Glad i found this thread!!!! OCKlasse this is the problem with my 96 c280. Good cold air outta the center vents and heat on the sides!! What did you come up with to cure this prob. I have done some of the mentioned items in this thread but to no avail. I even disconnected the duo valve under the hood to lower the temp coming outta the side vents, that helped for about a 30degree reduction. But still didn't fix the vent prob.