Hey John, I have a euro 1984 635 that has the same problem. Already had the heat valve replaced, firewall grommets checked and still when I close all slider bars to the left, I GET THIS HOT AIR from the defrost and lower heat vents as the car is traveling, with what seems to be the outside air forceing the hot air thru. I'ts driving me nuts!I CAN'T SEEM TO GET THE CORRECT ANSWER TO THIS. If you figure it out please let me know. Summer is just around the corner. Thanks, Chuck