Gas mileage saga UPDATE (long again, sorry)

[Marteney, Steven J.]

As an update, I've gone back through most of my old Diacom data I've been
saving throughout my tuning process.  Interestingly, I can see my gradual
rise in gas mileage as I saw with data gathered at the pump.  Also
interesting is, even before headers or tuning or anything, the calculated
AFR is still in the low 12:1 range and only fluctuates between 12 and 12.5
over 4 months of data.  Also interesting is the theoretical gas mileage from
the latest data set is the highest yet.  So, I think the recent decrease is
probably lead foot syndrome in town.  (I'm just trying to decipher why
18.5mpg in this car and not 21mpg.  Right now, I'm just toying with the ECM
data to make sense of it.)

I now have two questions:
1) If I go in and adjust the MAF calibration table to reflect more air flow,
the BLM should drop because the ECM will think it needs more and the oxygen
sensor feedback will say it needs less than commanded.  This should result
in the same injector pulsewidth, but a lower BLM and a calculated air fuel
ratio higher than before.  In theory, I haven't done anything to the car's
operation, such changed a digital number in the computer.  This approach
assumes the oxygen sensor is always right.  Is this the way to go?

2) What if I modify the commanded air-fuel ratio to 16:1 in the ECM?  (If
anybody knows where this is in the $6E code I'd like to know.  It's not a
parameter in TunerCat.)  The ECM should then lean out the mixture by
reducing the pulsewidth.  Calculated AFR again goes up.  This assumes the
MAF is actually right and the ECM is being fooled by the O2 sensor into
richening the mixture.  Would this even work?  (Can't try it cause I don't
know where to make such a change.)  This approach would actually change the
car's operation.  Is it the right operation?  Don't know.

So my over-riding question is which is to be trusted more, the MAF sensor
reading or the O2 sensor output (in closed loop mode), or neither?  I'm
pretty sure the pulsewidth reading is fairly accurate because I'm able to
predict my gas mileage rather well.  The other two are mysteries to me.

I'd like to squeeze 21-22mpg out this car and have been told I should be
able to.  I've just been trying to figure out why I'm so far off.  Timing
advance has helped raise me from high 16's to low 18's.  Maybe I just need
more and more timing.  But these AFR calculations are confusing me.  Any
help or suggestions are appreciated.

Steve
----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org