I'm in the process of installing my PLX R500 Wideband Lamda meter etc. I've replaced the stock O2 sensor with the supplied wideband unit and I am going to use the R500 narrowband output to feed the DME with a valid O2 signal for closed loop.

Question: The DME wire for the O2signal is thicker than most of the other wires, and when i removed the insulation i noticed that there was another smaller cable in the big one. Should I remove the insulation on the inner cable as well and tap into there to connect to the signal wire, is the outer part just a shielding against noise?

Question: The PLX manual states that you should install a load resistor on the disconnected narrowband wiring heater cirquit, do our cars even use a heater cirquit for the O2 sensor? The sensors 3 wires indicate that it is being heated, but if you look at the DME I can't find any pins for that?

General info about replacing stock narrowband

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YOU CANNOTT INSTALL THIS SENSOR BETWEEN THE HEAD AND THE TURBOCHARGER.
Change it if you have done so because it will provide you with useless wideband info, and totally screw up the motronic.

Also - the narrowband is better at 0-1 volt supply, it is faster.

You should use both sensors.

The outer is only shielding.

 

Can you explain this in further detail?

 

Wideband sensors work in a specifc heat range -- it won't work correctly and will get damaged if you put it too close to the exhaust ports. It needs to go after the turbo -- there's a picture of where I installed mine on my site linked below.

 

Plus the pressure before the turbo is too high for the sensor, makes jibberish of the output voltages. Additionally, you need to run the heater on the O2 sensor, wide or narrow. The heater is necessary to burn off contaminates.

Tool has it right. I also would not use the output of the wideband logger. These units are designed for logging, as a gauge, not as a replacement. PLX or WBO2, both programmers will tell you to stick with the stock narrowband sensor if using the DME.

The interpolation between data points in the sensor data table (coarse maps are provided with both units) and the software (Simulated Narrowband "NBsim") that turns the signal from 0-5V to 0-1V is just not up to par or compatable with Motronic systems, in my opinion. Use a 0-1V sensor and skip the processing.

 

Both the NB and the WB use the same technology. The difference is that the WB adjusts the sensor to see were the lambda =1, then measures the offset and reports the correct AFR reading. Once it sees how much it had to shift the sensor, it now can calculate the correct AFR. The problem in using the WB as a NB is that the WB is slower and has a limited operational range for pressure and temperature.

Bottom line, the NB is much better in setting the closed loop AFR to 14.1/1 (needed for the CAT to operate correctly) then a WB.

 

Ok, thank you guys, looks like I will have to get another bung welded in there.

Do I get any faster response from the WB if I put it as close as possible to the turbo outlet, or could I just as well put it a little further down the pipe?

 

How can the pressure be too high, or the temperature? Porsche designed the car to have the stock oxygen sensor in place on the uppipe to the turbo. Clearly there is not too much pressure or temperature for a NB oxygen sensor to determine oxygen content for the DME. Can anyone explain why then Porsche placed the NB oxygen sensor here and how it can work effectively if the pressure and temperature are too high?

Also, is it even necessary to use any oxygen sensor in these cars if you no longer have a cat? I still have an oxygen sensor in place, however from the research I have done it seems like if you just unplug it the DME will function as though it does in Europe without a cat or oxygen sensor. The oxygen sensor is only in place to take fuel away in the DME programming correct? If the car is running too lean it will, not add fuel, the DME will only take fuel away if the oxygen conent is too little in the exhaust stream.

Thanks in advance.

 

Also, how important is the oxygen sensor input in the operation of the Vitesse software? Is it just to assist in emissions like the stock operation or is it used as a crucial input in determining fuel mixture?

 

Having that narrowband on your 951 is a LUXURY for both tuning AND emmisions. We are lucky in the US that Porsche supplied it.

Also - you guys will apprreciate.... My 951 is up for the strict Connecticut emmisions test soon. I am currently making my second SMT6 map into the emmisions map. I want to see if I can beat a new Honda Civic WITHOUT my catylist. We'll see - its a challenge.

 

Chris -

I think the pressure issue is more of a problem with the WB sensors because of the cell design they use. From www.wbo2.com:

How 5 Wire Pump Cell Sensors Work

The Bosch LSU sensor requires a controller because it is more complex than a standard switching type sensor. It can be thought of as being made up of a heated narrow band oxygen sensor (comprising the Reference cell & Nernst Cell in the image) coupled to a pump cell in contact with a small chamber with a diffusion gap to the outside (exhaust gas). The electronics (in the WB unit case) are represented by the yellow (op-amp) and green (resistor) symbols. The actual electronics is much more complex and not shown is the feedback loop that maintains the heater to a precise temperature.

The pump cell, in conjunction with a catalytic reaction at the surface of the cell's electrodes, can either consume oxygen or consume hydrocarbon fuel in the pump cell cavity, depending on the direction of the Ip current flow.

During normal sensor operation, a small sample of the exhaust gas passes through the diffusion gap into the pump cell. That exhaust gas is either rich or lean and both conditions are sensed by the reference cell which produces a voltage Vs above or below the Vref signal (this voltage has the characteristics of a narrow band switching type sensor as shown in the image below left).

A rich exhaust will produce a high Vs voltage and the electronics produces a pump current Ip in one direction to consume the free fuel. A lean exhaust produces a low Vs and the electronics sends the pump current in the opposite direction to consume free oxygen.

When the free oxygen or free fuel has been neutralised, the Vs feedback signal goes to about 450 mVolts (the same as the Vref value). The pump current (which is a measure of the number of electrons used in the chemical reaction) required to produce this equilibrium is a measure of the Lambda or Air Fuel Ratio. The electronics in the WB unit converts the Ip into a number of signals including a Linear Voltage which is the output of the WB unit. Not shown is the Rcal, or calibration resistor, in the sensor's connector which compensates for manufacturing variations between sensors.



Perhaps the OEM style NB sensor was designed to work in an envoronment under pressure?

As for placement, I'm guessing that the headers and crossover probably hold abour 3-4 liters of air. 2 revolutions of the motor should push this air to the sensor, so a very minimal delay on that end. I think that the delay is more an issue with the sensor itself.

 

No, the O2 sensor and DME work together to keep part throttle mixture near 14.7:1. It is constantly adjusting (up and down) the duty cycle as needed (within limits) to keep it as close to 14.7 as possible -- no static map can account for all environments, so the O2 system helps keep the motor smooth. The O2 system makes the car drive smoother around town (under all part throttle driving conditions), and it saves gas, and it reduces emmssions. With or without a catalytic converter, the O2 system is nothing but good. As soon as someone comes up with a similar closed-loop system for WOT, we'll all want one.

 

The NB has a higher preasure/temperature operating range then a WB, basicly it has fewer parts so it is more robust. Just look at the data sheets to prove it for yourself. The entire operation of the O2 sensor and the cat is to dither the AFR around 14.7/1 ratio. This allows the Cat to oxide and then reduce the exhaust gas. It has to enrich and lean the AFR to do this.

 

Thanks!