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In this tech article, we will discuss replacement of the oxygen sensor in 1984-91 E30 3 Series BMW cars. While this article displays replacement of the sensor on my 325is, it should be noted that this article applies to most other fuel injected BMW models as well.
Why replace your oxygen (or O2) sensor? It is an essential part of maintenance on your BMW. The O2 sensor is used to monitor the fuel-air mixture of the car and send vital information to the cars on-board computer (OBC), which in turn adjusts the mixture and timing of the car. It does this by measuring the oxygen content of the exhaust. Over time, O2 sensors wear out, due to heat and carbon buildup from the exhaust. When the O2 sensor begins to wear out, you may notice the car runs a bit sluggish, fuel economy may be down, and in sometimes, it may cause an annoying intermittent surging idle problem, (as was the case on my car). Once I replaced the O2 sensor, I noticed a increase in gas mileage as well as a mid-range increase in power and drivability.
BMW recommends that the O2 sensor be replaced every 60,000 miles. This article is written with the home mechanic in mind. With a few tools and the correct parts, you will be able to replace the sensor yourself.
The first step of this article is to disconnect the battery. You will want to do this anytime you are working with electrical components. Plugging an electrical component into the electrical system with the battery connected can cause a voltage spike. Voltage spikes can destroy your OBC. That said, lets begin.
Open the hood and look down on the passenger side of the engine, you will see the exhaust manifold. Now, if you follow this down a bit, you will see a small sensor threaded into the side of the manifold. This is what we are replacing (Figure 1).
You will want to locate the electrical connector that the O2 sensor plugs into. On my 325is, this is located under the panel shown in the photo, right behind the front passenger side shock tower. Look at the top near the shock mount. You will notice a small plastic clip, holding the connector under the panel (Figure 2). You will want to rotate this clip 90 degrees, so the pin on the side lines up with the indexed hole, then use a screwdriver to carefully push the sides of the clip in. Once you have the sides of the clip pushed in, it should simply drop through the panel (Figure 3). With the connector free, you will want to remove the wire harness from the two clips shown in this picture (Figure 4)
Next, you will want to remove the small heat shield around the O2 sensor, this simply pulls off (Figure 7 and Figure 8). Remove the wiring harness from the clip as well. Now you will see the nut around the O2 sensor.
With the engine cold, spray a good penetrant spray (such as WD40 or 3 in 1 oil) around the O2 sensor where it plugs into the manifold. This will prevent the sensor from stripping out the threads when removing it. Let the penetrant soak for a few hours before continuing. Now is a good time to grab a beer or fruit juice, depending on your taste, of course.
Once the spray has worked itself into the threads, it is time to remove it. There are some tools on the market such as this O2 sensor removal socket that will assist in the removal of the sensor, however I found there was not enough clearance to use it (Figure 9). I recommend using the O2 sensor removal socket whenever possible. In this instance, I used a large pair of vice-grips around the nut to loosen the sensor. Simply reach down into the engine bay, use the vice-grips around the sensor and give it a few strong tugs. It should loosen up. Be cautious to avoid stripping out the threads (Figure 10).
When the sensor is loose, remove the vice-grips, and thread out the sensor by hand. Once I removed my the old sensor, I noticed that there was some carbon buildup on the sensor face, indicating that the car had been running rich for a while (Figure 11). The new sensor should correct this problem.
Now that the old sensor has been removed, its a good idea to spray a little bit of the penetrant spray on the inside of the threads inside the exhaust manifold. This will clean up any rust and soot that may have built up. Usually a quick little spray and then a few wipes with a rag will suffice.
Look at the new sensor (Figure 12). You will see a protective plastic cap over the tip that fits into the exhaust manifold. This is the part that actually senses oxygen in the system. It is crucial that you keep this tip clean. You will also notice a bit of anti-seize compound on the side of the sensors threads. DO NOT allow any of this to get on the tip. It will damage the sensor (Figure 13 - shown with plastic protective cap).
Now remove the protective cap, and carefully thread it into the manifold, making sure not to get any of the anti-seize compound on the tip. Hand tighten the sensor until it stops. Use the vice-grips around the outside nut to tighten it. BMW recommends that the O2 sensor be torqued to 55ft/lbs., however since I cant fit a torque wrench on there, I got it as tight as I possibly could when cold, then re-tightened the sensor with the engine and exhaust hot. This should secure the sensor in place.
With the new sensor installed and tightened, place the protective heat shield back around the sensor, and clip the wire in place. Next, plug the sensors plug back into the connector above, slide the threaded collar around the outside and hand tighten.
Once the sensor is plugged into the connector, line up the securing clip under the panel behind the shock mount. Line up the pin, then rotate it 90 degrees for the boss to slide through the panel. Once it is through the panel, rotate the clip 90 degrees to lock it in place.
And thats it, youre done!
If anyone has any questions regarding installation feel free to email me at The Pelican Parts Message Center
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