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2009 chevy malibu with a 2.4L P0172

posted May 22, 2016 11:48:35 by RassielCotoMedina
2009 chevy malibu with a 2.4L P0172 system too rich STFT at idle -20% LTFT at idle -8% upstream o2 oscilating good downstream responds to WOT test going rich and going lean on decel Maf reporting 3.1 gm/sec at idle and 2.4 khz specs are from 2.3 to 3.0 khz at idle it increases smoothly when accelerating baro 28.6 map reporting 10.1 inhg at idle iat reading 113 F im in florida but i also had a PID called airflow gm/sec reporting 6.2 at idle thats clearly a problem but maf was reporting 3.1 half of that airflow pid so i went ahead and replaced the maf sensor STFT ascilating from -4 to 3 % and LFFT at 0% after, i did call this vehicle a fix but i still want to know how is this airflow pid calculated
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7 replies
PaulDanner said May 22, 2016 11:54:40
That is wierd. The airflow gm/sec pid IS calculated directly from the MAF. I honestly didn't think there was a different between a MAF gm/sec and an AIRFLOW gm/sec data pid. Probably because when I've seen both on one vehicle they were always the same. Also a lot of manufactures only give you one or the other. You fixed it, so that's cool. But why the difference between the two? Can't answer that one bro. What were the after numbers?
RassielCotoMedina said May 22, 2016 12:00:01
the maf after about the same 3.2 gm/sec and 2.4 khz at idle but airflow went down to 3.2 gm /sec thats really weird for me im thinking maybe this airflow is a calculated value from maf, map and iat? but it makes no sense to me cause all i replaced was the maf, but hey lesson learned computer listens to the airflow pid
RassielCotoMedina said May 22, 2016 12:08:01
a little history on the vehicle it had the MAF sensor replaced not long ago somewhere in ohio they also did a tuneup i ask the customer why the MAF sensor was replaced and he said that the dealership had replaced it due to a CEL on i dont know wich code was there but he said that it was not too long ago, maybe this vehicle comes back and bites me in the a.. hope not
[Last edited May 22, 2016 12:08:44]
PaulDanner said May 22, 2016 12:15:24
I don't think so. Very strange one and a good one to remember moving forward. Something I've learned. When you see it once, you'll see it again and maybe the next one we can connect the dots :-)
Tyler said May 22, 2016 19:13:44
im thinking maybe this airflow is a calculated value from maf, map and iat? im thinking maybe this airflow is a calculated value from maf, map and iat?

I like the sound of this theory, especially since I frequently find calculated airflow PIDs on speed density systems.

Very surprising that this system appears to use the calculated value over the MAF signal for fuel delivery. Has to be, right? That's the only way this could result in a rich condition.

Now I want to go back through all my late-model GM data and see if there was ever a discrepancy, lol. Thanks for bringing this one to us, sir!
James Machalik said May 25, 2016 02:30:00
Airflow and Airmass Calculations
The VCM generally uses two airflow related parameters for its calculations: Airflow (grams/sec) and Cylinder Airmass (grams/cyl). If a Mass Airflow Meter (MAF) is fitted, the airflow is measured directly by the MAF and converted to airmass (g/cyl) using RPM and the number of cylinders in the engine. The VCM also has the ability to calculate the incoming airflow and airmass by using speed density calculations involving the MAP, charge temperature and RPM. In most cases this is used as a backup mode by the PCM in case the MAF fails, however, some vehicles are not fitted with a MAF and use speed density mode all the time. On some vehicles the PCM also uses an older method of calculation that uses Throttle Position (TPS) as the "load" indicator/axis rather than MAP. This is known as Alpha-N however, the calculation remains very similar to speed density.
Hope this helps.
Dylan said May 25, 2016 16:19:29
Cool. Thanks for the feedback James.
Belgium, Europe
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