When cold, O2 not used for fuel mix, so I'm thinking whatever long term adjustment due to wear etc are being used to estimate fuel mix and with new MAF it's sending a very different value. I'd definitely reset adaptations if you haven't already. I'm not sure if the same trick works on MAF as O2, but I had very good luck unplugging O2 to reset them wondering if the same will work on MAF.

Also rich when cold can be a flow restriction make sure you don't have an insect nest in your snorkel and that your air filter is clean

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