I have an 84 300d. The heat works great if I keep my speed under 45 mph. Above that, the fan still blows air but not hot. When I slow down, the air instantly gets hot. I have searched some threads but have not read anything relating to this problem. If I read correctly, the water valve is vacuum operated but the default is open. If I have a vacuum leak would not the valve default to open?

Thanks in advance for your input.

Steve