I don't believe my 02 sensor has ever been changed. When i had a valve job done, the fuel mixture was way off. I got in back to about where it should be by feel. I know this is not the proper way to do this, but it had to do for the time being, and it seems pretty decent (before this it would hardly accelerate).

My question is, if the 02 sensor regulates the fuel/air ratio, then should I have not been able to adjust this?

One more bit of information: the car accelerates much better before it is up to operating temp, then once it is warmed up, the engine does not feel as strong. The O2 sensoe does not play a part until the engine is at operating temperature, correct? This would lead me to believe that it is not funtioning properly.

Any thoughts on this would be greatly appreciated.
Mike

If the O2 sensor is faulty and produces no current then the engine would tend to run rich, was your gas consumption high?

What did you change to try to get it running better?

Hi,

Get a multimeter with a duty cycle function and read the fault codes on your car. You'll save a lot of money by not replacing perfectly good parts.

After buying a simple Craftsman meter for about $30 and getting the Service CDs, you'll be in good shape for narrowing down your problem.

I tried it the other way. Replaced a lot of parts in trying to solve my hesitation problem. Reading fault codes showed a fault with the 02 sensor.

You are correct in the the 02 sensor doesn't work until the car is warm.

You'll need a multimeter to adjust the mixture anyway.

Good luck,

Hi Mike!

The o2 sensor is actively used when the system is in closed loop operation. Then the ECU is correcting the mechanically adjusted fuel (hex tool adjusted through air filter box) injected. This regulation is carried out by a small pressure regulatior (EHA). That means that if your basic setting is way off, the possibility of correcting maybe insuffient even though your sensor is acting correctly.
As indicated already you should get hold of a DVM with duty-cycle setting and connect to the X11 plug (on the left fender) connecting to pin 2 (ground) and 3 and see if you get a reading that fluctuates around 50%. If it is stuck at either high or low reading, then try to bring it back by adjusting the mixture screw.
High reading -> set leaner by anti clockwise adjustment and vice versa for low reading. If you get a fixed 50 % reading (no fluctuation) when engine is warmed up, you know you have faulty o2 sensor.

Alan! I don't agree! You will not by default get a rich mixture, you will have what you have mechanically adjusted it to, lean, rich or correct.

Good luck!

__________________
Tore Bjorndalen

1990 300E

Alan, Chicago124, and ToreBj,-

Thanks so much for all of the info! I will be picking up a multimeter this afternoon, and hopefully I can make some progress. I will fill everyone in as soon as I make some headway. Mike

WHAT makes a multimeter able to do "duty cycle", I have a meter that measures DC milliamps (DCma), is this what is used to measure the pins?

Thanks

__________________
1991 300SE (my ride, 279,000 miles, looks almost new
1954 Cadillac (21 yo son's car, he bought when age 15)
1972 SeaBird 19 ft runabout (old but solid, slant six, Volvo sterndrive perfect condition, undergoing complete overhaul and refit)
1998 Toyota Rav4 (my sons daily driver when he is in the Continental US, PROUDLY serving in US Navy)

Do not confuse Duty Cycle with DC current. You are talking about current (DCma) it means Direct Current in milliamperes.
The Duty Cycle is something completely different, it is measured in %.
Read the Diagnosing Engine Controls article in the DIY section.

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Hi
I went in search of a multimeter, but was not able to find one with a duty cycle. Looks like I will be going online.

If I understand correctly, the O2 sensor, if functioning properly, will adjust an "out of whack" fuel mixture adjustment, as long as it is in the ballpark? Please excuse my technical jargon, I am just learning :0)



Thanks for all the help, Mike

The O2 sensor is just one of a number of sensors sending signals to the fuel injection system. It can influence the mixture by about 15%, if the system is working "closed loop". The system can work "open loop" with the sensor out of the system. To work open loop the O2 sensor is uncoupled with the engine running.

With the sensor in position it normally produces a small voltage 0.2 to 0.8 Volts. If the voltage is above 0.5V this means that the exhaust gases are low in O2, ie the mixture is rich and the control unit tries to deliver a weaker mixture (within limits).Below 0.5V the converse is true.

As I mentioned above if the sensor is failing it produces a lower current or none, the control unit gets a weak(no) signal and tries to richen(within limits again) the mixture. Hence the heavy gas consumption. If the sensor is taken out of the system this will not happen, but in open loop, the mixture will in any case normally be on the rich side.

In your case I don't know what you did when you altered the mixture, presumably by the mixture screw and whether or not you took the O2 sensor out of the system.

Sears has a multimeter with duty cycle for about $30. That is where I got mine. It works great!

__________________
J.H.
'86 300E

Our friend from Norway has it right.

The system is in open loop until the controller recognizes a level of activity in the O2 sensor. Until it goes closed loop all mixture correction are mapped (programmed) and are made irrespective of what the real mixture is. Or in other words, in open loop the mixture will be the sum total of all mechanical and programmed enrichment.

This capability can be confusing. Many times a car runs better open loop than closed due to mixture irregularities cylinder to cylinder. A car with air leaks into individual cylinders or restricted injectors can find a rough idle or even single cylinder misfires when the average mixture is brought to lambda. The mixture that satisfies lambda has no room for error in the lean direction (plenty in the rich direction - remember this). Thus when the average is Lambda the lean cylinders can be in lean misfire. Some times the resulting misfires can add so much oxygen (unused due to the misfire) that the mixture gets richer - all confusing stuff when one is monitoring.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thank you for clearing up my mis-understanding of the operation of the CIS injection also for the insight into the confusing symptoms which can arise in practice.
I am now wondering what output is required from the O2 sensor to trigger closed loop operation. For example, is it a minimum signal or is a fluctuating signal required?

I think that it is a fluctuation which indicates it is alive. Remember that the O2 sensor is like a small generator with a one volt potential. The bias voltage from the controller is around 0.45v as the sensor creats current flow this is pulled toward the reading. I think the controller sees this as a randow change in voltage. It quickly heads to the potential generated by the mixture being tested and the controller reacts.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I finally had some spare time pick up a multimeter and test things.

I plugged the positive into pin 2 and the negative into pin 3, and got a reading of 38%. Have I done this correctly?

According to Steve's article, this indicated a rich running engine. No flucuation would indicate a dead 02 sensor.

As I indicated before, my car runs great before it warms up, but once it reaches operating temp it becomes sluggish. What should my next step be?

Thanks again for all the advice, it has been very helpful, and will be to everyone else with this problem in the future.


Mike

Hi fahrgewehr,

I believe you have them reversed. Positive in 3 and neg in 2.

Let's see what you get from that reading.

Regards,