Mg zr blowing cold air all the time.losing water over period of time and temp gauge goes up into red and comes back down again.could it be head gasket?

change the thermostat again , at a different shop . make sure the rad fans are coming on and make sure your water pump is working properly

Sounds like the water pump is cavitating . Replace water pump. Check coolant level also. Open the bleeder valve usually located on thermostat housing to remove air from system. close valve when coolant is present while filling. close radiator cap and top off recovery bottle to correct level. These engines are prone to overheating, do not let temp gauge get in the red.

If temp gauge is going into the red on a 3.4 engine it must be shut down or head damage can happen within minutes. I have a Monte Carlo that was overheating. I changed everything, thermostat, water pump, belt tensioner, radiator, all with no result. It would start out okay on a cold start but then I would come to a red light and the temp would creep up. Higher and higher until it neared the red zone. The heat would run cold. Most of the time, as soon as it was about to touch the red it would start to drop, the heat would blow hot again and all seemed okay but not for long. This would get worse and worse until the red hot light came on and I found myself on the side of the thruway. This problem is a blown head gasket. I had no other obvious signs, no white smoke from the exhaust, no oil in the antifreeze. I even changed the intake gaskets. Head gasket issues are common on the 3.4 liter engines.

i had something similar once, it ended up being the intake radiator hose, it has a metal coil inside, mine was broken and when you revved the motor it would **** the hose shut and overheat, but under 2000 rpm it was fine .

3.4 liter?

Sounds like you may have a head gasket that has gone bad. with the engine cold remove the radiator cap and start the engine. If there is a constant peculation of small bubble coming out of the radiator and the smell like gas or exhaust you have a bad head gasket.

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I think you could be right about there being air trapped in the system.
Or the heater core(radiator) is plugged.
Either way you need a pressurized radiator coolant pump to put the coolant in without air being trapped.
Although,i have seen it done by jacking the vehicle up high enough that the radiator was now higher than the heater core level, and then the coolant was put in.
But this is dangerous to try.

First when vehicle is cold like in the morning check and make sure that the vehicle has enough coolant/ antifreeze in radiator and reservoir. If coolant is need use a 50/50 mixture of water and antifreeze. You will need some time to do this let car run at idle until the temp guage is in the middle NOT+ normal operating temperature. At this point if the thermostat should be in the open position. Let vehicle run and check to make sure that radiator fan comes on. This should come on when guage is about midway/half way. If temperature rises above half going to h/hot and fan doesn't come on, you either have a BAD FAN BAD FAN SWITCH BAD FAN RELAY OR BLOWN FAN SWITCH FUSE. You will need to check all of the following I will help if its needed. If fan comes on and doesn't go off and vehicle continues to get hot even with fan running. Feel radiator hoses and see if the lower radiator hose is cool/cold. If it is you have thermostat that stuck closed. Replace thermostat and fix problem. If all the above is good and vehicle continues to over heat. THE BAD NEW IS THAT YOU MAY HAVE A BLOWN HEAD GASKET /CRACKED HEAD /BROKEN WATER JACKET. Basically you will need a HEAD JOB at least a head gasket. ONe tell tail sign of this is oil looking like milkshake/chocolate milk-a creamish while residue in oil or antifreeze. Contact if you need hel0p. Good Luck and thanks for using FIX YA

Let your engine warm up with the rad. cap off and see if the coolant level drops. and make sure your overflow is full. It blows cold air because there is no water getting to the heater core. And it shows a normal(ish) temp because the water needs to be touching the temp. sensor. I really do think there is air in the system and there is not enough coolant but hard to tell from here. Also as with overheating any time make sure to check your oil and look for any coolant as the head gasket seldomly holds together if the motor overheats.

Sounds like you are either low on coolant, or a head gasket failure.

No you have air trapped in the cooling system, rasie the front of the car up as high as you can get it, fill the coolant bottlre to the top and run the engine for about 15 minutes at a high idle, let it cool down, it should purge itself of air once it is cool, make sure your radiator cap is good.