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!

Weak O2 sensor or MAF.

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.

Correct, 4 sensors.

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?

Has anyone had any success with CRC MAFS aerosol spray cleaner?

Is there a relatively easy way to test them?