So I've been having this issue recently with my thermostat where it would peg up to being overheating but then would randomly drop down to the normal range or even drop to the below normal range. Today it was completely in the red I was close to home so I got it home opened the hood. The engine didn't seem to be hotter than it normally is. The reservoir has coolant in it, fan is spinning.

One thing that is also weird if it I put the heat on full blast the needle spikes, when I turn the fans off it drops back down. Any ideas? Anything I should check?

 

R&R the thermostat .............

 

That's what I'm going to do. Staying away from the crap autozone one...

 

Never trust the factory temperature estimation gauge. Well known to trigger false readings. Get a mechanical water temperature gauge to verify what is going on. In a pinch I have used an infrared heat gun to measure the temperature of the thermostat housing. Seems to be pretty accurate.

 

and yes I agree with above too...I use a meat thermometer...old fashion I know, but it works and is very accurate

 

I think you mean dash temperature gauge when you are saying thermostat, right?
sounds like you have an electrical problem not a cooling problem. But since cooling could be deadly it is best to rule that out first. The thing with your heater controls leads me to believe that you have a short somewhere in your wiring and all you can really do is try and trace everything as far back as you can to check for exposed wire, signs of heat damage, or whatever else it could be. I hate electrical because it is always so 'finicky' like this. however 90% of the time it is a ground issue. Rule out that temp problem and good luck!

 

There is a sending unit that goes to the gauge...the thermostat sits at the top radiator hose & controls the temperature of the engine with a thermo-coupler type device that opens & closes a flapper inside the thermostat to control the temp.