A good friend of mine has a 2000 Chevy Cavalier, manual transmission. We live in Iowa, so we NEED our heaters to work - it’s below zero about half the year around here!
Anyway, my friend’s Cavalier warms up fine if she’s just driving around town. But once she gets on the highway and drives faster than 60mpf or so, the toasty heat coming from the vents turns to an icy gust, and the temp gauge drops to COLD. It stays this way forever - we went on a trip today that took two hours, and the gauge dropped to “C” the minute we got on the highway. Our feet and fingers were frozen by the time we got to our destination… BRRR. Once we got back to ‘town’ speeds of about 30-40mph, the car heated up again.
This has stumped four mechanics so far - they can’t figure out what’s wrong. All fluids have bveen checked and rechecked, the car is serviced religiously every 3000 miles.
Any help and/or insight would be GREAT, our toes will thank you! 
Leah
What have these four mechanics done, or tried, to solve this problem? You didn’t tell us. Without that information we are helpless. I will assume the thermostat has been replaced. If not, it should be, ASAP.
I’m going to guess: vacuum. If the HVAC system is vacuum controlled (if), the blend door, which controls interior air temperature, may not be sending incoming air through the heater core.
Check for vacuum leaks.
If the HVAC system is electrically controlled, it’s not a vacuum issue.
Sorry about that - I asked my friend, she said that the thermostat has been replaced, and two of the mechanics looked at the HVAC system. They’ve found nothing wrong with it.
Thanks for your reply!
Cut a piece of cardboard and put it in front of the radiator in the winter. There is a good chance you have seen school buses with covers. If you are careful, you can put a tie rap between some of the ribs in the radiator to hold it in place. Just be carefull that you do not restrict too much of the radiator. I suppose it could potentially make the care overheat.
If you really want to fix the problem, then I’d check the thermostat again. It is supposed to close to let the fluid warm up and reopen when it gets too hot.
If you can swing it, pull it out and put it in an oven set at 220 degrees. The spring should expand and move what is essentialy a valve.
Check the thermostat housing for defects too. I guess there is a slim chance a bad housing could make it stick open too.
Hope this helps.