At first I thought it strange that a 103 motor had no monovalve. I then looked up the heater valve for your car and sure enough.
Look at the photo of it below. Picture on the left has what appears to be a vacuum hose connection on top.
Vacuum issue or perhaps the valve is defective? Just my 2 cents.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1U40N9POF1UI0W5PAW&year=1991&make=MB&model=190-E-005&category=R&part=Heater+Valve