You guys ... have me laughing. It's like we have a comedy routine going. I see where I went wrong, let me fix things up a bit. Do follow the link to the explanation if you want the full story. RB It (the BMSK computer) uses the Oxygen Sensor and sees that there's too little oxygen in the exhaust, indicating a too rich mixture. A process of learning (managed by the BMSK and called Mixture Adaptation) creates correction factors which takes a tank or two of fuel to fully remove the BoosterPlug effect. Mixture Adaptation is documented by Bosch and BMW, but is not well explained or understood. If you'd like to read about the measurements and detail of how mixture adaptation works, start at post #358: Mixture Adaptation Measurements.