The temp stat in the Fiat Panda 169 as well as most other cars on the market have a bleed hole in it. This is to equalize pressure between the two circuits so as you incorrectly state, a small amount of coolant leaking across this state does NOT cause a huge issue. Without the bleed hole, the wax driven valve would get into a state of occilation where the heat would open it, and the cold the other side would close it. The bleed hole reduces this intermittent stage.
It is more likely that your stat was leaking pressure to the atmosphere or somewhere else in the system, maybe the resevoir cap. Rubber does perish over time which get accelerated the more it is flexed - like opening and closing the resevoir cap.
Most cars have a presserised cooling system that have three circuits -
- The engine block (which includes the pump)
- The internal radiator - which can be controlled by valve (to set temp) either mechanically or electrically driven.
- The radiator.
The pump running pumps coolant around the engien block to cool the oil. Engine blocks made of aluminum also disapate a large amount of heat a lot faster than cast steel blocks. This coolant runs past the engine temp sensor as well as the internal stat which is the path to the external radiator. #2, the internal radiator is an aux circuit usually on smaller diamater pipe that, for most cars, is linked to the main block. #1.
There is a bleed hole in this stat to equilize the pressure between the block coolant circuit #1 and the rad coolant circuit #2. As a car heats up, the metal in it expands and it is designed to expand equally. This is not usually the case so piston rings are given the flexibility to compensate. A cold engine has less clearance between the pistons and the block. Also, the oil is a little thicker. As a result, it is when the engine is cold is when the most wear and damage occurs until the oil comes up to pressure AND the metal expands to allow clearance. The coolant system is designed to requlate the engine temp to reduce / minimize damage.
Numerous issues occur when one replaces coolant with water - by misreading ratios etc. Coolant is a form of lubricant and can withstang greater temp and pressure ranges than water. Water freezes at 0, and boils at 100. It also expands by 2.5% volume for every 30 odd degree increase. As a result, a high water content in the engine coolant will increase the pressure at joints / seals and is one of the common causes of leaks in a coolant system. Water also is one of the elements required for rust, which is also another common issue with water rich cooling systems. Thirdly, water and oil seperate, and across the temp range an engine runs at, it is possible for the two to seperate. Rust in the cooling system can block bleed holes, small pipes (normally in the internal radiator system) and cause the blades in the pump to cavitate - which looks like some acid etc has ate the blades in the water pump.
Unless you are 100% sure that the ratio of water to coolant is correct in your car for the engine / model, I suggest that you don't add water to the mix, but just coolant. If you have an issue where there is leaks, it is time to flush the system and replace the complete coolant. By Fiats specs, the coolant needs to be replaced every 30,000 to 100,000 miles depending on where you see it. I suggest replacing same at the same time as changing the cam belt / water pump AND mix it kinda rich - ie more coolant than water.
The 2.5% expansion of water for every 30 odd degrees was handled in old cars with a pressure release valve and hose at the top of the radiator cap. My Fiat 169 dosen't have any pressure release compensator in the circuit... so if the pressure gets too high, a hose will normally blow off (hopefully) or one will damage the head gasket. The second, and more costly damage is that most / all the rubber seals will overheat and become brittle. This means an engine rebuild replacing every rubber gasket as you are going.
When a car has an aircon system, there is usually a second radiator internal to car as well as a second radiator behind the main rad in the engine compartment plus aux components (compresser, condensor etc). This air con system is sealed and totally seperate from the three circuits mentioned, and needs both skill, experience, the correct equitment plus tools plus qualifications / cerificates to address any issue within it.
