Please Help My Overheating ford fusion

Hello, I have a Ford Fusion 2013 4cyl ecoboost turbo 1.6L and been running into a over heating issue with it. I will start off by saying I know nothing about cars, and bought the car about a year ago. When I bought it was in the middle of winter so had no sign of overheating, but the first time i turned the ac on and drove 20 minutes, the temperature gage slowly crept up. This has been happening ever since and if i ever turn on the ac, within 10-20 minutes of it being on it will overheat. It will overheat faster when idling or going up a hill. Usually it will return to normal levels with the ac off, but it will occasionally even overheat without the ac on if im driving it for long enough. I brought it into the shop to have them fix this issue. They replaced the radiator, a sensor expansion valve, and the water pump but after getting my car back it was still over heating. I took it back in and the mechanic has had it for over 2 weeks now and says he literally cannot find what is wrong with the car. He says the last thing to replace would be the thermostat but also said he doesnt know for sure if thats the issue or will fix it. Im already 800 dollars in on repairs and really dont wanna drop another 600 for something that might not fix it. As a broke college student I also dont have the funds to buy a new car right now, and dont wanna sell it and scam someone else out of their money. I really need some help, has anyone experienced this or know what i should do. Thank you please ask any questions about it you may have.

Did they verify our cooling fan is working, you might have 2 electric fans both at the front of your engine attached to the radiator. At least one should come on automatically every time you engage the AC. If it only overheats when the AC is on that is where I would look first.

What comes to mind here is Murphys law...

The most expensive part in any engine will overheat and expire first, to protect the simplest thermostat - even though it was the thermostat can made the part overheat and fail.

Usually that replacing of the thermostat is to remove any doubt, and is the easier and cheapest approach to an overheating vehicle - you simply check the hoses and how long it takes to feel heat in them.

Water pump failure also can cause leaks and by that action, forces a timing belt change too, also in order.

The Heater core absorbs some of that heat - but the upper coolant hose not heating up would be a strong clue.

Simply undo the cap on the coolant tank and start the car and let it warm up - (idle down and blowing heat in the passenger compartment) - using thick insulated gloves, squeeze the upper radiator hose to feel pulsing or pressure of the fluid moving thru the hose to tell if the thermostat opened up enough or not - usually after about 10 minutes of warm-up, this test can indicate this. Even the top of the radiator would get too hot to the touch - means the thermostat is working - if you don't get these results - then the mechanic knows what to do..

Did you get this fixed? If not, it is likely that the active grill shutter assembly has failed (assuming your car as one - the one on my 2014 has failed). It is designed to shut when traveling at highway speeds to maximize fuel economy. When it failed, it fails closed (STUPID) and prevents air from going through the radiator and a/c coil. This is not a problem at highway speeds as the radiator gets air from under the car, but at low speeds in hot weather or with A/C on it can cause overheating.

Search the internet for active grill shutter fail or repair.

Ron