So if you guys didn't already know I am replacing a blown engine in this A4. I came across something interesting today. I've already drained the coolant out of the car and it was a clear watery red/pink liquid (normal). I pulled off the front of the car today (leaving the refrigerant lines alone this time!) and the coolant left in the pipes/tubes when I disconnected them was an oily brown. Not nearly as "oily" as oil but not watery. I remember reading in the Bentley manual somewhere that mixing older (blue fluid) with the newer (red fluid) coolant causes the fluid to turn brown and loose it's cooland properties. Anyone know anything more about this? Wondering if this could be related to the exploded engine. I do find it very odd that the fluid coming out the radiator drain plug was clear and clean.

<center><img src="http://www.prestone.com/products/19.gif"></center><p>

It's almost definitely from mixing coolant types. I second MikTip's Prestone SuperFlush idea - I used that at some point when I switched from the original glycol-based coolant to Prestone's OAT-based coolant and had zero problems. Bonus - you can get the stuff at pretty much any FLAPS, and it's pretty cheap.

I don't know what Audi uses these days but if I had to pick a bottle off the shelf to top one off I'd go with an OAT-based coolant.

Saying "run this radiator flush through the system" is a bit deceptive though :)

It takes several cycles of flush, fill, drive, repeat, repeat, etc if you follow the instructions on the bottle (which I did).

(shameless plug follows :)<ul><li><a href="http://www.12v.org/maintenance/index.php?section=ss&amp;ss=9">http://www.12v.org/maintenance/index.php?section=ss&amp;ss=9</a</li></ul>