Originally Posted by Stamina

To understand the MAF's role in fuel trims think of this exaggerated example:

Pretend that the pipe going into the intake was really big... like a couple feet wide. And the MAF was sticking through the pipe and coming out of the left side of the pipe.

Now if you've got a fan, and hold it up and blow air through the pipe, the MAF would detect the flow. If you pointed the fan to the right side of the pipe, the MAF would detect maybe some flow but some of the air is getting past without getting read correctly (and would result in positive fuel trim). Now if you pointed the fan at the left side of the pipe and blew the air right on the MAF, the MAF would read a lot of air, but since it thinks that that much air is flowing through the whole pipe, it overestimates the air going in and the ECU needs to correct (negative fuel trim).

Sometimes irregularities happen inside the pipe, and the airflow may not be uniform inside the pipe (why putting the MAF along a long, straight section of pipe is a good thing). For instance, maybe more air is hitting one side on the inside than the other as it goes into the pipe.

Total airflow and fuel trims are two related yet different things. You can have a lot more airflow and still have a zeroed fuel trim.

Originally Posted by SSlobalt

I got a little misty, reading that.