Quote:
Originally Posted by humfrz
The reason I was suggesting it would be OK to "top off" the coolant level with distilled or demineralized water is that a little bit of water is not going to wreck up the glycol/water balance.
* the concentrated Subaru super blue stuff is 5% water to start with.
* most of the loss of coolant is due to evaporation and the water is the first to go.
So, there ....... 
humfrz - @ Tcoat has learnt me well .....  |
The glycol component of coolant is hygroscopic. Depending on the relative humidity
at the breather hole of the overflow bottle, either evaporation of the water component of the coolant or absorption by the glycol will occur.
Factors I can think of varying the relative humidity
at the breather hole are ambient temperature, engine compartment temperature, ambient humidity, air flow past the hole, how long the hot and cold periods experienced by the bottle are and there may be other factors.
I know many people have never had to top up for 5 years whereas I need to every few months and put it down mainly to our average humidity is much lower than the world average. I am confident I have zero leaks (other than at that breather hole).
To minimise losses I have heat-shielded the bottle with Al foil and put some foam over the hole to soften air flow past the hole, and reduced the hole size itself. My dad's old Mazda had only a pinhole.
The overflow bottle idea is a good one but a sealed variable volume would be ideal, e.g. a bladder or piston but with a sealable opening for additions in cases of leaks. There would be zero corrosive oxygen from the air getting in and consuming certain coolant components.
I agree that for the most part top-ups with PURE water is fine but the makers play safe by stating fresh coolant in case there is a true leak and it also means fresh additives are introduced.