I've been so stressed about my car over the past 8 months, I have a 2006 G35 coupe and i've replaced every part of my cooling system (Thermostat, radiator, water pump, and bypassed the heater core) It never fully overheats (up in the danger area). but the coolant temperate fluctuates all over the place when I drive, until eventually coolant starts boiling out of the reservoir, (THERE IS NO WHITE SMOKE COMING OUT OF THE EXHAUST AND THERE IS NO COOLANT SMELL EITHER, OIL LOOKS COMPLETELY FINE AS WELL). Took it to the mechanic today to have them diagnose it because i'm done trying to myself, they said they found exhaust in the coolant, which is weird because I had recently done a block test and there was no exhaust in the coolant..... this has been going on for almost 8 months, and I drive this car all the time. From what I'm aware of if this is a head gasket leak, it would've blown and the car would've been unusable by now after 8 whole months of having this problem, I genuinely have no clue what to make of this situation. Is there anything else that can cause exhaust gases to get into coolant? Or is the mechanic full of BS?

 

Has anyone pressure tested the system first?
You could have a leak from a hose or hose clamp..or even the radiator.
Also make sure your fans are running and not dying...check you battery.

I would check that first, before jumping on the head gasket idea.

 

The pressure tests came back fine. The radiator, thermostat and water pump were replaced about a month ago, and the fans are just fine. I even used a boroscope to check the combustion chamber and I didn't notice any abnormalities in anything down there besides just some carbon deposits on the pistons, none of them looked steam cleaned, which they should be if its a combustion leak. I've even done the block test on my own multiple times and it contradicts what the mechanic told me, never had a positive test.

 

Maybe remove the tstat and see what that does.

When we had a small head gasket problem, the car ran fine with the tstat removed. We ran it a few months like that and the temp was steady but cooler. I guess the idea is that it put less stress on a leaky head gasket..dunno.

But we live in FL where it is pretty hot.

Too long running cool like that can burn out cats..I believe.

I still think there might be a coolant leak somewhere.