This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
The modern fuel injection systems utilize a form of closed-loop feedback to regulate the air/fuel mixture within a specific range. Beginning in 1980, Porsche installed an oxygen sensor (sometimes called a Lambda sensor) to better regulate the mixture of air and fuel being injected into the CIS fuel system. Not only does this sensor enable the catalytic converter to work at its maximum efficiency, but it also helps the car run more efficiently with more power.
The oxygen sensor is one of the most important elements of the modern fuel injection systems. Early fuel systems that utilize carburetors can see the air/fuel mixture ratios vary as much as 20% over the operation range of the engine. In comparison, a finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02%. Keeping the engine at the stoichiometric level (14.6:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensor is located in the exhaust system of the engine, and senses the oxygen content of the exhaust gases. The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.
The oxygen sensor is manufactured out of a ceramic material called Zinconium dioxide, and the inner and outer surfaces of this ceramic are coated with platinum. The inner surface of the sensor is exposed to outside air, while the inside surfaces are exposed to the exhaust gases. A small voltage signal is produced due to the difference between oxygen contacting the inner and outer surfaces.
If the amount of oxygen in the system is low, then the sensor will produce a high voltage signal (around 900 milivolts) and the ECU will compensate for the mixture being too rich. The amount of fuel delivered to the engine will decrease.
If the amount of oxygen is too high, then the oxygen sensor will send a low voltage signal (around 100 milivolts) indicating a lean mixture, and the ECU will adjust by adding more fuel to the injection system.
There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are:
- Irregular idle during warm-up
- Irregular idle with warm engine
- Engine will not accelerate and backfires
- Poor engine performance
- Fuel consumption is high
- Driving performance is weak
- CO concentration at idle is too high or too low
In general, if the oxygen sensor is not working, the car will be running very poorly, and will also be outputting a lot of harmful emissions. If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running really lean, and will try to richen the mixture. At the other extreme, if you disconnect the oxygen sensor, and replace it with a small AA battery that supplies 1.5V, the ECU will think that the car is running really rich and attempt to adjust the mixture to be leaner.
Needless to say, troubleshooting the complete fuel injection system is beyond the scope of this project. If you think that the oxygen sensor may be causing some of your fuel injection problems, it should be replaced. In general, it is recommended that you replace the sensor every 30,000 miles. The sensor is located on the left side of the car, right before the catalytic converter. Access is made much easier by removing the rear left wheel. The sensor is simply unscrewed from the converter. If there is a lot of rust in this area, it may be advisable to spray the area with some lubricant and let it sit overnight prior to attempting to remove the sensor. When you install the new one, it's a wise idea to place some anti-seize compound on the threads so that they will not rust and seize up. Some of the new sensors even have the anti-seize already placed on the threads right out of the package. It's also important to note that the rubber grommet that is attached to the wire on the sensor usually comes preinstalled with each new original equipment sensor.
The oxygen sensor (O2 Sensor) is attached to the top of the catalytic converter. The sensor measures the amount of oxygen in the exhaust prior to the catalytic converter acting on the exhaust. The sensor is one of the key elements in a 'closed-loop' feedback system that regulates the amount of fuel injected into the engine. The test port, located directly below, allows the measurement of exhaust gases to be made before they enter the converter. The catalytic converter pictured is an aftermarket unit, and not an original Porsche catalytic converter.