sealed you shouldn't get any in the car no matter what. Use this link to find a good shop near you. https://repairpal.com/repair-shops Alex

Set HVAC control to RECIRCULATE for test purposes.. Possible oil leak under hood dripping on hot exhaust manifold - or other engine bay fumes.. Should be MORE PRONOUNCED while vehicle is stationary with engine on. RECIRCULATE should stop outside gases / fumes from entering cabin - ALL windows and rear hatch tightly closed of course.