Have an 87 Fbird that overheats only when the interior heat is off. Is this a sign the thermostat is bad? Btw, warmed up the car with the heat on yesterday and both upper and lower hoses got very warm (upper one first) and fan kicked on when coolant in expansion tank reached "hot" level. Any help appreciated.

 

what does your temp gauge show? if it gets up near the max, i'd definately check your thermostat, stat.