The air recirculation system is designed that way. It will remain on for 30 minutes, then shuts off automatically. However, after shutting down, you still receive 80% recirculated air and 20% outside air if I remember correctly. This is I believe to prevent mold/mildew from forming inside your AC system.
According to the CD manual, it is possible to switch the system to 100% recirculated air for an unlimited period (quoting) if with outside temperatures above 20c, the temperature selector wheel is engaged in the "Min" position.
The MB AC systems are very advanced ... and do alot more than you think they might -- even the older models like your '87. The CD manual has a 35 page PDF document explaining the intricacies of the system, just to give you an idea.
'91 300E, 207k miles
Last edited by pmizell; 02-12-2002 at 11:59 AM.