I have searched for this question but cannot find it anywhere. My issue is strange so i thought i would post it hoping for some help. I have a 1998 Ranger 2WD, 3.0 Liter with a 5 speed manual. It has been below freezing here in Missouri and this morning when i started the truck to warm it up everything was fine. The temp came up normally and the heater blew hot air as it has always done. As i was driving it however the temp started to rise. I turned off the heater and the temp dropped down to normal. When i turned the heater back on the temp started to rise again. It also rises when i climb a hill. What gives?? The water pump and thermostat are brand new with less than 1000 miles on them and as far as i can tell the radiator is full. Any suggestions would be helpful.