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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
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
- 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
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 an 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
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
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