Interesting problem. I have a 81 300 d non turbo. Heat works fine when driving around town in a city type speeds. Temp stays right around 80 degrees Celsius. When I get on the expressway, at roughly 55 mph it cools off after about a minute. Temp gauge still reads 80ish. I added some cardboard behind the grill still with no improvement. I'm currently running diesel in it even though I have a Lovecraft waste oil kit. Any help would be great. It's going down to negative numbers tonight here in MI so it should be a cold ride to work in the morning. Thanks in advance.
Brian