Yeah, not so likely for a Pierburg pump to give out. Does it seem noisier than it should be? Heater valves are always open by default so disconnecting should provide for heat and help to eliminate the IHKA.

Any chance that hoses/connections got messed up with a rad replacement?
Otherwise you could be back to a possible head gasket, I'm thinking when you did a pressure test it wasn't to find a head gasket issue?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)