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Excessive Fuel Consumption and Poor Overall Performance

Recently we had a 996 Coupe in the shop, the customer’s concern was that the fuel consumption appeared excessive and the engine seemed to be less responsive than normal. We ran a complete controller interrogation, meaning we scanned all the control modules (computers) in the car for faults with the PIWIS (Porsche Integrated Workshop Information System) scan tool. Primarily we were searching for faults or fault codes in the DME (Digital Motor Electronics) controller AKA the ECU (Electronic Control Unit), there were no faults present. At this point most technicians would conclude that since there are no faults present, there must not be a problem, however, this is not always the case. Through experience we know and believe that the customer is usually very familiar with the operating characteristics of their car. With this in mind we continued our diagnosis.

We performed a test on the pre-catalyst (before the catalytic converters) oxygen sensors utilizing a DSO (Digital Storage Oscilloscope) to visually inspect the electrical signals coming from the oxygen sensors. Oxygen sensors do exactly what their name describes; they sense oxygen in the exhaust stream. When there is an excess of oxygen in the exhaust, the engine is running lean (too little fuel). Likewise, if there is a lack of oxygen in the exhaust, there is a rich running condition (too much fuel). Normally, when a lean running signal is sent from the oxygen sensors to the DME, the DME then richens the fuel mixture via a wider injector pulse width (keeping the fuel injectors open longer). The mixture eventually get’s too rich, the oxygen sensors identify this change and adjusts the signal sent to the DME, the DME in turn leans the fuel mixture again. This cycle repeats over and over in a process called closed loop operation. It should be noted that closed loop operation doesn’t occur immediately after cold engine start, the oxygen sensors must first reach at least 600°F to operate properly, if they are not faulty.

As oxygen sensors age, they deteriorate and are considered lazy, their reaction time slows down. As they switch from rich to lean or lean to rich, this process begins to take too long. SAE (Society of Automotive Engineers), the organization that helped develop OBD-II (On Board Diagnostics-II), determined that the threshold for an oxygen sensor to switch from one extreme to the other should not take longer than 100ms (1 tenth of a second). The only way to accurately measure this is with a meter that not only measures voltage but also time. This is why we utilize a DSO for this test procedure.

Here is a DSO screenshot of the two pre-catalyst oxygen sensors on our clients 996 coupe;

The technician performed a test, where they induced propane to create a rich mixture and then quickly generated a vacuum leak to monitor the oxygen sensors switching speed from rich to lean. This data indicates that these O2 sensors are on the verge of setting a check engine light. The top of the screen represents a rich mixture and the bottom of the screen a lean mixture. The far left to far right borders represent one second total in time. Look at the timeline, these two oxygen sensors are exceeding the 100ms switching time from rich to lean.

Oxygen sensors are a wear component and should be checked even if there are no computer fault codes or Check Engine Light. It’s good practice to replace modern oxygen sensors at least every 100k. Other than operating time and or mileage causing an O2 sensor to wear out, other enemies are Oil, Coolant and Silicone contamination.

In our client’s case, the oxygen sensors had aged and were responding in closed loop operation much too slowly (lazy). When the O2 sensors were replaced, the system returned to normal fuel consumption and engine response.

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