I will try that tomorrow morning. It is so slight it is hard to feel without a load on the engine. I will have my wife put it in drive and hold it with the brake and see if I can tell which bank. CEL hasn't even flashed and this has been going on for month or more. I haven't reset the MPG monitor for a long time. I checked it yesterday and it was showing 12. Usually it is a little over 15. I reset it and will check that when I have run half a tank through. Once in a while if I rev it shortly after a cold start I will get a small pop from the passenger side exhaust which would indicate a rich condition. I haven't been able to make it happen after it is up to temperature. I have checked the vacuum lines, air filter, MAF, and throttle body so it appears to be downstream from those.

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