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Here is my problem:   First of all.  My water pump's weep was leaking and over heating but even when the coolant level was good the car started to loose heat from the vents/only cool air.    My mechanic, flushed the radiator, burped it and also replaced the water pump.   I drove the car a bit had heat,  and then lost the little heat that I had and the car was over heating and boiling back.  I took it back to him and he revved up the engine and saw that the lower hose quickly vacuumed.  He then let the car cool down,removed all coolant,  and added it again.  He was told by someone that there is just some air in the lines.   He apparently let the car run at idle for over an hour and he had a minimal amount of heat and the car never over heated. It also seems if you turn the heater off, only maybe 60 percent of the time you can get the heat back.    I took the car home, next morning.   I had heat, very little and drove the car half way into my journey hot/but not on red with the gage running up and down.  Of course no heat. When I turned off the car, the engine did not appear to run hot, no coolant boiling back into the resivour, which was what I was used to hearing prior to him burping all the air out this last time.   I have asked about possible head gasket/warped head.   He says that there are no signs of that.   Which I do not see either however, I have heard about combustible fumes entering the coolant which can also cause the car to over heat. “But what about the heat?" My car is not running rough either, been running really smooth, after the spark plugs were change.
The heater core has been checked, the radiator has been checked. Again no signs of internal leak also he said that he thinks that the radiator cap is leaking.  So I replaced that.  New radiator cap made for my make and model.   Now, there is a couple thing that my contribute to this.   The cool air/heat knob is busted.  I have to go underneath the dash to turn open and close.   When I do have heat, I can hit the air conditioning button and get cold air, I will go back to the heat and its not there and I cannot get it back. Also I do hear a belt chirp when starting the car.  He had changed only the air conditioning belt when changing the water pump.  He said that the timing belt was loose but not damaged at all.   Since the car is running hot “on and off”, where is the heat going?   Should I have the pressure test? Check the coolant for gases?  After the car ran hot, the reservoir is still full and the hood of the car is not even hot. It’s now fall and I have no heat. I got in the car this morning let the car idle with the heat on high, got in my car and nothing but cold air. I can't afford to take it to another shop.  It seems that there is a head scratcher.  If anyone has any sugggestions pleas assist! I can't keep losing sleep and money.  

Have you replaced the thermostat? it can stick open(running to cold) or closed (overheating).