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Apart from a laptop computer, a wideband oxygen (commonly referred to as an O2 or lambda) sensor is the most important tool in a tuner’s arsenal. Unfortunately, it is also one of the most overlooked tools in the amateur tuning community. Tuning a modern vehicle without a wideband oxygen sensor is impossible regardless of what anybody says. Historically, these measurement devices were only utilized by OEMs since they cost several thousands of dollars. However, with the improvement in technology, they have reduced in cost such that any enthusiast can afford them.

Purpose
The main purpose of the oxygen sensor is to inform the DME what the air/fuel ratio is in the exhaust stream. In older cars with only a single pre-catalyst oxygen sensor, this is its only purpose. In newer cars with sensors before and after the catalytic converters, they also serve as a diagnostic tool for the catalyst efficiency. The DME uses the signals from the pre-catalyst sensors to make small adjustments to the opening time of the injectors such that all of the injected fuel is completely combusted. This is commonly referred to as ‘closed-loop’ control since the output of the oxygen sensor is used as an input to the fuel injectors. The engine actually operates in this manner during the majority of the time. The only time it typically deviates from this strategy is during startup and full throttle events. The DME then relies on absolute fuel tables to directly guide the injector duration.

How it works
Before discussing the installation of a new wideband oxygen sensor, it is important to understand how the sensor itself works, and more importantly, how the DME uses the signal coming from this sensor to control the amount of fuel going into the engine. Most engines today have what is called a two-step oxygen sensor. In the tuning industry, this is commonly referred to as a narrowband oxygen sensor. These sensors are constructed in such a way that they can compare the residual oxygen content in the exhaust stream against the oxygen content inside the sensor body itself which is located outside of the exhaust. These sensors generate a voltage differential when the oxygen content is different on both sides of the measuring element. When the exhaust gases are considered rich (too much fuel, lambda < 1), the sensor generates a voltage of approximately 1 volt. In contrast, when the gases are considered lean (too much air, lambda > 1), the sensor generates a voltage near 0 volts. There is actually a very narrow window centered around the perfect air/fuel ratio where the sensor outputs approximately 0.5 volts (lambda = 1). This region is known as stoichiometric combustion where all of the fuel is completely combusted.



As seen in the image above, the sensor voltage changes quite drastically about the stoichiometric point. This is what enables the DME to easily and quickly make small changes to the amount of injected fuel during normal driving. The voltage actually transitions above and below 0.5v continuously during closed-loop control. The normal operating window of this transition is approximately between lambda values of 0.99 and 1.01.

When speaking in terms of lambda, it is always important to remember that lambda is equal to the current air/fuel ratio divided by the stoichiometric air/fuel ratio. This makes it easy to talk about stoichiometric combustion for any type of fuel. For pure gasoline, the stoichiometric air/fuel ratio is approximately 14.68:1 which means it takes 14.68 kg of air to fully combust 1 kg of fuel. To transform lambda into air/fuel ratio, simply multiply the lambda value by 14.68. Therefore, a lambda range of 0.99 to 1.01 translates into an air/fuel ratio of 14.5:1 to 14.8:1. This is where the term “narrowband” comes from when talking about oxygen sensors. When looking at the narrowband sensor output above, notice how flat the voltage response is outside of this narrow window. What this means physically is that it is impossible for the DME to accurately measure air/fuel ratios outside of this window. This is where the purpose of the wideband oxygen sensor should become apparent because it is necessary for the engine to operate much richer than 14:1 during full throttle, especially if the engine has forced induction!

Difference between narrowband and wideband
A wideband sensor is constructed slightly differently than a narrowband sensor such that it can measure air/fuel ratios far richer and leaner than stoichiometric combustion. This is primarily due to the sensor response curve which is much flatter, making it easier to interpret. However, the special construction requires a precise controller which is why wideband systems are more expensive than narrowband systems. The sensors themselves are quite comparable in price (~$50) but the electronics required to control the wideband are the expensive part. This is where it is a good idea to invest in a quality wideband system because the controllers vary between manufacturers. Purchase (or rent from your dyno operator) the best one you can afford because you’ll regret it later if you destroy your engine due to a cheap controller.



Exhaust Temperature and Pressure
The output pump current from wideband sensors can vary depending on several other factors other than the air/fuel ratio. These include primarily the exhaust gas temperature and pressure. Most wideband sensors operate most efficiently between temperatures of 600°C and 900°C. Because of this, most oxygen sensors (including narrowband) have a heater element built inside of them to facilitate a short heating time after the engine has been started. This is one reason why the DME operates in open-loop during the moments just after starting the engine. The sensor needs these elevated temperatures to have a good response time to the DME. Temperatures below 300°C can take seconds to respond whereas temperatures near 600°C take less than 50ms. After the sensor has reached its operating temperature, monitoring the actual temperature is very important (especially under rich conditions) because the output signal must be scaled appropriately. This is where cheaper systems usually falter and give sporadic readings.

The other important factor that cheaper wideband systems usually ignore is the effect from exhaust backpressure. Most wideband sensors are calibrated to operate near atmospheric pressure. This is why it is extremely important to install the wideband sensor after the turbo on forced induction applications. A high exhaust pressure will make rich condition appear richer and lean conditions appear leaner than actual. There is no effect on the reading at stoichiometric conditions. Some more expensive systems actually employ a separate pressure sensor that is installed in parallel with the oxygen sensor to obtain very accurate results. However, for most enthusiast applications, this is overkill as long as the sensor is installed appropriately in the exhaust stream.

Installation
After choosing a wideband controller and oxygen sensor, the final step is installing it into the exhaust. Careful planning is also necessary during this step to yield good results while using the wideband sensor.

The first step is choosing a location along the exhaust to install the sensor. As mentioned previously, oxygen sensors operate best between 600°C and 900°C. According to the documentation of the Bosch LSU 4.2 sensor (attached below), the maximum operating temperature of this sensor is 950°C. Therefore, it is desirable to have it fairly close to the engine, but not directly after the exhaust valves. Having it closer to the engine also reduces the latency in reporting values. This is especially important for data logging and trying to match lambda readings with engine speed during full throttle runs. If the oxygen sensor is located near the muffler, the reported lambda could be delayed drastically compared to when the engine actually experienced the measured event. For naturally aspirated and supercharged E36 cars, near the stock narrowband oxygen sensor is a good location. For cars with turbochargers, the wideband sensor should be placed AFTER the turbine somewhere in the downpipe.

After choosing the location, the next consideration should be put on the angle of the sensor within the exhaust. This is important because of the condensation that builds up in the exhaust shortly after starting the engine. If this condensation touches the pre-heated oxygen sensor, it could cause thermal shock which can damage the sensor element. Bosch recommends installing the sensor at an angle greater than or equal to 10° with respect to the horizon as shown in the image below.



Another important factor when installing the oxygen sensor (especially in setups with two exhaust manifolds) is making sure the sensor output represents what the whole engine is experiencing. In other words, unless you have two separate sensors, it is best that you place the sensor after the two exhaust manifolds converge. Otherwise, the lambda readings will only represent one bank of the engine. On older cars (OBD1), this isn’t much of an issue because the stock DME doesn’t have control over individual cylinder fueling. Regardless, it is generally better to place the sensor in the collective exhaust stream.

A final thing to consider is exhaust leaks. Any type of leak before the sensor will cause it to produce false readings. If excess air passes by the sensor, it will generate a lean signal because of the increase in fresh oxygen which will then cause the engine to inject more fuel. Care must be taken while welding the bung into the exhaust tubing to ensure that there are no leaks. Don’t forget the exhaust manifold gaskets either as this could be a source of a small exhaust leak upstream of the sensor. In addition to leaks, this problem can also happen on aggressive cam timing where there is a large overlap between the intake and exhaust valves. During idle and low load conditions, fresh air and fuel can enter through the intake valve and escape from the cylinder before the exhaust valve closes. This will have the same effect on the lambda reading and should be anticipated if the cam timing is aggressive.

Below are some images from an actual wideband installation on our test mule which is a '95 E36 M3. Although it isn't necessary to completely remove the exhaust, it makes it much easier to weld the sensor bung. This ensures that there are no leaks around the weld. However, as you'll see in later pictures, this can lead to some other difficulties when removing fasteners that are this old!



Notice the location of the stock narrowband oxygen sensor here. If keeping the stock exhaust, we recommend installing your wideband in a similar location.





It makes it much easier to get the drill started if you use a hammer and metal punch such as this.



Be sure to use a metal hole saw that has a diameter similar to your exhaust bung. You'll see that we chose one that is slightly larger than the bung such that it will actually sit inside the exhaust tubing a bit.













Here is a test fit of the exhaust bung. This is exactly what you should try to achieve.



We then removed it to file away the paint that rubbed off of the hole saw. Anything such as this or surface rust should be removed to help the weld stay clean.



Yes, even a MIG welder can be used if you're careful! However, if you or your exhaust shop has access to a TIG welder, then that should be used as it is much easier to control the bead and will create a cleaner weld. Regardless, you'll see that a MIG welder can perform the job just fine. We left the threaded plug in the bung while welding to ensure that the bung did not deform from the heat. Be sure to put some high temp anti-seize on the threads just in case. Also, be sure to not accidentally weld the plug to the bung!







And this is what happens when trying to remove exhaust manifold fasteners that are 18 years old! Although we didn't try it prior to removing them, some people have luck by spraying the nuts liberally with PB Blaster or a similar penetrating lubricant. If this happens, don't fret too much as it's still possible to remove them; it just takes time and A LOT of patience! You'll notice that we've already successfully removed one of the sheared studs in the rear manifold.





Here is one of the studs that we removed after about an hour. You'll notice that the end which was protruding is now marred beyond belief, but what we actually did was make the cross section oblong-shaped such that the vice grips could get a good hold of it. In hindsight, it would have been much easier to use a small grinder to do this instead of repeated slipping of the vice grips and channel lock pliers!



Here are the tools we used to remove both studs. Instead of using standard propane, go down to your local hardware store and pick up a bottle of MAP gas in the yellow cans. It's usually right next to the propane. The reason for this is because MAP gas burns at a much higher temperature than propane which allows for more efficient heating of the manifold. We also found that holding a small candle to the backside of the heated stud helped melt wax which wicked down into the threads a little. Not pictured here is a can of PB Blaster which we also used to soak the studs for a few days prior to attacking them. We also found another trick which helped: tightening the sheared studs into the manifold a quarter turn or so. Believe it or not, this actually helped get them started after continuously trying to remove them. Once you get the studs to turn a little, work some more candle wax and PB Blaster on the back side. They should be fairly easy to remove from the manifold then.

You may be curious as to the purpose of the hammer. We're not sure if it helped with the broken studs, but it definitely helped relieve stress! Feel free to give them a good blow from time to time as you can't damage them anymore than they already are. Don't worry about the exhaust manifolds either as they are built strong enough to withstand the apocalypse.



Here is the new hardware that will be going back on after we get that other stud removed. It's always a good idea to use new hardware when reinstalling the exhaust. Don't forget to put some high temperature anti-seize on the threads which will protrude from the manifold.



Here you can see the torch being used to heat the manifold. Try to keep the flame concentrated on the manifold and not the stud.



And voila! The second one was a little easier to remove than the first.







This actually makes it a bit easier to install the new studs. Take a similar sized nut and tighten the copper one against it. This will give you something to hold onto while installing it into the manifold. After the stud is fully seated, remove both nuts.





Here you can see how the wideband oxygen sensor fits in the exhaust. We've secured the wires to those from the narrowband to keep the driveshaft and road debris from interfering with them. The wires from the wideband are then routed up the transmission tunnel toward the front of the car on the driver's side. The idea is to keep them (and the wideband controller) as from from the hot exhaust as possible.



Here you can see the wideband controller securely fastened next to the transmission. We found some existing wires and connectors which provided a good attachment point. Although now outdated, we chose to use the LC-1 wideband from Innovate Motorsports because it had good reviews and an independent study by FordMuscle.com showed that it was the best performing wideband available for enthusiasts. Click the previous link or see the attachment below which details the results.

it is possible to read the WIDE by the ECU? I can do closed loop with readings WIDE ??

MayconBarbieri wrote:it is possible to read the WIDE by the ECU? I can do closed loop with readings WIDE ??

The ECU can only read the signal from a narrowband sensor. We are using the wideband sensor to display the A/F ratio to the driver. It might be possible to replace the narrowband with a wideband only if the wideband has the option of simulating a narrowband signal. However, we've never tried this and have installed both sensors in the exhaust.