[mention]Overboost [/mention]

How does your oil temp compare? Also radiator outlet temp vs. inlet? It's strange for tstat to fail hot, but you may be onto something with the water pump. If it's impeller is worn and not pushing enough volume it will cause your symptoms but you will notice in the outlet temp being too close to the inlet temp (can be caused by any blockage including dirt in the radiator fins)

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)