Coolant temp issue

what would make my temp go down when the A/C is on and go up when the heat is on??

 

the thermostatic coupler on the cooling fan... when the AC is on additional heat is pulled through the radiator from the AC condensor and this triggers the cooling fan to lock up slightly more, if that makes sense.

 

A/C on also increased the idle RPM, providing more cooling while stopped or slowly moving.

 

What they said.

 

How are you monitoring your coolant temp? Stock gauge or some other type of reader? It does sound like your fan clutch might be getting tired, but if you are running your heater on high with the fan blowing full blast, your coolant temp should drop.

Start with the simple things and check your coolant level. Also, check the condition of your radiator cap. The OEM caps are known to fall apart and drop a spring and a plastic cap inside the radiator. A compromised cap won't hold the proper pressure on the cooling system. If the cap and coolant level are fine, I'd check the function of the fan clutch.

Here's a video showing how you can test your fan clutch:

<iframe width="560" height="315" src="https://www.youtube.com/embed/WzhlACxqeqQ" frameborder="0" allowfullscreen></iframe>

 

I just replaced the cooling fan about a month ago, but as soon as I turn the truck off and if I just turn the key right back to acc the gauge is reading in the middle where it should normally run