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The oxygen sensor is 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 level (14.6:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensor is located in the exhaust system of the engine, and senses the oxygen content of the exhaust gases. 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 light 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. On most BMWs, the engine's computer will give you a warning signal that lights up the Check Engine Lamp if the signal received by the computer is out of it's normal range. For more info on reading these fuel injection codes, see the the Pelican Technical Article: Reading Fuel Injection Fault Codes.
If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running really lean, 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 article's scope. If you think that the oxygen sensor may be causing some of your fuel injection problems, it should be replaced. In general, I recommend that you do this every 30,000 miles. The oxygen sensor is located on exhaust manifold and/or exhaust pipes that lead to the muffler. It's location changed and varied from year to year, but for the most part, it is easy to reach and remove. Figure 1 shows an O2 sensor from and E36 3-Series BMW. You will typically have to jack up the car to gain access to the sensor. See the Pelican Technical Article: Jacking Up for more details.
Using a 22mm wrench, simply remove the sensor from the exhaust pipe. On the E36 318, the sensor is located in a very difficult to reach spot - you may have to use a special deep socket with a slit cut in the side to remove it (Figure 2). Figure 3 shows the electrical plug for the O2 sensor - simply rotate the plastic retainer counter-clockwise and the plug should come right off. New O2 sensors should have the same exact plug - ready to attach to your car. On the E36 3-Series, the plug is located towards the right side of the rear of the transmission. Figure 4 shows the plug being removed.
When you remove the O2 sensor, you will probably find it to be black with soot, as shown in Figure 5. This is normal for an old, worn out O2 sensor. Figure 6 shows the mounting "bung" for the O2 sensor. Install the sensor into the hole, making sure that it is snug tight. 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. Check the sensor first though, as new ones sometimes come with anti-seize already on the threads.
I have found that often the new O2 sensors come with the correct plug, but the cable is way too long. If this is the case, then secure and tie off the cable, as is shown in Figure 7. Make sure that the cable is not located anywhere near any exhaust components - you don't want the heat melting the cable to the O2 sensor.
Well, there you have it - it's really not too difficult at all. If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs. If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one. Your continued support directly affects the expansion and existence of this site and technical articles like this one. As always, if you have any questions or comments about this helpful article, please drop us a line.