87 300E runs great but mpg suddenly bad
 

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?

High probability of O² sensor...

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

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.

Winter Fuel Formula ??
 

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...

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

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

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

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.

Will try to measure on Sat.

More.
 

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

http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html
http://www.peachparts.com/shopforum/831810-post15.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.

90 mustang O2 sensor
 

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

The sensor is identical... the wire length and connector plug are different.

Raw data
 

I will post raw data first
The connector I used is behind the ABS and horn on the driver's side. It unscrews and pin 3 is clockwise from pin 2. The com part of the multimmeter was connected to pin 2 and volts to pin 3.
With key on but engine off I got 3.76 on the first measurement but then 4.06 on all subsequent ones.
With engine idling i got 11.35 steady. I measured first after the car cooled a little and would get 5 for a flash then 7. Repeat measurements were flash of 9 then 11. I drove the car 2 miles and the got 11.35 steady.
Battery voltage engine off was 12.08 and 14.06 engine on.
So Engine off test 1-v(pin)/v(max)= 1-4.06/14.06= 1-.29= .71= 71%. However this is engine off pin 3 and the battery voltage when engine on. Value for battery voltage engine off would be 1- 4.06/12.08=1-.34=.66=66%.
Values for engine idling would be 1-11.35/14.06= 1-.81=.19=19%.
I do not see fluctuation in digital multimeter other than for a split second when engine may have cooled some.
What does everyone think. Correct me as needed. Was car hot enough?-water temp was up off the stop.
I will add I thought the exhaust had a different odor for the last month or so.
Is this fun or what!

Was the water temp gage at 80+? that would be hot enough. Until then I'm not sure there arent other enrichment factors that would through off the signal. The fluxuation of the voltage usually starts between 40 and 60 but the test calls for 80c and ideling. The type of fluxuation your looking for is over a range of 1v or less.

I just ran the test on my 86 sans duty cycle
Battery = 13.2v (engine off)
KOEO = 3.91v (key on engine off)
Temp 80+ ideling = voltage hunts back and forth between 7.2 and 7.7
Temp 80+ ideling o2 sensor unplugged = 7.1v constant

Thanks for the info!
 

I don't have anything to add other than THANKS for the nice detailed info. I'm printing for future ref. I've done 02 sensors on every Benz I've owned but it was after a CEL indicated failure. I should check my '92 (I'm assuming it would work the same way even though it's an 8 not a 6) to see if there is anything I need to address there.

Again, THX!

My temp guage does not seem to go to 80 this winter. I will check after 15 min of driving. I thas been 35-45 here. Would a problematic thermostat keep the water temp cool and fool the O2 sensor?
I saw no fluctuations at all-perhaps 11.35 to 11.36 occ.
Thanks. I will measure again.

By Jove I think youve got it.
 

I believe it would!
The system enriches when cold to warm the engine.
What temp does it get to?

will drive hard tomorrow.

water temp
 

I drove about 7 miles today. 2 miles at 35 mph, 4 at 60 mph and 1 at 25 mph. The temp guage only got to the line below 80 which I assume to be 60. The guage has 40, 80, 120 C marked and lines between which I assume to be 60 and 100 C. If I remember correctly the guage got to about 60 when I drove it longer too. Should the temp get to 80 even in winter? It is about 40F here.
Perhaps my thermostat is bad.

That was the next thing that came to my mind. If the car is running too cold that could account for some of this sudden drop in milage. Running too cold is also considered bad juju for engines. I'd at least look into having it tested/replaced (I figured to test it you have to remove it so why not just replace it -- of course it could be a $1200 part. :D )

I agree with Holeshot's assesment, except about the price of the part ($23.00 or less plus shipping on this site): http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1OH0MY2181OT0KACVY&year=1987&make=MB&model=300-E-002&category=G&part=Thermostat+Kit
Depending somewhat on the length of your trips and the average temps, your car is spending too much of the time enriching (adding fuel) to warm up the engine.
When I get a chance I will try to look up the specific temp. at which enrichment stops. But it is not good for the engine to run cold you should change the thermostat at the earliest oppourtunity and hopefully that will fix all symptoms.
Running rich for too long has other deliterious effects on the catalytic converter (which is an expensive part) and can kill the o2 sensor as well.

Thermostat was broken in open position
 

Thermostat had structural failure. One of the copper struts broke and the diaphram was open 1 cm wide on that side.
The fact that the engine was heating up did not register with me at first.
I put the thermostat in so the diapham opens down (took old one out fast to avoid extra coolant leak) and put gasket between thermostat and top-hopefully correct.
Now car heats quickly to just above the 80 C line- makes sense with 87 C thermostat.

Now for the O2 sensor- pin 2 and pin 3 on the X11 still give 11.35 v. constant. So does this means the O2 sensor is bad? I will remeasure today after a longer drive though water temp was 80+ C.
I do not get a constant 7.1 v (with O2 sensor unplugged) as A264172 says.

Did running cold for a month or so bother the O2 sensor?(if it is bad)
Thanks everybody so far esp A264172!

Nothing like a farily easy fix. The O2 could be damaged or dirty. Running it for a while and checking it again is unlikely to do any more harm than already done.

The last time I replaced the O2 on my 300E I took the sensor into the big-box auto stored, had them cross-ref the Bosch number and picked the lest expensive one and soldered on the MBZ connector to the end. Done.

The first time I did it I went to the dealer and.. the only one they have for me had not plug on it whatsoever (and of course at more than twice the price) so when I did it the next time I just got the proper sensor and did not worry about the plug.

(now if only I can sort out the short in my 500.. :( )

My drive to work has been reduced from 15 miles one way to 1.5. The heater hardly gets warm.

my average mpg has dropped drastically. I would say from the very low 20's to maybe 15 mpg... however, in a week I am driving very few miles.

I'll take the lower mpg for now. I do have to consider having my wife drive this car to work once a week, however. She has a 30 mile commute which would be better for the car.

This is one of the few processes I am a little familiar with so... glad to lend a hand.
A reading of 20% duty cycle with no fluxuation (when the car is fully warmed up and ideling) indicates a problem with the 'throttle valve switch' (part of the throttle body) or it's wireing. The switch is located on the lower portion of the air intake manifold.
And only controls fuel delivery under full load and at idle so your gas mpg should not be affected by it.
This thread: http://www.peachparts.com/shopforum/tech-help/58398-86-190e-8v-throttle-valve-switch.html?highlight=throttle+valve+switch
...has a few thoughts on that switch... but you might want to triple check your duty cycle under 'proper' conditions before you go digging at it. ;) Are you sure there is no fluxuation of the voltage when the care is ideling and the temp is up to above 80?

When the needle first hits the contact there is occ a reading 2v lower for an instant. Once the pins are in 2 and 3 the voltage is steady.
I will check again this PM after driving 7 miles.

Just in case, if I buy the Ford O2 sensor do I replace the part in the exhaust or near the seat? In other words does the exhaust part break or the part near the seat(is this just a connector?)

The part that screws into the exaust pipe gets replaced and the connector under the floorwell stays... ...cut the wires closer to the connector on the new unit so the splice is inside the cabin...

also you don't need to drive the car to test it... you can just let it sit and idle until 80 + temp is reached.

That is what happened with my wife. She used to go 30 miles each way to work and drive our C230K Coupe and get 27MPG average. Now she goes less than 1.5 miles and the car gets 20MPG now. But alas I do take it once a week for a run to blow it out so to speak. If not for the heated seats she would never be warm by the time she gets to work. lol

But hey a tank of gas lasts more than a week now.:D

Voltage still 11.35 no fluctuation after ccoming home- 7 miles. I guess I will drive for a week or so and retest to see if proper temps help things.
A26 says the reading does not make sense re the O2 sensor. Will see how mpg changes. Worst dase scenario I go to the Jan tech session and ask the MB shopforeman.

Final Result to this saga
 

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.

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.