Mixing coolant

[swansea v6]

Just about to do a coolant flush on my x-type and just wondered what would happen if you mixed the green and pink anti freezes? (Not that I plan to:-) does it turn to a brown st sludge?

[dmitsi]

Green anti freeze contains silicates and phosphates as it's corrosion inhibitor. The orange stuff uses organic acids as its corrosion inhibitor. The two don't mix and the organic acids cause precipitation of the silicate which can reduce the overall corrosion protection in your engine.
The precipitation of silicate essentially creates a white material which coats the cooling system, clogging it up and reducing flows leading to a loss of heat transfer and therefore overheating and potentially blocking the cooling system totally, resulting in a generally unhappy car.

[Locknut]

The first thing to remember in any discussion on anti-freeze is that the colour codes are not universal. In Britain and Ireland blue anti-freeze is for two years, pink or red antifreeze is for five years and green is somewhere in between. However some members on here could be from other countries and any particular type of anti-freeze could be a different colour in their country.

For instance I have a four-year-old BMW. It has blue anti-freeze coolant so in it's third year I enquired about having the coolant changed. Turns out blue BMW coolant lasts ten years, not two, so here's a major difference between two European countries.

When the red (pink) longlife antifreezes were introduced there were horror stories of them turning into a brown sludge when mixed with the blue stuff. I've heard that the ingredient that caused this reaction has been removed and there is no danger of the brown sludge now. However I would not be happy mixing them. I would go for a complete flush if changing types (colours)

[theshrew]

Drain it, flush it then fill with whichever is the type for your car.

P.S. the blue stuff tastes the best

[VinceM]

The colour is a dye only, it does not indicate at all whether it is a silicate based or OAT based version. Do not use the colour as a guide, no matter where you live.

[swansea v6]

Well after finding a brown sludge in the expansion tank (car did not overheat, lose water, have any bubbles in the otherwise clean coolant, or give off any steam) I decided to flush it. Emptied all the system, filled with plan water and run engine for ten mins, emptied and filled again this time with a coolant flush. Then emptied again and filled with 'pink' coolant and water 50% mix. Sludge has completely gone, so lets how it worked and especially hope the sludge wasn't due to a head gasket issue.