Well that is interesting, this now sounds a little bit like the other thread with the 3.0L engine. Does the V8 have a manual water pump and not an electric like the 3.0L? It does sure sound like in this case that the aux pump would be suspect. Any history of cooling system work? DIS would give choices for diagnostics and bulletins based on symptoms even if there are no codes maybe ISTA will too??

Heater valves and secondary water pump should be in the ISTA as well.

OK I see you listed some history in the first post, anything else maybe? Was this the same when the aux pump wasn't working ??

Head gasket can give those heater symptoms but you would think there would be more going on and I think both those engines are pretty bullet proof as far as that goes ??

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