Your dealer is a moron. A common way for the electric water pumps to fail is internally, they will leak into the circuits and short them...that's most likely where your coolant is going.
That's why it seems fine when they do a diag.
There should be shadow faults stored indicating any issue with the water pump, common one being reduced flow rate.
I don't understand why everyone insists on continuing to drive after they get a coolant system warning. If the car didn't limit you, you would be replacing your engine right now.
Yes it's a pain to sit on the side of the road waiting for roadside assistance, but that's better than dishing out thousands for an overheated engine on what I'm assuming is an out of warranty N52B30.
Btw price for parts alone are around $800 for the pump and tstat. It's normal for dealers to charge upto $1000 for labor making this job aroun $2k. Look for an Indy to do the work, will most likely be 2/3 the price.
Last edited by Alpineweiss e90; 05-02-2012 at 11:49 AM.