what are the signs of a bad O2 sensor on the M104 engine? I don't like the smell of the tail-pipe of my car......the sensor is original from what i can tell, so I am sure it needs to be replaced after 180,000 miles

thanks,

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1993 300E, 2.8 M104 .....

no check engine light?

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

most w124's dont have check engine lights.

well, if the o2 sensor is dead, you're on the high road to clogging up a very expensive mercedes cat. converter.

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

No check engine light, and i still need to build a code reader....

I have a feeling like its eating gas like crazy, and the smell of the tail pipe is really bugging me...I cleaned the MAF sensor and the IAT sensor..not much left to do on the M104...

So how much impact does the O2 have?

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1993 300E, 2.8 M104 .....

A lot.

They are warrantied for 75K miles from the factory because they are vital to keeping emissions down.

A symptom of a bad O2 sensor is excessive fuel consumption.

The O2 sensors tell the engine management if it's getting a good fuel mixture. The default output is to dump fuel.

If you have a bad or dying sensor the car will never run right.

O2 sensors are easy to replace and they should not be overlooked. They will ruin a catalytic converter like somebody has said.

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1993 W124 300D
-297K on the clock as I type this.

Hey Latief - I think the only way to test the O2 sensor is with a scope or a voltmeter. The sensor is supposed to put out a range of voltages depending on the O2 concentration in the exhaust stream. Appareantly a bad sensor only fluctuates slightly or not at all. There is some info on the forum here about it - try a search. I thought I had the link marked but I can't find it.

Or you can just replace the sensor with the generic Bosch four-wire new sensor for about $60. (the factory sensor is about $130.) The right one to use (four-wire) is out of like a '94 Dodge Caravan. I have the number at the house for the right one and can get it for you if you want it so let me know. There is a ton of info on this topic on the forum here. You have to cut off the connector from the wire on the Dodge sensor and also cut the wire from the car to the original sensor and then splice in the wire to the new sensor because the connectors don't match. No big deal though; it's about an hour job total. The splice goes under the carpet in the front on the passenger's side, up against the seat base. Once you get the carpet out you'll see it; it's very straightforward. I soldered the splices in mine but you don't have to do that. The hardest part of the whole job was feeding the wire coming from the new sensor through the weather seal grommet located in the trans tunnel and then reforming the grommet back into a seal.

My '93 124 has a CEL but that might be because it is a Kalifornia kar.

So you could spend the $90 on a diagnosis or $60 on a new sensor and you know that is done. But, that might not be the original problem at all...

If it truly is original, you have probably spent more money in lost gas mpg than the cost of a new one!!

Get it changed asap! It should be changed every 60k miles.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k

I think it is original or at least very old from the way it looks. I bought the car in august, so not sure what the PO did .........

so will it cause the mixture to be off (i.e the exhaust smell)? and will it reduce performance enough to be noticed?

thanks for the info guys....

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1993 300E, 2.8 M104 .....

You can disconnect the O2 sensor and note any difference in engine performance.
But if your smelling fuel, the cat is likely the culprit.

Here are some other things that cause the exhaust to smell weird due to too much fuel. One is a knackered fuel pressure regulator. If the rubber diaphragm inside the regulator is leaking by/shot, raw fuel will be pulled into the the intake manifold via the vacuum line. So pull the vacuum connection off the regulator and see if fuel comes out. Then pull a suction on the nipple of the regulator using your Mityvac and see if fuel comes out. If so, you found the problem and the regulator is toast. You can get a Mityvac free using the free loaner tool program at AutoBone (AutoZone).

I had this happen on my Ford Exploder and it filled the crankcase and cylinders with fuel to the extent it hydrolocked the motor with a big BANG when I tried to start it. The dipstick was WAY up past the full mark - I bet I had two gallons of gas in the crankcase.

Next, get under the car and pull the vac hose off the trans modulator and see if there is trans fluid. If so it is getting sucked into the motor.

Last, same goes for the vac line off the brake booster - see if there is brake fluid in there. Let us know what you find.

I'm putting my timing cover back on tomorrow morning. Cover me, I'm going in...

Pete

Elsinore,

These are excellent points, I will look into this further....I have a feeling it is either burning oil or coolant. There is Zero smoke coming out of the tail pipe at any time (except for vapor when the weather is very cold). the smell seems stronger at first start which really worries me, but again no smoke. I just finished the head gasket in late august so i am hoping i did not screw something up.....there is some oil in the intake being sucked up by the cross over tube (that is supposed to be normal i guess on these engines as it has no PCV valve).
Other symptoms i have are terrible gas millage, and a high rpm at first crank that settles within a second or two.....i do feel it is a bit slugish, but i don't have another 300e as a reference point ....

as for the tranny vacuum line, can i inspect it at the top where it enters the manifold, or does it have to be at the tranny?

I pulled the vac line at the fuel pressure regulator and it seems dry....

I am open for ideas.... !!!

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1993 300E, 2.8 M104 .....

You can test the O2 fairly easily with a digital volt meter...find the splice to the O2 power wire (usually black) under the passenger seat carpet and disconnect the two halves. Then stick a small shunt into the female half of the connector (short piece of light guage bare wire). Then reconnect the two parts with the shunt pinched in place.

Using an alligator clip from the DVM, connect the positive clip to the shunt (red wire), and the negative clip to a ground connection (seat bolt head). Using the < 2V scale take the DVM readings after warming up the engine...take the readings at or around 2500 rpms. If the O2 is good, the values will cycle up and down across 0.5V. If they are fixed at a value above or below that, then the O2 is likely no longer functional. If you do not connect the meter in parrellel, then the reading will be fixed at around 0.42V (DVM hooked up wrong in series...don't ask me how I know)

You can also remove the O2 and bench test it if you have a propane torch and a DVM plus bench vice...Heat the bulb until it is incandescant, then read the values between the bulb (black probe) and the other half of the O2 (red probe)...Do this test heating continuously for around 3 minutes...the DVM value should never drop below 0.9V..if it drops to .89V even then your O2 is bad...Remember to continue heating the whole time of course.

If you changed the head gasket due to burning coolant, then the O2s and the cat have both had it- The materials they are made of are poisoned by ethylene glycol.

excellent point

actually, i changed the head-gasket because it was leaking oil down onto the exhaust manifold and into the coolant in the typical mercedes fashion......no leaking coolant into the cylinders.......

thanks for the feedback, I will look more into this

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1993 300E, 2.8 M104 .....