[Smhoney]

We have an 87 300 E 68,000 miles.
The o2 sensor light came on and it was causing the car to not be able to go over 20 miles an hour or so. So we bought a new one and replaced it about a month ago. Last night on my way home from work the o2 light came back on an the car slowed down and it's done it again!!!!
It can't be bad in just a month! Any tips on what else this could be??

My dad just bought me this car in August, it was an old mans that had past away, it has Ben sitting in a garage for years. The son cleaned it up and had it checked by a mechanic and then sold it to us. So we really know nothing about this car. Other then what is in the owner manual and receipts and stuff in the car!!

 

[stormtigers]

You've entered the TWILIGHT Zone....LOL but seriously...

It is commonly known as "LIMP HOME MODE" designed to allow you to keep on going rather than stopping, and get your car home or to a shop.

What is happening is with a failed O2 sensor the engine has no way of metering the fuel/air ratio properly and it goes into a protective mode (cause it don't know what the hell is going on). Are the leads and connections intact, make sure there is break in the leads from the sensor !
Oh and have a bit of look to see if you have any holes in the exhaust area BEFORE the O2 sensor as that could be stuffing the sensor readings also.

You don't mention other problems however I would also think about replacing the OVP relay (near the battery with 1 or 2 fuses on top - if it's a 1 fuse it's an oldie and should be replaced anyway).

Have a search on the forum about limp home mode as there is a variety of things could be causing your troubles not just the O2 sensor !

Good Luck

 

[Smhoney]

We haven't had any other problems with the car. It has Been great. Just this o2 sensor light!

And we have never owned a Mercedes before so this whole thing is new to us!

 

[stormtigers]

We haven't had any other problems with the car. It has Been great. Just this o2 sensor light!

And we have never owned a Mercedes before so this whole thing is new to us!

I haven't experienced it myself. Unfortunately with MB's amazing over engineering often you get deceptive indications and it's another part in the chain that is letting you down.

By the way when you changed the O2 ensor the first time what condition was it in ? Some of the posts I have read indicate that excessive oil consumption can lead to clogging of the sensor and when it was removed it was quite dark and covered in junk. I don't know if you did the change yourselves but if you did and yours was in a similar state then have a look at the new one you've just installed to ensure it hasn't failed for the same reason.

Apart from that I can only suggesting start reading, build up a list of suggested items to check. Some will require checking voltages etc.
Otherwise maybe someone can recommend a good MB indy in your area.
A good experienced MB indy could save you hours and hours of frustration !

 

[Sam Fuller]

Also look at battery voltage 12.+ with car off 13.2 to 14 when car is on. A lot of time electrical problems stem from simple things that screw bigger things up! If the car doesn't show 13.2 + while the car is running its probably a bad voltage regulator on the alternator. easy fix but messes with other things. +1 on the OVP after checking the voltage check the OVP its in the same area. Not knowing the history on the car I'd replace that just for kicks

 

[Savannahbenz]

Man does that sound familiar, I fought this problem for six months. did a new O2 sensor all tuneup parts, OVP. the problem turned out to be a failing ECM. Ideally, if you know someone you can borrow an ECM from, pop it in your car and see if the problem goes away. My indie used a computer from a 300SE to test the theory, then ordered the correct part.
Historically, the computers have been relatively trouble free, but now they're getting old, and probably starting to suffer from the degraded solder joint and component gremlins that have affected fuel pump relays and cruise control amps. Personally, I'm going to keep an eye out for a junkyard find spare computer to keep in reserve.

 

[stormtigers]

Yeah I would back the boys on those suggestions above !

Voltage is extremely important in the W124 so make sure your getting what you need through the system.

I hesitated at suggesting the ECM because it's such an expensive unit to replace. If you can get a cheap one from a wrecking yard the only hassle is knowing whether it was intact - bit of a cross fingers solution.

 

[Ivanerrol]

????

O2 sensor should have minimum effect on the car's running.
It's there to adjust the engines fuel mixture via the EHA.

The car should run fine without the O2 sensor or even the ECU in the circuit.

No limp home mode on a old 300e

There's something else wrong with this car. Check the Crank Position sensor.

Disconnect the EHA wire connector. The car should run fine.

 

[stormtigers]

????

O2 sensor should have minimum effect on the car's running.
It's there to adjust the engines fuel mixture via the EHA.

The car should run fine without the O2 sensor or even the ECU in the circuit.

No limp home mode on a old 300e

There's something else wrong with this car. Check the Crank Position sensor.

Disconnect the EHA wire connector. The car should run fine.

Ivan where you been mate ??

All the posts I have read from the US the 300E did have extra crapola on them for emission issues including the 87 model.
Perhaps the US boys can clarify ?

 

[Savannahbenz]

????

O2 sensor should have minimum effect on the car's running.
It's there to adjust the engines fuel mixture via the EHA.

The car should run fine without the O2 sensor or even the ECU in the circuit.

No limp home mode on a old 300e

There's something else wrong with this car. Check the Crank Position sensor.

Disconnect the EHA wire connector. The car should run fine.

North America market 87s do have limp home mode, (I limped home many times troubleshooting my car).They also have Fault Recognition circuitry in the ECM. which is a very primitive diagnostic tool. Faults with engine management components can be read using exactly the same process used to read mixture ratio with a duty cycle multimeter. The system does not store codes but will display them when the fault is occurring. For example, meter is attached to the x11 connector, car is running well, lamda reading is fluctuating between 45-50. suddenly the engine starts running like crap and the mixture reading drops to a steady 30%. A reading in the 30% range is indicative of a fault in the coolant temp sensor circuit. A steady reading in the 50% range would indicate a fault in the O2 sensor circuit. It works, but you gotta catch the fault codes when the engine is misbehaving. In my case, one time it would throw a code 30, another time it would be 50, later it might throw 70. when that happens, it's not likely that all the sensors suddenly went bad, it's a sign of a bad computer.
You are right, the car will run with the EHA disconnected,

 

[sbaert]

You are right, the car will run with the EHA disconnected,

The only thing that does is turn your Bosch CIS-E driven vehicle into a CIS driven vehicle.