Matt Marks

Hi all,

I just had the front oxygen sensors replaced on my 2.5L 1999. They had failed with p0131 and P0151 errors (low voltage at both right and left banks) after 4 or 5 years. I'm hoping the replacements help me to scavenge some added power as the DME will now be able to use the O2 inputs to tune emissions/power in "closed circuit". As running the circuit with the failed sensors (to my understanding) causes the DME to go into open loop and go fat rich using an A/F ratio coded into the DME to protect the motor.

On my previous Porsche (944), at WOT the car goes by design into open loop and ignores the 02 sensors, instead using a pre-programmed value for A/F from the DME.

Any idea if the boxster behaves in the same way as the 944? or does it always use closed loop/the full range of the O2 sensor regardless of throttle position? (unless of course the sensor(s) fail per the above)

Thanks in advance.

mikefocke

I snagged this years ago...

Brett posted on PPBB:

The O2 sensors do just that, sense the amount of O2 in the exhaust gas relative to the amount of O2 in ambient air. Perfect combustion of a perfect mixture of air and fuel (around 14.7/1 air/fuel ratio) leaves behind only CO2 and water as products of combustion. All the oxygen gets consumed in the combustion and combines with all the carbons and hydrogens. If there is not enough fuel (lean mixture), then all the fuel gets burned leaving some oxygen left over. Conversely, if there is too much fuel (rich mixture), then all the oxygen gets burned leaving behind extra hydrocarbons (fuel). Now an oxygen sensor outputs a voltage between 0 and about 1 V depending on the difference between the amount of oxygen in the exhaust and the amount of oxygen in normal air. If there is a lot of oxygen in the exhaust (lean mixture condition), the sensor outputs close to 0 volts. Conversely, if there is no oxygen in the mixture (rich condition), then the output is close to 1 V. These O2 sensor voltages are read by the computer. This is the feedback loop that tells the computer how the engine is performing with regard to air/fuel mixture. It's impossible for the computer to hold the exact perfect air/fuel mixture constantly, so the way mixture control is designed is for the computer to continually adjust the mixture from very slightly rich to very slightly lean and back again using feedback from the pre-cat O2 sensors. This means that the pre-cat O2 sensor signal will oscillate back and forth from high to low to high to low voltage as the computer adjusts the mixture. In a normal running engine at idle the signal goes from low to high voltage and vice versa about every 1 second, with a transit time from low to high (or vice versa) being about 200-300 milliseconds. This transit time is important because as an O2 sensor ages, the transit time gets longer, and eventually it can get too long such that the computer will call it a malfunction and signal a check engine light and fault code for a slow responding O2 sensor. O2 sensors need to respond to mixture changes quickly so that the computer can keep up with the proper mixture adjustments.

So the bottom line is that the pre-cat O2 sensors should oscillate between about 0.2 to 0.8 volts regularly (about every 1 second at idle) in a healthy engine.

The post-cat O2 sensors are identical to the pre-cat O2 sensors (same voltage outputs). They are there only to monitor the performance of the catalytic converters. So, as discussed, the pre-cat sensor signals are oscillating between 0.2-0.8 volts. Once the exhaust gasses pass through the catalytic converter, most (all, in theory) excess fuel (hydrocarbons) will be combusted thus reducing hydrocarbon emissions. The cat uses oxygen in the exhaust to combust the fuel. So what you end up with in the exhaust after passing through the cat is a gas mixture that is reduced in hydrocarbons and reduced in oxygen relative to the mixture entering the cat. The post-cat exhaust gas mixture should be CONSTANTLY low in oxygen if the cat is doing its job of burning excess fuel. Therefore, the post-cat O2 sensor signal should be a constant lower voltage reading (not oscillating). So, if the post-cat O2 sensor is seen to oscillate just like the pre-cat O2 sensor, that means that the post-cat sensor is seeing the same gas mixture as the pre-cat sensor meaning that the catalytic converter isn't doing its job of burning excess fuel. The computer monitors the post-cat sensor and compares it to the pre-cat sensor. If the signals are similar, it assumes the cat is bad.

Matt Marks

Thanks Mike - great post!

Do you have any additional information on the fuel mappings? This is a race car so secondary O2 sensors are deleted. But I'm trying to determine if at some point in the throttle mapping (WFO?) does it go open loop and use a pre-saved A/F map from the DME, or are the O2 sensors "in the loops" and adjusting mixture 100% of the time.

My theory is that my car is significantly down on power on the dyno based on pulls from March. but just found the O2 faults the other day. I've replaced the sensors and am hoping for a pickup in at least mid-throttle drivability and power/TQ, but am curious if they will help at full at full throttle when the DME may or may not go to a preset "safe" A/F mixture.

Will try to find a dyno this weekend and see if there is a measurable change - but curious in the meantime.

Thanks!

Matt

mikefocke

What I posted is all I've ever read. Hopefully someone else can help with your specific question. Perhaps one of the folks on one of the racer sub-forums.