My car has no oxygen sensor. It was removed a long time ago by previous owner. We used to have only leaded fuel in this country, so those sensors used to fail prematurely and where simply removed. But for some years now unleaded fuel is available (they call it the "green" fuel), so now I would like to try to refit a O2 sensor to correct a chronic over rich fuel mixture problem and lower emissions. There is always a lot of sooth in the exhaust tips, and exhaust gases itself smell way too rich anyway. Fuel economy is not really an issue (premium 95 oct unleaded costs only about 9 cents a gallon), but it would be nice too if it can be improved. As it is now I get only about 160 miles (of mostly stop and go city driving) out of a full tank.

My present exhaust system consists of dual pipes all the way down between the manifolds to the central muffler (no catalytic converter), and then a single pipe follows to the rear muffler. The downward pipe portion where the sensor used to be mounted is long gone, so I'm wondering what can be done in order to attach the sensor to one of the pipes between the manifold and the central muffler. Maybe have a muffler shop braze a big nut to the pipe so the sensor can be screwed on?

I'm already planning on ordering a 1990 Ford Mustang 5.0L V8, Bosch PN 13942, which should be equivalent to the original Mercedes sensor, but only 1/3 the price.

Any suggestions on how to attach the sensor to the pipe will be much appreciated.

-Alex

 

.09/Gallon??? So, how would one go about relocating to Venezuela?

 

Unfortunately that is pretty much the only thing that is still ok here. Our local madman that some even call "president" hasn't spared much else so far. And he has mentioned that a price hike should be in the works, but that was some months ago, and so far the issue seems to have been forgotten.

Getting back to the 02 sensor issue, c'mon guys, in such a knowledgeable forum I'm sure someone must have an idea of how to refit one in my car. Thanks.

 

OK, as far as installing a new sensor, you would first need to locate the wiring connector and make sure it is intact. On my car it is under a metal shield and the wire disappears through a rubber grommet. You would then have to weld a small fitting to the exhaust pipe that has the same thread size as the sensor. It would be a lot of work, and might never work correctly. These Lambda sensors are extremely sensitive.

A easier solution is to simply adjust the mixture screw slightly leaner to achieve the proper mixture. Yours seems to be running quite rich so any leaner you make the adjustment should be an improvement. See step 26.

 

I haven't checked, but that of course would only be a minimal problem as any missing wiring is rather easy to replace. I will inspect that area tomorrow to make sure what the status is.

You mean sensitive to being installed improperly? because the fitting would obviously be attached/brazed first to the pipe, and then the sensor could carefully be treaded into it. I'll have to check at the muffler shop and see if they would be able to do the brazing job. I'm aware of the metal shield you mention, I believe it is to physically protect the sensor, and might also prevent it from cooling off due to too much air flow around it. This is just my assumption, I might be wrong, or there might be other additional purposes for the shield. In any case I'm guessing that a small shield similar to the original shouldn't be too difficult to improvise after the fitting has been attached to the pipe.

Another concern would be if the tip of the sensor does not reach inside the pipe far enough if the fitting is not correctly positioned, or is simply not the right size. Any ideas in that respect are still a bit hazy, and someone else's experiences would be very valuable input.

I tried this, but the usable adjustment range is really too small in order to fully compensate for the open loop mixture condition that is now present without the sensor (according to the adjustment procedure you sent, step 26, the adjustment only affects idle mixture). If I go too far either way with the adjustment screw the engine idle turns very unstable, so I'm now pretty much in the middle of the somewhat tolerable range, but mixture still is way too rich. I don't have a CO analyzer either, so my adjustments are purely focused on trying to achieve the best possible idling smoothness given the current condition.

Thanks for posting the job 07.3-1100 PDF, much appreciated.

 

I have an idea. Yours should be be an ordinary, common 2-wire sensor. Maybe you could find a car with about the same size exhaust pipe in the junk yard. Pull the pipe out and cut out the area with the threaded part in it. Cut a piece out of your exhaust pipe and weld the other piece into it. Screw the sensor in, connect the 2 wires, and you're done.

 

I believe my sensor should have 3 wires, at least the Bosch PN 13942 has them, two for the heater, one for the sensor itself. Not sure if my car actually uses the wiring for the heater, and I assume that is why you mentioned the 2-wire sensor. I'll have to take a look, maybe today after washing the car I'll take a peek under the front passenger carpet.

I thought about the piece of pipe thing, but I would have to take it off from another Mercedes, and that might not be so easy. Junkyards down here aren't exactly flowing over with Mercedes. There are a couple of places that specialize on importing cut-up cars from the US to resell here as individual parts, but they charge an arm and leg for everything, never mind its just old used parts. That is why, except maybe with rare exceptions, I never buy any parts locally, I import everything I need for my car and that way it ends up costing me less then half of what it costs here off the shelf, or even for used parts, for that matter.

Is there any way of knowing what the tread size of the sensor is? I'll have to order it anyway, but hadn't plan to do so until I had worked out a plan to install it. If there is no other way then I'll go ahead and order it.

 

I was thinking the pipe could be from any car with a similar size pipe. Also, I didn't think they used heated sensors in '86. O2 sensors were new technology at that time. Also, the thread size should be the same on most sensors. Sounds like your on the right track. Good luck! Mail me some gas would you? I just paid $3.29/gallon for Premium 91 octane!

 

Any idea what size the thread actually is?

Sorry to hear about your fuel prices, that IS expensive!

92 octane is leaded, cost is about 7 cents/gal.

But who needs it when there is 95 unleaded at 9 cents/gal.

 

Today I checked under the passenger carpet, found two loose connectors, one is a rectangular one with two wires (a brown and a black), and another round connector with a green wire, this wire much thicker then the ones from the other connector. Looks like after all I do have the connections for a 3-wire O2 sensor.

 

was the original exhaust replaced? I asked because the o2 senson slot should be right underneath the passanger seat. The hole should be facing up so looking at the car from underneath you would not see it. If its the original exhaust system it most likely has the slot already. I just replaced my o2 on my euro Benz 300E-24v

 

Yes it was replaced. So I need to get a fitting brazed on to the exhaust pipe that matches the threads on the sensor, at about the old location. Thing is, I have no idea how far the sensor probe should go into the pipe, or if it has to be at any particular angle, etc.

Furthermore, I now have two exhaust pipes reaching all the way from the manifolds to the center muffler instead of being joined together just before where the catalytic converter used to go. I understand the sensor used to be attached to the pipe after the joining "Y". But I guess just putting one sensor on one of the pipes should also be adequate.

 

I'm running a true 2" twin exhaust that is pictured below.
Notice that on the far right side assembly the O2 bung welded in the down pipe from the rear three cylinders before the silencer.
That's all you need to do !!!

 

It would have been great to see a closeup picture of that specific spot. The whole system looks great, did you install it on a 300E?

 

I cropped the photo so you can get a better idea of the O2 bung in the stainless steel exhaust pipe.

The exhaust was supplied as part of a twin turbo kit that was recently installed on my 1988 300CE M103-12V.

A second O2 bung was later installed for tuning purposes...the one pictured accepted the stock O2 sensor

 

That dual turbo system of yours sounds very interesting.

Was it a tight fit, or did it go on easily? How many additional HP do you estimate getting out of that setup? Who sells it?

Thanks for the more detailed picture, it gives a good idea of the location of the sensor but unfortunately it still doesn't show much detail of the fitting itself. I have no idea how exactly the stock sensor fitting looks like. Is it just kind of a threaded fitting brazed onto the pipe, or is there more to it? I was going to look for a big nut that would fit the thread on the sensor and then have it brazed on to the pipe, but since I'm not sure how far the sensor tip has to go into the pipe, and if the distance (and maybe also the angle?) is critical, not sure this would work.

 

OK so I went to google and made a "02 sensor bung" search and found serveral places where they sell these. For example:



Or this other one that even includes a plug:




OK, now I understand. So this basically is a stainless-steel ring with a thread on the inside. Now the question would be, are 02 sensor threads in general the same size, or do I need to look for a Mercedes sensor (Bosch) specific thread size bung?

 

Almost all O2 sensors are 18MM ( M18 x 1.5 thread ).
Jut weld the bung into your pipe, thread the sensor in, connect the wires and you're good to go !!!!

 

Thanks for the info. I'm now finally getting somewhere. I just sent an inquiry about the thread size to both the bung and sensor sellers. See if they match up.

You didn't comment on your twin turbo system, was it hard to fit? how many additional HP's with it do you estimate getting out of it?

 

Kits for the M103-12V may still be available from U.K. Merc dealer "Hughes of Beaconsfield"



They had the kits made for them in the early nineties by TurboTechnics for install on new 300E/300CE Mercs at their dealership.
Currently selling off old inventory for the incredible price of 1000 pounds sterling.
Quite a few Forum members have purchased the kit.
I was the first to complete the install.

Do a Forum search for TurboTechnics and you'll find all the details.

Dyno numbers are just a comparitive tuning tool.
From stock baseline to turbo install power increased by more then 50%.
Moe importantly are actual performance results.
Best recorded 0-60 is 5.46 seconds in high temp and humidity.
Better launch technique to limit wheel spin and cooler temps/low humidity should help to pick up a few more tenths !!

 

Impressive!

 

So I did a bit of research and yes, it turns out most sensors do have a M18x1.5 thread. Some people even sell plain nuts in this size for $5+s/h as a bung. So after all my initial idea of using a welded on nut wasn't so far fetched as I though, I just wasn't sure what the proper name for the fitting was ("bung"), and the thread size of the sensor.

I can now order the Bosch 13942 sensor, and then just look for a appropriately sized nut, and might even get a plug, just in case.

Thanks for all the info.

 

Finally got the o2 sensor and new muffler installed. I had the workshop braze on a nut in the position where the sensor has to be mounted. I spliced the wires remaining from the long gone original sensor, and plugged all 3 wires back together. The sensor (while hooked into the CIS-E) reads 0.59V cold and rises to 0.84V after 2-3 minutes of warm up, which I believe is still a rich mixture reading. I haven't had a chance yet to replace the "Check Engine" bulb on the instrument panel to see if it now stays off, but I suspect it will not.

What else would I need to check in order to correct a rich mixture condition? All I did so far was to try and adjust "by ear" the mixture screw setting on the fuel distributor as per job 07.3-1100. I went both ways until the engine began to stall, and then I left the setting about half way in between. BTW had to make the adjustment with the air filter housing removed, as the extender is missing that enables the adjustment to be made while the housing is in place. I know one is supposed to do it while its in place, but does it make a lot of difference? If so, I'd need to find a really long 3mm hex key.

Thanks.
-Alex

 

Update:

After adjusting the mixture screw with a 3mm hex key I managed to get the system into "closed loop". Now the o2 sensor voltage fluctuates between 0.3 and 0.7 volts, which is the desired condition which means the ECU is actively controlling the mixture between slightly lean and slightly rich.

In order to reach the closed loop condition I had to turn the mixture screw about 3/4 turn anti-clockwise.

The Check Engine bulb was on all the time because it was the wrong bulb socket (by mistake I used the SRS indicator, which stays on all the time, and is a separate problem I have to deal with sometime). I'm now using the correct bulb socket and it comes on as it should only after rotating the key all the way to the RUN position, and extinguishes shortly after the engine starts.

Because the exhaust fumes still smell a bit off, I will play a little further with the mixture adjustment to refine the setting by connecting a scope to pins 2 and 3 at the diagnostic socket, and adjust for 50-55% duty cycle as MB calls for.

 

I believe that anti-clockwise is leaner, fyi.