This mornings drive to work in my 81 300SD was a cold one for most of the way.Here are the stats,about 30 degrees out, 22 mile drive,temp gauge reads between 80-90, fan blows on all speeds,selector seems to work fine.I didn't get any warm air until about 15 miles into the drive.Is this normal? This is my first Mercedes diesel and have heard most diesel vehicles run cold in the winter.Should a piece of cardboard be put in front of the radiator or not?I did a search on monovalve, read a lot of posts but I don't think that is the problem. Any thoughts?