Time to Have a Talk with the Jag

TOP LEFT I know there is a leak, but I do not see any smoke. TOP RIGHT Thar she blows! I knew there was a leak hiding somewhere. ABOVE LEFT Scan data of fuel trims with leak bypassed, confirming diagnosis With the leak source bypassed, fuel trims started dropping immediately. ABOVE RIGHT On my way to the source of the smoke.

Then I ask the PCM what failed. In the case of this Jag, the same codes were stored as the last time the car was in: P0171 and P0174. So whatever else was a cause the last time, it is not the only cause. There is still a problem.

Some Basic Tests are Next

Just like the doctor, I wanted to run some basic tests. I started the car and allowed it to reach operating temperature, then reviewed the current data stream for anything out of place. What instantly came to the forefront was a repeat of what I had seen in the freeze frame data: LTFT was at its limits, and Short Term Fuel Trim (STFT) was trying to add even more.

This car's fuel management system uses a Mass Air Flow (MAF) sensor to read airflow into the engine. Because a MAF sensor measures actual air mass entering the engine, that's a tool for me to use. I increased engine speed to 2,500 rpm and took another look at current data. LTFT was now closer to normal, and STFT was normal, switching nicely across "0." The oxygen sensors looked a lot happier as well, also switching normally.

LEFT My diagnosis was correct. The rubber boot was deteriorated. Now to repair and confirm" RIGHT Close up view of deteriorated elbow.

Is All the Air Being Measured?

A MAF sensor, unlike the Manifold Absolute Pressure (MAP) sensor in a speed-density system, measures actual air mass entering the engine. However, it can only measure the air that is flowing through the sensor. Air entering the engine from any source downstream of the sensor will not be measured. This air can come from a vacuum leak, a torn intake boot between the sensor and the throttle body — even a poorly sealing oil dipstick tube.

The PCM doesn't know that this extra air is coming in, and doesn't compensate for it by adding the appropriate amount of fuel. The oxygen sensors, however, are definitely going to tell the PCM that the mixture is too lean, and the PCM responds by increasing fuel trim. This cycle continues until the mixture is restored to lambda=1. If the LTFT corrections necessary to achieve this goal exceed a specified threshold, a "system lean" code will be generated.

Of course, there are other factors that can cause a "system lean" code. But all the evidence is there to point to unmetered air as the cause. Do you know what it is?