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I set up the Ford Mass Airflow sensor (actually NGK) with fan
on the bench.

You wet your finger and put it on the hot wire and the voltage
goes up. Turn off the fan and the voltage goes down.
Works like a charm.

It really is a hot wire. You put
your finger on it without spit and it will burn your finger.

There is a huge power supply filter capacitor in there as after
you shut the power off it takes a long time for the output
voltage to die down. Of course I am not loading the output.

The next step is get some dry ice or speed up the airflow
to test at higher air velocities. Pretty easy to hold it out
of the RX8 car window when doing 140 MPH :)

One way of leaning the mixture is put the mass air flow sensor output
on top of a 1 meg pot and connect the wiper to the op amp. The 555 will
think it is getting a lot less air than it really is and put out
less fuel.