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Originally Posted by MikeTangas
A bad O2 sensor can cause a major decrease in fuel mileage.
With the 560SEL I consistently post 15 mpg on an all city tank, 17.5 mpg on a mixed tank and 19-20 mpg all highway running 80mph.
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Emissions pieces are very tricky and fickle with fuel economy. If your ECM thinks you are in closed loop mode, it will richen the mixture and cause poor fuel economy. Does the exhaust blow black smoke, or smell gassy?
If your O2 sensors have not been replaced, change them, they have a service life of about 50K mi. and will throw off you ECM and fuel mixture real easily.
Good luck