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 PIPES AND THERE IS NO COOLANT SMELL EITHER, OIL LOOKS COMPLETELY FINE AS WELL). 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?
The mechanic told me they detected exhaust in the coolant, but I've ran the block test myself multiple times and I've never once had a positive test, I also checked the spark plugs, all of them looked normal, as well as I 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.