So my G35 kept overheating at idle. It eventually started overheating regardless. I had it towed to my mechanic, they replaced a hose. I got it back, then it started overheating again. Water pump replaced, fan module replaced. Lines bled a bunch and still had air in them.

He suspects the head gasket is warped and is saying he can replace the engine with a low mileage one cheaper than what it would cost to fix the head gasket.

My partners Dad who is a mechanic thinks I should get a second opinion but I'm torn because I don't want to tow it to another spot just to be told I'll have to pay 3k to fix the gasket.

 

Make sure to bleed the rear upper port. Spill free funnels help to bleed out the air.

 

Rent a coolant pressure tester from an auto parts house, if the system won't hold pressure it will overheat eventually.

Also make sure your fans are actually turning on when it's hot.

Did that previous shop perform a LEAKDOWN TEST, that's how you typically determine if a head gasket is blown. However you can also use a dye kit to see if there's exhaust gas in the coolant, they typically sell those at parts houses too. You use a baster, suck out some coolant, put it in the test tube, add the contrast solution, then check to see what color it changes too, there will be instructions in the kit.

You can also take off the radiator cap when it's cold, then fire up the engine and watch at the fill neck to see if there's a constant stream of little bubbles coming out, that would indicate a very large exhaust gas leak into the coolant system. A very small leak you might not see though, thus the dye kit or a leakdown test.