1998 ML 320, 106K miles... fuel economy has gotten noticeably worse over the past several months... now getting approx. 14 mpg.

Vehicle starts readily, drives well and is asymptomatic, other than the reduced fuel economy. Spark plugs and fuel filter were replaced at 90K miles, crank position sensor at 104K miles and ignition coil and wires for cylinder 6 at 105K miles (due to misfire). No fault codes are found, using M-B HHT.

Any ideas as to what to look for?

I would start by looking for a " lazy " oxygen sensor.
May not be worn-out enough to cause a check engine light yet.

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2007 C 230 Sport.

As well as the O2 sensor, I would look at the Mass Airflow Sensor. It may be at the hairy edge and telling the computer to send more fuel. This might explain the coil pack going out. Rich mixture carbons up the plugs, and it takes more amperage to fire a fouled plug. This might have accelerated the failure of the coil pack (which really only have a limited life anyways, though). I would check the CO and pull a plug or two and see if it is black instead of whitish brown.

No check engine light but all 4 oxygen sensors are factory original. In general, are the front sensors (pre-TWC) more likely to fail than the rear (post-TWC) sensors, because of their location?


Forgot to mention but air mass meter was also replaced at 105K miles... no CEL but adaptation value was drifting upward over time... 1.27 at time of replacement.