Hi Alberto,

Try not to panic. I don't think your head gasket is blowned. When the head or head gasket fails, you get oil and coolant mixed.

You could have a dificult to trace coolant leak. Coolant can leak from several places: hoses, radiator, water pump, exp. tank, heater system, etc.

Your glass experiment is not good because the system needs pressure to avoid coolant boiling. Boiling temperature depends on pressure.

If the car is overheating, start with the basics: new thermostat and new pressure cap. And try to locate the coolant leak. Maybe it's related to the heating core. Also see if the fan clutch is in good condition. When the engine is at working temprature and you turn the engine off, the fan should stop almost inmediately. If it keeps turning for some time, fan clutch is probably bad.

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Juan
1995 E320 Sedan
2008 Ford Escape
RC Helicopter