Since the issue is associated with cold mornings, first startup and goes away after a minute or less it could be an open loop problem. When the engine is cold and first starts up it is using a pre programmed air/fuel mixture. It does gather a little info from MAF and water temp but primarily it is using the pre programmed formula. Since your problem goes away and engine operates normally it is not likely the thermostat. I would eliminate the MAF as the potential problem. It is not likely the programming has changed but could have.

Sometimes a vacuum leak seals itself when the engine starts to warm up. I note you have done a smoke test. Was test done on a cold morning prior to starting the engine?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas