I arrived to a 2000 Toyota Tundra 3.4 running rough with a P0171 that was cleared by the shop. They had in hand fuel fouled spark plugs and no explanation as to why.

I connected my scan tool and observed the fuel control and feedback PIDs. I found fuel trim to be positive (indicating a lean condition) at idle and at 2500 RPM. This ruled out a vacuum leak.

The engine air flow was 7.56 grams per second (GPS) at idle. With an engine size of 3.4 lites I expected to see a GPS number close to that. A general rule of thumb: An engine flows about 1 GPS at idle per liter. For example a 2.0 liter engine would flow about 2 GPS at idle.

With a reading twice what it should be normally I wanted a close look at the sensor. Gaining access was easy, the mass air flow sensor is mounted in the air intake after the air filter near air box (arrow).

Once removed the mass air flow (MAF) hot wire was clearly covered in debris. This can lead to a skewed reading. When the hot wire is covered in dirt at idle (low air flow) the dirt acts like a radiator, cooling the wire and reporting a higher than actual reading. Off idle, the dirt acts like an insulator, protecting the hot wire element from the passing air and it’s cooling effect. This leads to a lower than actual air flow calculation.

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The dirt was cleaned and the MAF sensor was installed.

The reading dropped a few tenths of a volt but not quite enough.

After confirming the reference voltage and ground was good to the MAF sensor I suggested replacing it. The MAF sensor itself was faulty.