i think your problem is most likely a obstructed heater core. it could be plugged up internally, externally or both. if you have A/C that can get plugged externaly aswell. my truck runs at a constant 180* the outside temp was about 30* today and my heater was more than sufficient infact i had to turn it off a couple times. i had next to no heat in my truck untill about 2 months ago when i changed the core. not only was the core somewhat restricted internally the heater and A/C core where both externally plugged with junk. another thing to check is that the hot/cold valve is working and also be sure theres no air in the core. had a hell of a time getting the air out of the core until i figured out a little trick. i dont think its a good idea to try and make your motor run hot.
