Originally Posted by Gilles Kay

Also make sure that your system is giving you 100% recirc air when interior is initially hot on startup or as demanded by the recirc switch on the front panel. My '92 300E has one or both recirc valve which has a leak and will not give me 100% recirc air no matter what. Only 80% recirc air. I am going to pull the dash next week and replace these valves along with some other vacuum valves in the A/C system. You can check by pushing the recirc swtich up and getting the red light, then go outside where the air enters the A/C system at the back of the hood and feel for air draw or use a piece of paper. If there is any air draw at all, one or both of the A/C recirc valves has a leak on the high draw(100%) side.