My '89 560SL cranks out heat better than my 2011 BMW. The problem - the heater only works when I start the car with the heater on "Max," defrost and automatic. A monovalve issue? Any suggestions? I'd like to fix the problem so that I can run the heater as intended. I know - I should be happy that the heater even works . . . but, this car is in awesome condition and I'd like the heater to work as it is supposed to work.