Quote Originally Posted by leinahtan View Post
The culprit is the worn out foam seal on vent flaps. Vent flaps is located just inside your dash, which is also underneath the hood vent grills. You switch your AC system to outside air mode, the vent flaps opens thus outside air comes in, switching it otherwise, the vent flaps shut thus air just circulates within. But even when the flaps are shut, you may still experience outside air because of the worn out foams are no longer seating well.
+1 here. or you can have the vents sealed shut permanently. its not really useful here in metro manila.