I'm not sure WHY this is a mystery

You have the answer to your question in your original post....you have a leak that you need to fix, you shouldn't' have to look very hard if there is a puddle underneath the truck everytime you drive away.

Here's what you do, lay on your back underneath the truck, put the back of your head in the puddle and look straight up, there's your leak. Now get it fixed.

We can't answer the question "Is it my thermostat" without more information....

 

If you crank the truck, does the guage rapidly (within a few seconds) come back to the cold area? If no movement could be bad sensor (under $30) or cable to sensor. If truck has been sitting for two days it has assumed room temperature. This will have nothing to do with loss of coolant. The coolant loss is the Rover way of announcing impending doom and reduction in your bank account. The more you drive, the longer you wait, the more it will be.

 

I imagine as you are new to the truck you still have Dexcool (its Orange), you will want to replace that with Universal (Green) but that will come after you fix this initial problem. Probably the only good thing about Dexcool it is easy to trace leaks because it leaves a sticky trail and a typically white splatter from where it is leaking. I would wipe down the engine bay and also rip out the cover underneath the engine if it is still there (it is completely useless and actually keeps heat in, the reverse of what it was designed to do) which will give you a good view up from underneath the engine. Start the truck up and see if you can find any splatters, since you have a puddle I would imagine you should be able to find it. If you can't then the leak is probably behind the firewall, which is not awesome and a PITA to get to.

 

Thank you all for your feedback.

It could be a bad sensor...The Needle does shoot up and within a couple of seconds comes back down lower than the cold area. The dangerous thing about this, when would you know if your engine is REALLY overheating.

But I'm coming to terms that it is probably a leaking head gasket that over-pressurizes the cooling system, from what other people have told me.

As far as the Universal Green coolant, are there any specifications that I should follow, when I switch?

 

That is completely normal. Find and fix your leak first.

 

Ok. Let me rephrase that. The needle rapidly goes all the way up to the Red, then Red indicator illuminates, and the needle returns back down PAST the blue. The red indicator light stays on. The needle just sits there UNDER the blue and there is NO movement.

 

What everyone is saying is that you need to have some one on one time with your engine bay and try to locate where the leak is coming from, dont worry about the temp gauge right now. Like I said, wipe down the engine and all the hoses and start it up and look for splatters and leaks, you may find the leak just by wiping everything down, Dexcool looks like orange tree sap when it leaks. As for a thermostat, you should replace that regardless when you do the coolant swap AFTER you find the leak. The coolant swap involves letting the truck sit til cold, pulling top and bottom radiator hoses, flushing it with regular hose water, hooking the hoses back up, filling it up with water, letting it run for a few minutes, let it sit for a few minutes, and then repeat that process with water about 4 times and then filling it up with universal coolant. But that all should come later.

 

Pressure Test complete, Leak Found and fixed! Also, the reason for the temp gauge being out of wack was the ECT sensor. They let me know that it almost never goes bad. Thank you all for the help you have given me.

I am going to switch my coolant to the universal green. Any brand in particular that works the best?...

 

The Peak Global coolant sold at Walgreens is one of the best available, it is yellow and is compatible with other coolants. Just out of curiosity what was causing your coolant leak.

 

I second everything Buick said/asked... you gotta let us know what was leaking!