The 500 is obviously quite different to my 04 panda..
That method wouldnt work on a 16v either
There are various ways of achieving the desired outcome, none of which seem to be totally repeatable or guaranteed to get all the air out first time.
One point of note is that, unlike most cars, these Fiat's don't have a water control valve in the heater circuit. Water circulates continuously through the heat exchanger in the heater box; cabin heat is controlled by flaps which vary the flow of air, not valves which vary the flow of water. So the position of the heater controls maked no difference whatsoever to the bleeding procedure and isn't relevant. (But to get unrestricted flow through the heater circuit on most other cars, you do need to turn the heater to max to fully open the water control valve).
It's hard to get a good bleed when the thermostat is closed; there's a one way valve in the thermostat for this purpose, but it's tiny and doesn't work very well.
Part of the problem is that bleeding works best when the coolant isn't circulating, the thermostat is fully open and the system is at operating pressure. This means carrying out the procedure when the engine is stopped, fully warm, and all caps are securely in place. You then,
carefully and very slightly, open a bleed at the highest point in the system (the heater hose on the 500/Panda) and let the overpressure push the air out.
The risk of scalding yourself is significant, so don't try this at home unless you know exactly what you're doing.
It's vital not to unscrew the bleed to the point where the pressure blows it out or you'll spray scalding water all over the engine compartment and probably yourself.
Normal mortals are best off just keeping the reservoir topped up to the maximium and letting the system stabilise over a few hot/cold engine cycles. If it won't stabilise, you've got a leak and this requires urgent attention.
I'll share a little secret with you all, If you need to work on the cooling system, to change a water pump or thermostat for example, first check the level is normal. Then syphon the coolant into a clean container* and either put it back when you've finished, or replace it with an identical volume of fresh coolant. That way, you'll know exactly how much coolant you need to get back into the system.
*On those 500's with the expansion tank at the side of the radiator, you can get just as much coolant out by syphoning as you can get by removing the bottom hose.
