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

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

__________________
Thanks,
Mike

89 300E 175k (sold and missed)
89 Porsche 911 / 3.6 V-ram / #94

Want to make God laugh?
Tell him your plans.

High probability of OČ sensor...

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

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

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.

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

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.

means little... they seldom do.

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...

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

If the O2 sensor is bad, you will probably have a noticeably stinkier exhaust. One that old should be changed out anyway.

__________________
1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma

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

__________________
Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles

This link has a lot of the info:
http://www.landiss.com/mixture.htm

From the link:
The X11 diagnostic connector

On my 1991 560SEC the "X11 diagnostic connector" is mounted on the inside of the left fender, behind the ABS controller. It has a round screw-cap. On some cars (California?) there is also an "X11/4" between the two firewalls - don't get them confused.

The signal provided at pin 3 of X11 is called the "lambda on/off ratio" signal. It is convenient to use pin 2 of the same connector as a ground reference.

This lambda signal is not a replica of the voltage from the oxygen sensor. Instead, it is a constant 100 Hz pulse whose duty cycle indicates the hunting of the fuel injection for an ideal mixture by switching back and forth between very slightly rich and very slightly lean.

Mercedes has chosen the less common definition of "duty cycle" in this case. They are referring to the percentage of the entire pulse period during which the voltage is zero, not the time when it is near battery voltage (mine was +13.6 V when the battery was +14.0 V). In other words, if the pulse rests at ground for 7 milliseconds and then rises to +13.6 V for 3 ms, the duty cycle is considered to be 70% (see oscilloscope trace at right).

If you have an oscilloscope to measure this timing, fine. If not, it can still be estimated with a voltmeter. Since continuous "0 volts" would be considered 100% and continuous 13.6 V would count as 0%, just measure the voltage between pins 2 and 3 of X11 and divide that by 13.6. Next subtract that ratio from one, and convert the result to percent.

For example, if the meter reads 4.0 V, first divide 4.0/13.6 = 0.294. Subtract 1.0 - 0.294 = 0.706, or 71%. In equation form:

Duty Cycle = [1 - (V{pin 3}/V{max})] x 100%

Remember that if the oxygen sensor is doing its job and the system is operating closed-loop, the reading will jump around, so you might see readings from 5 V to 7 V and have guess at an average.

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

Will try to measure on Sat.

These links are to sheets from the factory manual. They also detail the process and give the readings that can be got at X11.

Duty cycle 84%? What does this mean?
Duty cycle 84%? What does this mean?
Duty cycle 84%? What does this mean?

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

Thank you for the encyclopedic info. I hope I am smart enough to use it.

It's not so difficult...
You want to measure the voltage between pins 2 and 3 at the X11 connector (drivers side of the engine bay - round, black plastic, screw off top)

Since your multi meter does not read duty cycle you: [1 - (V{pin 3}/V{max})] x 100% = Duty Cycle

When I read duty cycle I have to put the + terminal of the multi-meter in pin 2 and the - terminal in pin 3 or I get an inverse reading.
I have never tried it reading DC Volts and dividing by the max voltage... so I'm not sure which way you align the poles to obtain the correct reading. To determine which way to align the MM you should first take a reading, engine off, key turned to on position (assuming your car is not a california car)... then your equations correct answer should be 70% constant voltage (85% for california)

When you do the actual test your Engine should be fully warmed up and ideling...
If the reading fluctuates the o2 sensor could be functioning but may still be bad (lazy or out of range) note what voltage the reading starts at and where it moves to over about 5-7 minutes time.
If the answer is a constant 50% the o2 sensor is definatly bad

Post your results if you have any problem interpeting them or get unexpected results...


P.S. If you end up replacing o2 sensor you can solder a 1990 mustang 302 sensor (about $40.00 US) onto your existing connector and save $70+ over mercedes OEM.

P.S.S. I hope all my info is correct, if not I hope someone will jump in and correct it.

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html

Reading the thread its stated that a 90 Mustang ( 302 engine) O2 sensor can be used in place of the MB version. Is it the same sensor, or does it just operate in the same manner? or?

Thanks
Alan