2000 Toyota Tundra 3.4 P0171

2000 Toyota Tundra 3.4 P0171

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.

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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).

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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.

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

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