Cooling System???
 

I bought my son a '99 XLT Extended cab a couple of months ago. I noticed the temp gauge wasn't climbing very high so naturally I assumed it needed a new thermostat. We replaced the thermostat, it seemed ok, was running a little warmer according to the gauge. We went on a trip this weekend from Cincy to Columbus and I noticed the temp gauge was reading extremely low again. My son drove it around town this morning, seemed OK. On the way back to Cincy I noticed the gauge showing a low temp reading again so I decided to turn the heater on high to check things. Nothing but cold air coming out the vents! If the water pump was bad I can see getting cold air, but the thing would be overheating. What gives?

JA