Our Mk3 is almost 11 years old and so due a change to the coolant - FL22 stuff.
General opinion seems to be to change the coolant and the Cooling Radiator Expansion Tank at the same time as the tank can spring a leak around the neck and, ahem, upset the engine.
Is it simply a case of draining off the coolant, flushing a few times with distilled water and running the engine until warm and then re-filling with FL22 (or equivalent)
Is there anything else to do at the same time?
I’m in the process of doing a how to if that helps should be up this weekend.
I had to do four water fillups, run at 2000 rpm for say 10 minutes to get the engine up to temperature, at that point the thermostat opens and the trapped 2.5 litres of old coolant mixes and you can drain again.
I only got a clean drain after the fourth flush.
Next issue FL22 is only available ready mixed, that means that you only get a real antifreeze mixture of 33% antifreeze if you go the Mazda route, no I did not go to the hassle of removing the thermostat to try and get a lower trapped water content.
There are at least two better routes here, either buy Comma G30 which is a BASF product that manufactuered to the common modern Japanese standard used by Subaru, Mazda, Toyota, Suzuki etc., it is purple and I wanted a product with a similar colour to the Mazda FL22.
There is a German company that produce a product to the Japanese standard and it is available here in the UK in concentrated form and is a similar green shade to the Mazda FL22. I have used that product.
https://www.ravenol-direct.uk/ravenol-hjc-hybrid-japanese-coolant-concentrate.html
In the past 40 years we had the following temperatures
Scotland -27C
England -26C Shropshire
Wales -23C
Therefore, I would use a concentrated product at 55% concentration as recommended by Mazda.
If your car has done over 65,000 miles it is suggested you replace the expansion tank.
Using the Mazda FL22 premixed product with the existing trapped water you are protected down to -16C approx.