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Lancia Fulvia

The loss of water in the radiator?
I have a Lancia Fulvia, who seems to be losing water. I checked the radiator for leaks (none) compression test Cylinder head gasket (all right) I took the thermostat was stuck because she caused it to fire at once and replaced the rad, so it now has a new one. but after all these controls im sure it is still losing water. The level is very low anyway in the rad (knowing from past experience with other Fulvia) What else could it be?
I'm not familiar with your make of vehicle, so this is just a general answer. Sometimes heater cores develop small leaks that do not cause dripping inside the vehicle, but some of the steam will escape through time. You can have the slightest hint of a sweet smell in the vehicle interior. Pressure tests may not reveal this issue because refrigerant usually run through the heater core unless the heater or defroster is on and the temperature for heating. Moreover, in many vehicles, the coolant does not flow through the heater core until the thermostat opens when coolant is hot enough. Just a thought.