Stupid Audi coolant! Had to happen sooner or later.
Had put in (I should have known better) the 'original' Audi coolant during the last timing belt job on our 1998 B5 A4 1.8T.
Later, coolant needed some topping off, and 'regular' (yellow/green) was used.

Now, it says you should NEVER mix those 2. But that is from the same guys (Audi) who makes us believe the engine goes up in smoke when not using their ridiculously overpriced stuff (fact: aside from the longer lifetime (100k vs. 40k) of this type of fluid, modern coolants for aluminum engines protect from corrosion just as well, just break down earlier).

So: do I need to drain and flush the system?
If so, how best to flush? Just drain, fill with water, drain?

Also: I have 3 sources of water:
a) hard, iron loaded (NOT)
b) soft, no iron
c) reverse osmosis (but read it can be corrosive?)

What water to use for mixing with coolant?

 

I use distilled.. comes in small or large jugs at the grocery store. 99cents at the most.

 

never mix the two... there, I said it.

Flush with tap water... drain old coolant, add tap water, run to temp, let cool, drain water, add more water, repeat until water comes out nice and clean. Add G12 and distilled water, 50/50 mix.