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  #1  
Old 09-22-2008, 01:15 PM
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W210 E420 - sudden MPG drop

My sister-in-law's '97 E420 with M119 has suddenly been getting really low gas mileage on long freeway drives. Historically, it has returned about 20 to 21 MPG in mixed town/highway and 27 to as much as 30 MPG on long trips.

For the last two months or so, highway mileage is down to 20/21 but intown mileage has not really changed...in effect, no matter what type of driving is done, the car is returning about 20 to 21 MPG.

The car drives fine, engine has good power, idles smooth and accelerates normally. No CEL.

What could be wrong?

Thanks!

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  #2  
Old 09-22-2008, 01:44 PM
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Weak O2 sensor or MAF.
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  #3  
Old 09-22-2008, 01:49 PM
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Thanks Doc. Do you know off hand, how many O2 sensor a '97 E420 has? I believe it's an OBDII car...so I imagine there are two sensors before the CATS and two after.
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Old 09-22-2008, 03:21 PM
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Correct, 4 sensors.
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ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
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Old 09-23-2008, 01:15 AM
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Would I be correct to assume that the sensors in front of the cats are more likely to be causing the poor MPG? Don't the rear sensors just measure cat efficiency?
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Old 09-23-2008, 03:42 PM
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Has anyone had any success with CRC MAFS aerosol spray cleaner?
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  #7  
Old 09-23-2008, 03:47 PM
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Is there a relatively easy way to test them?

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