Barometric pressure

Jason Haines jhaines at lingenfelter.com
Fri Sep 14 17:03:09 GMT 2001


It is not just at WOT - that is why I said "WOT being one of them".


Jason


> I never knew the Baro value changed...until next key on. One reason is
that
> you can start an EFI down here, drive to Banff--and it's running on the
rich
> side till you stop for gas <G>... suddenly, it's fixed. The 3000 ft
> elevation change isn't updated till next key on cycle. If they're using a
> WOT run for an update, I dunno--IMO it's not the whizziest idea in the
park.
>


>
> > On the newer GM vehicles, (such as the 1994 application originally asked
> > about) the MAP sensor is used to determine the barometric pressure. In
> > addition to setting the barometer to the MAP reading at key up, the MAP
> > readings are also compared to the stored barometer reading under
different
> > conditions (WOT being one of them) and changed based on the expected
> > pressure drop in the system at different RPM levels, throttle positions
> and
> > air flows (in MAF based systems). Different vehicles, ECMs etc. have
> > different levels of complexity of this model.  It is my understanding
that
> > one of the reasons for doing this form of barometric pressure updating,
in
> > addition to whatever issues the OE may have, is to comply with EPA and
> CARB
> > requirements for altitude compensation (manufacturers are no longer
> allowed
> > to have separate high altitude calibrations).
> >
> > Some appilications do use a separate barometric pressure sensor but this
> is
> > rare (I only know of one GM application).
> >
> > The above information applies to GM vehicles but should be true for many
> > other applications. They all want to save money on sensors so they try
to
> > reduce the content on the ECM and in the vehicle (plus, less parts means
> > less parts to go bad).
> >
> > Just my $0.02
> >
> > Jason
> >


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