I'm dumb founded as to why the right passenger side heater only puts out cold air on my 89 735il. I checked the water valves under hood and both are open. However when I turn the heater on the left side water tube gets hot but the right side remains ambient temp. Is there a control valve of some type in the lines that regulate flow? All of the servo motors that operate the ventilation doors appear to work. Air comes out of the top, front and bottom vents on right side except that it never gets warm. I would appreciate any help.