I was driving down 217 here in Oregon, and I noticed that all of a sudden my heater is blowing cold air. When I slowed down, the air warmed up again. It seems that under load my heater is blowing cold, and when I get back down to under 50 mph it gets warm again. I am at a loss...head gasket?? Air bubble in the heater core?

 

Check the water level !

 

Opened the radiator shortly after I got home, and it was full...

 

Look to see if there is a mechanical valve hooked into the heater hose ??

 

Thermostat stuck open?

 

What is the temp gauge doing?

Have you checked the belt?

Does it have a water control valve (probably vacuum?)

I didn't bother to look up the heater in a 66, is this a factory AC car or heat only?

You have a shop manual?

Might be a blend door not working

 

Hey all...thank you very much for suggestions on where to start looking. Drag my thermostat was closed when I fired it up yesterday afternoon. I fired it up and drove about 12 miles today, and the heat was working. I never went over 50 though. 440...I do have a shop manual. I am going to check to see if it has a water control valve this afternoon. Its a factory a/c car, and the a/c works. Belts are new, and temp gauge is not accurate. New sender in my toolbox, just haven't had time to put it in. Started to yesterday, but my apartment complex received a complaint about my car so I had to stop . What I find hysterical is that my car is almost as quiet as the day it rolled out of the factory, having just installed new mufflers. Its timing is spot on, my carb is tuned in, and my old 383 is finally running as it should...with the exception of the heat. Nobody else has a 47 year old daily driver in my complex though...a lot of Volvos and Lexus. The neighbors should be happy my daily driver isn't some open header model A, or something equally loud...