98 ML320 has 220k miles, I have had a recent drop in milage from @20 to @12MPG. Seems to run fine - no loss of power or skipping. Rencently changed fuel pump. Any ideas? Thanks

There are several keys to good fuel mileage.

O2 sensor is most important. Fuel pressure as well, controlled by fuel filter.

Air mass sensor inputs are also critical.

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