Coolant reservoir empty when sitting overnight

pressure test the cooling system , look for the leak .
Pressure Testing Cooling Systems EricTheCarGuy

Check the water pump and see if coolant is around it.

The A/C causes the engine to work harder (that's why fuel economy suffers when it is run).

If your car has an electric fan - make sure it runs when the engine is hot. If it does not - you may have trouble w/o the A/C running if is gets warmer outside. Look for fail fuses, disconnected plugs, etc.

You may also have flaky thermostat that is not opening fully when needed to exchange hot water in the engine block with cooler water in the radiator.

First, check the coolant level. You should find a semi-transparent reservoir / tank located under the hood that has a cap marked "Coolant". The side of the tank should have two indicator lines - one for HOT and one for COLD. If the engine has been run up to normal temperature (so that it can provide heat in the car), you should see the coolant level at or slightly above the HOT line. If the engine is cold - first thing in the morning or after sitting for several hours - the coolant should be at or slightly above the COLD line. View these levels while the engine is off and the car is parked on a flat, level surface. If there is not enough coolant - you will need to add the CORRECT type for your car.

Hot Engine
(1) With the engine hot, add enough coolant to the tank to bring the level to the HOT line. You may have to check the tank several times after driving if the coolant level in the radiator is down a great deal. NEVER open the radiator cap when the engine is warm or hot!

or:
Cold Engine
(2) With the engine cold, add enough coolant to the tank to bring the level to reach COLD line. Additionally - with the engine cold - you can add coolant directly to the radiator by removing the cap. Start the engine, set heat in car to HOT and add coolant mixture to radiator until level comes up to the bottom of the filler neck. Once the engine is HOT (getting heat in the car) check and top off radiator as needed and put cap back on radiator.

Check the levels in the TANK several times over the next few days when cold. If the tank ever empties -you'll need to have this looked at by a mechanic.

start engine until get hot enough for coolant get hot pressurize then turn off engine check for leaking coolant hose.

better change out your Plenium (Intake) before it ruins your engine ! Part is about $125.00 and comes with gaskets and a new Intake and new EGR Tube you replace also, what it is doing is leaking internally and burning it, it will eventually start to miss and quit on you Hydrolocking the engine. water does not compress in cylinders. If you are lucky you won't have to do lower intake gaskets or head gaskets......But get it fixed before too LATE !

Look in the radiator in the morning when car is cold take off the cap if it is full then look at the reservoir and make sure you see to lines on it one you fill to when cols the other you fill to when how it will say hot and cold fill lines fill yo cold when it is cold then next day check and see when cold if it is at the same mark

sure,. sounds like a good plan to me.,. you may want to also inspect to be sure it's not broken or otherwise cracked. and fill the the respective level line (when engine is cold or hot). You may be able to simply use a 50-50 prestone blend or full strenght if the existing coolant is low on antifreeze depending upon your needs climate-wise...
Ck

It needs a thermostat and a coolant flush, in the worst case scenario a head gasket may be blown or leaking, have your mechanic do a coolant flush,thermostat, and leak check.

I guess the burning question is , where is the coolant going.
Is this a 4.3 engine? How many miles does the vehicle have?
Has the intake gasket been changed?
I'm guessing a can of "Bars Leak" may cure your problem.
Let me know.
Regards,

water pump