Let me preface this thread by saying that I have changed the thermostat twice with the OEM part from 2 different cities while on the road. The Dodge Ram 2500 hemi has 85k miles on it and will always overheat from cold. Usually I will turn on the heater with fan at full when I see the temperature gauge go above half way and there will be no heat until the thermostat opens. If I am driving the temperature gauge will climb to hot and radiator will boil over. If I pull over to the side of the road with the heater on, the temperature gauge will return to normal (about half way). Once the temperature returns to normal I can drive the rest of the day without issue. No leaks from water pump or radiator or hoses. No milky looking oil. Coolant goes from 200 degrees when it enters the radiator (top) to 125 degrees when it leaves (bottom). Have burped the system a couple of times. Fluid levels at overflow container stay full. I'd like to think I didn't have 2 stickey thermostats out of the box but haven't ruled that out. Any suggestions?

 

find an IR temp gun and shoot various areas of the radiator when the system is at running temp, look for hot/cold areas. It sounds like there may be some plugging going on, and by adding the heater coil into the mix, you're getting a little extra heat rejection area to help get the engine cooled off. It could be bad t-stats (not like that hasn't happened before), but the IR might give you a better clue up front as to what the radiator is doing. Overheating is really detrimental to these aluminum block/heads engines. Get it figured out soon or you may doing some unintended engine work also.

 

GF the radiator looked consistant side to side and top to bottom when I used the IR temp gun. Was a little concerned until I figured out it was a cross flow radiator.

 

If you suspect bad thermostats, drop one at a time into a pot of water on the stove with a thermometer in it to determine exactly what temperature they open at...

 

Impeller loose on the water pump shaft caused similar problems for me last year. When the engine rpm's increased the water flow did not.

But your symptoms seem odd that once it cooled back down you were able to drive the rest of the day without overheating.