Losing coolant at varying rates, sometimes substantial amount, sometimes just a little. Engine always runs hot (between 220 and up; even hotter at high speed and warmer weather) but does not overheat. When the engine gets really hot, the heat will start to blow cold; sometimes it will even cool down while still driving and begin to blow hot again. No visible leaks (except sometimes there is small amount of coolant on the underside of the hood just above the overflow reservoir). Pressure test done at reservoir did not detect leaks. Block test showed exhaust fumes in coolant reservoir. I was told it is a blown or leaky head gasket but I DO NOT have any of the following symptoms: white smoke from tail pipes, sweet smelling exhaust, rough idle, loss of power while driving, unusual engine noise or hissing, contaminated oil, engine overheating. Used Bars leak gasket and cooling system sealer but problem persists. The car is running great. Water pump was replaced shortly before sysmptoms began. Radiator was replaced about two years ago. What can it be and what diagnostics should I have done?
You had the diagnostics done and it showed a leaking cylinder head gasket. There's a dozen places it can leak and each one results in different symptoms. There's no other way to get combustion gases in the cooling system. When you do it is common for it to pool under the thermostat and cause it to not open so the engine overheats. Thermostats open in response to hot liquid, not hot air.