As you are aware, I am sure, the Fiat engine cooling system is prone to become air bound if the block is drained as it might during a anti-freeze change. At the least, if air is in the system, the heater does not work. It the worst, no water gets into the engine block until the thermostat opens. As the thermostat is mounted externally, your engine might hit 260 before it opens. In fact the engine might self-destruct before water gets past the thermostat as one side of it might be dry. Trying to follow the factory instructions to add water through that small bolt hole in the radiator hose tee can be time consuming , frustrating and a pain. My solution is to add a fast flush adapter made by Prestone at the high point in the heater hose by the distributor. Through this, when you change the antifreeze, it fills the block first up to the back of the thermostat and then flows into the radiator. No air is trapped in the block and when you first start the car every thing works normally and your blood pressure will not go through the roof as your Fiat melts before your eyes.