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Credit to Brett 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.
Thanks. Added to Porsche Acronyms List. Model to model they change, location to location too. I only mention the MAF possibility because suppose the ECU has some built in parameters that say at certain revs it should do thus based on the inputs it is getting from the MAF and the O2 sensors. If the characteristics of a different MAF give the ECU different readings at a certain airflow, then the ECU could get all confused and suggest the wrong air/fuel mixture. Any codes? Got access to any Porsche specific diagnostic hw/sw?
There are two possible MAFs for a 2000 (in both Bosch and Porsche parts) and the latest one requires a ECU reflash to update the standard parameters if used as a replacement. (see table towards the end of this) Was the new MAF the exact same part number as was on the one replaced? I presume the old one matched the ECU and at one time the pair worked together over 6k RPM. By the way, what does FPSH translate to?
A classic cause of not starting when hot is the crank position sensor. If it is not working the tach will not bounce even a bit when the key is turned. Repeated attempts could have killed the battery. Good luck.
A 3.2 of what year to a 3.6 of what year? Year to same year swaps of 996S to 986S are relatively easy but different engine generations result in some wild and lengthy threads on the forums with sometimes no resolution.
1. CPS sensors are notorious for failing at high temperatures, cooling off and working again until ..... 2. The tach not bouncing is another sign of a probable CPS sensor failure. Several of the forums have folks who posted the diagnostic methods of checking voltages on the CPS.
Several options depending on your budget. Double DIN Pioneer with kit can give you a modern bluetooth interface, XM, sat nav, etc. On the other end there are adapters. How deep are you wanting to go? Browse a crutchfields web site to see some kits and prices for the former.
What maintenance history do you have for the car? Done things like leakdown test, scanned for current or pending codes, pulled the oil filter and pan and inspected for debris? Alignment? Cam deviation? Has a professional Porche experienced mechanic driven the car? How old is the clutch? Condition and age of tires? Brake fluid age? Oil and filter? Where I'm going with these questions is you need to get where you really really know the condition of the car. What is your height relative to the rollbar? Have a hardtop? What are the rules of your local track and race sponsoring association?
By housing I presume you mean the circuit board like thing that the bulbs and wiring plug into. Your description says there are two functions that may not be working. One is a fog light and I presume you know that is only one side of the car and not both. I forget which. But if one is lit, you ignore the other. The other is what are sometimes called running lights. The light that is lit when the headlights are on and the brake lights are off. That is likely to be a connection at one end of the harness or the other assuming the other side is working. Could also be a chewed wire in the harness.