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Jon Pell
Jon Pell, ASE Certified Technician
Category: Car
Satisfied Customers: 155
Experience:  ASE certifications, Many years in the field, and graduated from Wyotech with degree in Auto, diesel
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Cadillac Deville 1999, Northstar engine. Heater not working.

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Cadillac Deville 1999, Northstar engine. Heater not working. Last week was burning hot on passenger side. Now blowing cold air. A little warmth on driver's side. Has coolant fluid; gets up to 196 degrees. Disconnected battery to reset programming. Water pump is circulating water. In and out heater hoses are warm but not hot after idling car for 10 min.
It sounds like you could have a one of two problems; first the heater core could be partially cloggled- disconnect both heater hoses and put the water hose to the core to flush the system out. Second, check the door acuater under the passenger side dash to make sure that it moves when you move the temperature control switch. This is just a door on the heater core that basically opens when you want heat to allow the blower to move air across the core to heat it up and when the door is shut (this blocks all air circulation across the core) you will have air conditioning
Customer: replied 7 years ago.

I don't know if my reply went or not or just got erased. This may be a duplicate.

The door actuator is working. In trying to adjust the temperature control to check the door actuator, heat started coming when temp was 60 degrees. Nice and warm. As we moved temp control up above 80 degrees, passenger side blows cold air, driver's side cools down. 90 degrees both sides are cold. It is not totally consistent, but we have heat now, just not what it should be. Any suggestions?

From here I would take the radiator cap off, start the engine and let it run to make sure there is not an air bubble stuck in the system. Have you replaced the thermostat?
Customer: replied 7 years ago.
We were going to replace the thermostat, but it didn't appear to be the thermostat. Now that we have heat, we know the core is working, the door actuator is working. Such a process of elimination. Just tried the radiator cap off, temp up and down, now no heat on passenger side, but warm on driver's side. 60 degrees is warmer than 90 degrees.
The only other things it could be are one touch the upper part and the lower part of the radiator after engine is at operating temp, if both are hot that rules out the radiator. Two, The water pump may be flowing insufficiently, look at the coolant and put your hand on the radiator hose to see if you can see/feel water circulating. But I would most definately replace the thermostat. After doing all of this you still don't have heat then there is an air bubble trapped in the system leave radiator cap off, turn on max heat and let it run about 10 to 15 mins to purge all of the air out.
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