I have a 98 ram 1500 same problem. Ive swapped the thermostat, water pump flushed the heater core, and replaced all the coolant. I also have the radiator completeley block off with card board and my engine temp wont get over 150f. Mine normally doesnt get warm when its below -10f. I live in alaska and its not uncommon for it to get to -50f. If its above 0 then my truck will warm up to about 200f after running for 45 mins. Ive run out of ideas. Any ideas?

 

Test your thermostat. See what temp is actually starts opening at.