Chevrolet Repair: Overheating, electric cooling fan, chev

Question
Yesterday it was about 5 degrees outside - My son was driving his 1993 Chev S-10 PU - 4.3L V6 - Automatic - 2WD, on the freeway

When he would run the heater the PU would overheat. Shut off the heayer PU ran Normal Temps, we can not see any leaks - Heater on Overheat - Heater Off Normal Temps, This does not make sense. I ran PU in my driveway, for 15 min, heater on, did not overheat.

Answer
Hello Carl,
I would check the engine to see if it was really overheating, or just the gauge indicating different when heat is selected.
Also, make sure there isn't air trapped in the system.

If that truck had an electric cooling fan for the radiator, I would almost think it was running backwards, stopping the air flow through the radiator caused by foreward motion.

Van