Runs hot while driving

Tiny
RMDEMOUY
  • MEMBER
  • 1999 CADILLAC DEVILLE
  • 4.6L
  • V8
  • 2WD
  • AUTOMATIC
  • 125,000 MILES
My car started overheating when I ran my A/C only so I just replaced my thermostat and did a radiator flush but my car is still overheating while driving. Even with the A/C off now. Also my blend door actuator doesn't work so the heater doesn't blow through the vents. But both houses to the heater core still have same heat. What do I need to do to get my car back at normal coolant levels?
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Wednesday, July 15th, 2020 AT 5:07 PM

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Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
The fact that changing the thermostat seems to have made this worse since it overheated only when the A/C was on and now it overheats all the time, we need to make sure the thermostat is installed correctly. It is easy to put these in backwards. I attached the image of how it needs to go.

Let me know if you can confirm this then review this guide for what we need to move onto:

https://www.2carpros.com/articles/engine-overheating-or-running-hot

The next major thing we need to do is make sure there are no air pockets. If we have an air pocket then it will overheat pretty easily. Let me know what you find and we can go from there.
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Wednesday, July 15th, 2020 AT 5:31 PM
Tiny
RMDEMOUY
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My thermostat is installed correctly. Also it's a fail safe thermostat. How do I determine if there are any air pockets in the system? Also, as I stated my Blend Door Actuator is not working. Someone told me if my heater isn't function properly it may also cause the car to overheat. Is that true? I plan to change it this weekend since it's right under the glove box. Will I need to blow out my Heater Core once I replace the Actuator?
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Wednesday, July 15th, 2020 AT 5:44 PM
Tiny
KASEKENNY
  • MECHANIC
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Short answer is no. The blend door not operating properly will not cause it to overheat. This is most likely being confused with a trick to try and keep the vehicle from overheating. There are times that you can turn the heat on inside the car and it will pass air over the heater core which will cool the coolant as it passes back to the rest of the cooling system. However, it will not cause an overheat.

The best way to tell if you have an air pocket is to use this type of coolant funnel. It will help force out air pockets as the air moves it will actually work its way to the highest point in the system. It will also help if you use a floor jack to lift the corner of the vehicle that the radiator cap is closest too. This will again cause the air to work its way to the highest point.

The other thing we need to check is make sure both fans turn on when the engine is warm and the A/C is on. I attached the wiring diagrams for this so we can start working through it if one or both are not coming on.
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Wednesday, July 15th, 2020 AT 7:03 PM
Tiny
RMDEMOUY
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Both fans are new and working properly. They usually come on when the temperature is high 220's or as soon as I turn on the A/C.
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Wednesday, July 15th, 2020 AT 7:13 PM
Tiny
KASEKENNY
  • MECHANIC
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Okay. That is great and exactly what they are supposed to do. So at this point I would recommend getting one of those funnels and jacking the vehicle up so that the radiator cap is the highest point. Then with that funnel on there, you should see a little rippling in the coolant in the bowl of the funnel when you are running the engine but as it comes up to temp and the radiator opens, you will see some bubbling come out if there was an air pocket. Once this settles down and is not bubbling any more then you can plug the funnel, remove it and then install the radiator cap prior to shutting the engine off.

Lastly, while you are letting this run, you can use a laser temp gauge and check the temp around the engine and hoses. The temp should be even all around except for the upper and lower radiator hoses. Clearly these should have a temp difference as the coolant as been cooled as it goes through the radiator. If you find a place that is hotter then you have a blockage.

https://www.2carpros.com/articles/how-to-use-an-infrared-temperature-meter
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Thursday, July 16th, 2020 AT 3:15 PM

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