87 300E runs great but mpg suddenly bad - PeachParts Mercedes-Benz Forum

lizem100 · #1 12-14-2005, 05:37 PM

Car runs normally. O2 sensor light not on. Do not smell gas leak- changed fuel filter 30 d ago. Mpg tumbled in last week from 22-25 to 17. Perhaps one time event /bad measurement but this tank is dropping fast too. Any ideas? Thanks

MikeF · #2 12-14-2005, 09:24 PM

When mine did that I replaced the O2 sensor and the problem was solved. How old is your sensor?

A264172 · #3 12-14-2005, 09:50 PM

High probability of O² sensor...

Do you have digital multi-meter? And does it 'read' duty cycle (i.e. duty%)?

lizem100 · #4 12-15-2005, 12:29 AM

Should O2 sensor light come on if it goes bad? That is why I said in the first thread that the light did not come on.

lizem100 · #5 12-15-2005, 12:32 AM

Have digital multimeter but no duty cycle.
O2 sensor is 340,309 miles old.

A264172 · #6 12-15-2005, 11:04 AM

Quote:

Originally Posted by lizem100

Should O2 sensor light come on if it goes bad? That is why I said in the first thread that the light did not come on.

means little... they seldom do.

Quote:

Originally Posted by lizem100

Have digital multimeter but no duty cycle.
O2 sensor is 340,309 miles old.

I will have to look up the math for you to test at the X11 connector without duty cycle... can be done... will post back with more info if you don't find it you may be able to verify bad o2 sensor there...

STUKA · #7 12-15-2005, 01:47 AM

Perhaps your area has switched over to a winterized fuel formula,-meaning more ethanol, less BTU, and less MPG. Just a thought. One tank full is not a very accurate measure also. Let us know what happens.

lkchris · #8 12-15-2005, 01:44 PM

Quote:

Originally Posted by STUKA

Perhaps your area has switched over to a winterized fuel formula,-meaning more ethanol, less BTU, and less MPG. Just a thought. One tank full is not a very accurate measure also. Let us know what happens.

Yes, ethanol is welfare for farmers--nothing about it is good for car owners.

lizem100 · #9 12-15-2005, 01:54 PM

Will try to measure on Sat.

lizem100 · #10 01-15-2006, 10:49 PM

I went to the tech session yesterday thinking my O2 sensor was bad. Lead tech at dealership hooked up their analyzer to the X11 connector. It read 2%(I am not sure units) and did not vary. He said it was too rich and needed more lambda. He took a 3mm allen and adjusted (near the air intake) the mixture more lean. The final result was 4.5 varying up to 5.5. Thus the sensor is fine. (I let a shade tree style indie touch the richness several years ago after fixing a water pump.) Perhaps the broken thermostat and the excess richness caused the apparent sudden drop-will need to monitor mpg and report.
For A264172's benefit I checked my X11 with my multimeter. I got 6.75v up to 7.5v. So it varies now.
Re O2 sensors- both Bosch and NTK have toll free number to call for tech info. The 13953 sensor used at one time one the Ford Mustang(Parts people now rec 13942 shorter cord for the Mustang) now rec on E 150 econoline can be spliced in though it has a 6 ohm heater instead of 4.5 ohm. Not sure if the ohm diff matters. NTK had a $29 one with a short cord(theoretically bad as solder will be outside cabin) and correct 4.5 ohms.
Many Thanks to all who helped.

A264172 · #11 01-27-2006, 09:02 AM

Thanks for the follow up.
That's something I haddn't considered; running so far out of adjustment range that there is no visable fluxuation.
Glad to here it's runing good and not fouling your cat anymore.
Congradulations.