Heater blowing hot, cold, hot
 

My 2004 cummins is having the following issue. When I start the truck as the temp gauge starts to go up. I turn on the heater and it begins blow hot. then its like the heat just dumps and the temp gauge goes back down to 140 and the heater blows cold. I turn off the heater and the temp gauge will slowly begin to climb and will go over 200 and continue to climb. I then turn the heat back and it may or may not blow hot or cold. If it blows hot then the temp will go back to normal and the rest of the trip it will operate as normal. If it blows cold I will turn the heater off and on until it blows hot and the temp gauge goes back to normal.
I can't tell if there is a funny smell or if I'm imaging it.

I already changed the thermostat.

Any thoughts

sounds like your blend door actuator might be goin out

Thanks for the suggestion. I'm not very mechanically inclined. Could someone explain how the blend door sticking would cause the engine temp to dump back to 140 or the engine temp to go to 230.

Any ball park on the what it would cost to fix blend door? Where is the actuator located?

Any other possibilites beside the blend door?

it shouldnt make engine temp go abnormally hot but, it can pull your temp back in cold weather especially at idle. if your coolant level is normall and there are no air pockets in the system causing the the hot and cold switching then blend door going out wont be directing the air over your heater core so you wont have heat. if your loosing heat and your gauge is at 140 degrees i would think its something in the hvac and you also said that you can cycle your heat on and off and it will eventually blow hot.