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The oxygen sensors (also called O2 sensors) are one of the most important elements of the modern fuel injection systems. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric ratio (14.7:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensors are located in the exhaust system of the engine, and they sense the oxygen content of the exhaust gases. There are two on this Mercedes Benz M104 engine: one on each side of the catalytic converter. The sensor located just in front of the catalytic converter measures the mixture of the exhaust gas exiting the engine. The sensor located after the catalytic converter is used to measure the performance of the converter by comparing the O2 levels before and after. The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.
There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are:
- Irregular idle during warm-up
- Irregular idle with warm engine
- Engine will not accelerate and backfires
- Poor engine performance
- Fuel consumption is high
- Driving performance is weak
- CO concentration at idle is too high or too low
- Check Engine Lamp is illuminated
In general, if the oxygen sensor is not working, the car will be running very poorly, and will also be outputting a lot of harmful emissions. The cars computer will usually give a warning signal that lights up the Check Engine Lamp if the signal received by the computer is out of its normal range. Sometimes the computer may output an error code stating that the oxygen sensor is reading out of range, when in reality the values registered by the O2 sensor are accurate because there is something else wrong with the fuel injection system. Prior to replacing the oxygen sensors, make sure there are no other codes being recorded that may affect the O2 sensor readings..
If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running lean (not enough fuel), and will try to richen the mixture. At the other extreme, if you disconnect the oxygen sensor, and replace it with a small AA battery that supplies 1.5 volts, the ECU will think that the car is running really rich and attempt to adjust the mixture to be leaner.
Needless to say, troubleshooting the complete fuel injection system is beyond this project's scope. If you think that the oxygen sensors may be causing some of your fuel injection problems, they should be replaced. In general, I recommend that you do this as needed. You will have to jack up the car and secure it safely on jackstands to gain access to the sensors.
Using a 22mm wrench, simply remove the rear sensor from the exhaust pipe. On the front oxygen sensor, you'll need to get a little creative. The front sensor is mounted in a really tight space and at a odd angle. To loosen it, you'll need to use a special type of crow's-foot wrench with a slit cut in the side to remove it. The electrical plug for both O2 sensors unplug from the chassis plug. New O2 sensors should have the same exact plug - ready to attach to your car. When you remove the O2 sensor, you will probably find that it is coated with black soot. This is normal for an old, worn out O2 sensor and usually indicates a rich running condition.
Install your new sensor snug-tight, or if you have the proper slit-tool and a handy torque-wrench, then tighten it to 40 ft-lbs (55 N-m). It's also a smart idea to add some anti-seize compound to the threads of the plug before you install it, but make sure the anti-seize doesn't get into any of the slits on the head of the sensor. Check the sensor first though, as new ones usually come with a dab of anti-seize already on the threads.
There are two different types of sensors you can purchase: generic ones that allow you to snip the connector off of the old sensor and put it on the new one, and original OEM sensors with the correct connector. On older cars, I used to use the generic sensors, but I've had problems with using them on these newer cars. Researching further, I discovered that the wires and connectors are very important on these O2 sensors.
Soldering wires together can interrupt the reference air signal and lead to problems with the sensor. I've also had problems with the Bosch factory Posi-Lock connectors that are supposed to work with these newer sensors. Because the relationship between the connector and the O2 sensor is so vital to the proper reference signal, I recommend that you only use the correct OEM sensors with the proper plug.
Jack the car up and secure it on jackstands at all four factory jacking points. You'll need to get under the center of the car to gain access. Shown here is the rear oxygen sensor in place for a Mercedes-Benz with the M104 engine (green arrow).
Use a 22mm wrench to loosen and remove the rear oxygen sensor. Once removed, pull the cable free of the clips that hold it in place and unplug it from the electrical connector. Place the new sensor in the exhaust, torque it to 40 ft-lbs (55 N-m) and plug the connection in after routing the new cable along the clips holding it to the car.
Shown here is the top of the front mounted oxygen sensor (green arrow). As you can see, it's a tight fit in there. What makes it even more difficult is the angle at which the sensor is mounted. It faces into the exhaust stream right at the merged collector between both header banks. This angle will prevent you from fitting a wrench over the sensor.
This is the 22mm crowfoot wrench you'll need in order to loosen and remove the front mounted O2 sensor.
Slide the crow's-foot wrench over the sensor and use a ratchet with extension as shown here. It's tricky to get it at just the right angle that will allow you to loosen the sensor but it is possible. NOTE: if your transmission mount has failed, there will be almost no way to get the wrench on the sensor. Just something to keep in mind if it seems like you just can't reach the sensor. See our article on transmission mount replacement for more info. On our project car, the transmission mount had failed, causing the whole transmission to sit about 2 inches lower. Once the mount was replaced, it became evident how to get the crow's foot wrench over the sensor.
Here's a shot of the oxygen sensor being removed from the side. As you can see here, you'll need to reach the ratchet over the exhaust pipe and then fit the extension into the crow's-foot wrench. Once removed, take the new sensor and carefully thread it into the exhaust. Torque down the sensor to 40 ft-lbs (55 N-m) and plug the connection in after routing the new cable along the clips holding it to the car.