the N54 is prone to carbon building on the intake valves, this leads to generally bad performance.
i suspect that at your mileage, this may be a factor in your misfires and stumbling
unfortunately, a walnut shell blasting of the valves is not cheap- 5-600 dollars, but the only way to clean it if you have the buildup. its visible if you pull the intake manifold.
Past: 2001 330i ZSP/ZPP/Step
Past: 2004 S60R 6MT Sport
Past: 2007 335i 6MT ZSP/ZCW/322/655
Past: 2010 335xi ZMP/ZCW/ZPP/PPK/PE
Present: 2013 335i 6MT ZMM/ZDH/ZTP/3AG/494/508
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