Simple Air Mass Sensor Check for DIY car buffs

While having the air filter assembly off and car running, lean over and with your mouth, blow air on the sensor. Idle should change with an operational sensor (mine dropped, I think it fooled the Electronic
Control Module and enriched fuel delivery without get more air with throttle still closed, temporarily flooding engine) . It's a simple check for a working or non-working AMS. Could anyone verify my theory?
Thanks, Derf
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When you blow into the MAS, you simply blend the natural air with air from your lungs. The engine still sucks the same amount of air per revolution unless your change the pressure into the system (which you don't do by simply blowing).
The reason why the idle changes is, that the ECU assumes that the airflow measured by the MAS has normal oxygen content for atmospheric air. The air from your lungs has less oxygen content, resulting in too rich mixture (fuel/oxygen ratio).
The air drawn by a 3 liters engine at 600 rpm idle is 15 liters per second, so the air you add by blowing is relatively insignificant, so the fact that idle changes only demonstrates how sensitive the engine is to correct mixture.
So, your experiment only shows that your lungs work (and your body consumes oxygen), not wether the MAS works.
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Correction: Air drawn at idle is only some 2 liters per second, depending on how much power is required to keep the engine at idle. So, the air blown has more significance, but still the same conclusion.
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Because there was a fluctuation in the idle from blowing on the sensor would indicate that the MAS was at least sending info to ECM and functioning.
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Well, a little techical discussion just for the fun of it.
The MAS works by measuring how much current is required to keep a platinum wire at a constant temperature 180C above ambient air. The more air drawn, the more the wire is cooled (depending on the air's specific heat - therefor the name air MASS sensor), and the more current is required to keep up the temperature.
When you blow at the sensor, you merely replace some of the ambient air with air from lungs, and unless some air is reflected from the sensor wire (which I hardly believe), the sensor will still measure the total air mass drawn by the engine.
Since the air from your lungs most probably has higher temperature than the ambient air, you add a little heat to the air drawn through the sensor, and consequently less current is required to reach the 180C just as if less air was drawn. So yes, in this way you will cheat the sensor (only because intake air temperature sensor is located at the air filter and thereby bypassed when MAS is exposed).
But at the same time the ECU assumes a certain amount of oxygen per mass unit of air. The air from your lungs has less oxygen contents than natural air, and in this way you cheat the ECU no matter whether the MAS is cheated or not.
In any case, the ECU should immediately compensate for this through feedback from the oxygen ("lambda") sensor at the exhaust, although the RPM may fluctuate slightly due to delays in the control.
The point is, that you don't know for certain whether the fluctuation is due to cheating a working MAS or not.
In fact, if the MAS does not work, the ECU will operate in "limp back" mode, where it uses absolute manifold pressure instead of MAS to sense the engine load and control the mixture according to a preset table disregarding oxygen sensor feedback. In that case you would still be able to cheat the engine control by supplying air with less oxygen content, and the ECU will not even try to compensate for it.
In this way you can also conclude, that the fluctuation in RPM is because MAS is not working. Or in other words: You cannot conclude anything from your experiment.
A fresh idea anyway :o)
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