I just recently bought my 07 g35 coupe, on the drive home after purchasing it, it decided to overheat I was in park not even going anywhere, so I took it to a local shop and they changed my thermostat and it was perfect.. I went on a trip 3 weeks after and my car was fine the whole way until I started going threw the mountains. while going uphill it shot completely up and overheated.. I thought maybe I need some oil so I put a quart and a half in and still nothings working.. any suggestions?

 

Check your coolant when the car is cold via the radiator cap, it's most likely very low. Even if it's topped off, you could have air stuck in the coolant system. Search up how to burp air out or pay a shop to due it well. The quick way I do it is open the cap (cold), squeeze the two big hoses directly under the cap and next to the intake airbox in an alternating pattern to force air out of the open cap. It's going to take a few times to get any air out. Try it again after the car has gotten up to temp and then cooled almost entirely down (hoses are slightly warm). Be very careful opening the cap when it's slightly warm, as the coolant will have a bit of pressure in the system. This won't get all the air in the pipes behind the engine, but it'll get some out (a shop can pretty quickly get all the air out, or the actual burping method; I never had luck with that one). You can identify air bubbles in the system when the car makes a gurgling sound from the engine bay while you're in the car (lightly rev it when cold/with the heater on to really test for it).

As for oil, that's not really gonna cause overheating. If you're overheating due to low oil, you have bigger problems (check the oil dipstick while cold as well, and make sure it's between L and H). If you were actually a quart and a half low, the engine may have taken some damage being driven through mountains.

 

Find a radiator shop in your neighborhood, they'll easily pinpoint what is causing your over heating issue. Just because you had a new thermostat installed doesn't mean it's working as intended! I've had them fail right out of the box, which you'd never expect!
Gary