Haven't backflushed the core yet because it's been freezing. Raining now so i might try today... my coolant looks clean and new though... if the core was full of crud wouldn't my coolant look nasty too?

In other news, opened up the monovalve and the diaphram is ripped. It pops out when I turn the CCU off, and pops back in with any of the air buttons. I'm assuming it pops out to seal off the hot coolant, and the ripped diaphram lets hot coolant into the system... so this isn't my problem.

I could feel the aux pump running with the key in the on position. I tried with the monovalve open but didn't notice any coolant flow...

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1984 300CD

No, not necessarily. The heater cores are normally made from Copper or aluminum, and as a consequence, tend to corrode differently than the steel of the block. You made not see much in the rest of the cooling system. Also, water flow is truncated away from it when it is off, therefore it is more sedimentary in the core vs the rest of the system.

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1998 VW Cabrio
2001 Audi SomeRoad A/T (no air suspension)
2003 Audi DeadRoad M/T (no vroom, for later)
2002 Audi NoRoad A/T (nothing under the hood, being rebuilt)