2011 A6 3.0t 101k miles.
Wife came home in her A6 a couple weeks ago and I smelled coolant. Checked the reservoir and it was about halfway empty. Went to dinner (in the S4), and when we got home the res was empty and coolant was still dripping off the belly pan.
Worried that it was potentially a plastic water pump issue, I threw it on the lift and chased the leak for a while. Finally found that it was coming from the front crossover coolant pipe between the heads from the driver's side head flange.
What ended up happening, near as I can tell, is that the inner race of the o-ring holder failed and fragged, allowing the o-ring to swell until it failed and leaked. Luckily it retained all the chunks in the flange and nothing was sent through the coolant system. I don't know if it had been replaced and over-tightened in the past or what.
$236 for a new coolant pipe from ECS, I'm changing the t-stat while in there, as well as carbon cleaning the intake valves with walnut shells.
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