I’m trying to help a neighbor with their car.

Their G37 started getting hot, went to the red at one point. I hooked up my gauge and noticed that it ran around 204°. It would get up to 213° and the fans would kick on, until it returned to 204°, when they’d turn off. Rinse and repeat.

It was low on coolant and they stated they had to keep adding. I filled it up and ran it without the cap. Coolant kept rising up and coming out of the radiator. No bubbles. They’d said that the overflow would keep filling until it overflowed, but wasn’t drawing back into the system.

Is this simply air in the system? I have a vacuum tool I can use to evacuate, test and refill the system, if that’s what’s needed.

 

This is almost always a sign the radiator cap has failed.

It sounds like the main cooling system is doing it's job but that the radiator cap is not allowing coolant to draw back into the system after it has cooled.

I'd try replacing the cap, top the coolant off, and see how the car behaves.