no heat on the highway
 

'85 300 TD

For some reason I have no heat on the highway. I get warm air once the engine is warm, but when I begin to drive at highway speeds it just blows cold air.

What could cause this? Could the thermostat be stuck open?

-Erik

Good guess, it sounds like your t-stat is stuck open to me.

Well... what is the coolant temp gauge reading? Does it go drop down close to 80 or lower once on the highway with a previously warmed engine?

Yeah, my engine temp looks to be around 70 on the highway. Is the thermostat easy to change? Do I need to drain the system first?

-Erik

Sounds like a bad mono valve.

How does a bad mono valve behave differently than a thermostat that is stuck open?

Besides inspecting the monovlave, the easiest way to tell is to drive the car for 15 minutes around the neighberhood, shut the car off for a few mins, then start it again. If it blows hot for a minute or so (heat built up in the engine comp't), then starts to blow cool air, it's most likely the monovalve.
It usually fails because the rubber seal tears on the valve or the rubber diaphragm.