I'm new to this site. So, let me introduce myself. My name is Bill Capetanos and I own an 84 300D Turbo. I use this car as my daily driver. My other car is a 67 mustang race car. (at the moment, completly disasembled.) I've had this car for a couple of years now. I have done a few repairs. The glow plugs, radiator, and brakes. The problem that I am having is with the heater. At idle, and not moving the heater works fine. Once I get going, sometimes it works and sometimes it blows nothing but cold air. I have checked the vacum lines and found no cracks. What else could be the problem? I'm learning as I go here. I'm used to working on nothing but Fords. Any help would be much appreciated.