MAF vs MAP

[Jennifer and Brock Fraser]

> > > > Priority:      Normal
> > > > To:            diy_efi at efi332.eng.ohio-state.edu
> > > > From:          "bruce plecan" <nacelp at bright.net>
> > > > Subject:       Re: MAF vs MAP
> > > > Date:          Mon, 23 Feb 98 13:43:45 PST
> > > > Reply-to:      diy_efi at efi332.eng.ohio-state.edu
> > >
> > > > ref KV:
> > > > GM (OEM) specific
> > > >   IMHO it's as much about application as anything.  Both use
> > > > feedback from the O2 sensor for the final trimming of injector timing,
> > > > so at less than WOT, there is about 0 difference.  Now at WOT,
> > > > and for off-highway use racing the MAP is the winner since there
> > > > is no MAF restriction.  As I understand things now, on the newest
> > > > GMs some use both a MAF, and MAP.  Does anyone know if they
> > > > do this for averaging, or backup, or what conditions they use each
> > > > for??..
> > > I would say atmospheric correction, most flow meters are calibrated
> > > at STP(std temp press 29.92"Hg  68F) you need intake air and
> > > atmospheric pressure to get actual corrected flow.
> > > alex
> >
> >   And backup.  Unhooked MAF of running 96 Corsica 3100 V6, engine never
even
> > hiccupped.  smooooth and snappy.
> >
> > Shannen
> >
> >Yes, I accidentally did the same thing, must say a liitle surprised
> >and alot of comments like throw it away, who needs it.
> >Rather busy with other problems at time so did not look in to it
> >further.  How does it run under load?
> >alex

>  I was in the middle of a service job, so I didn't have time to check.  Wild
>thing about these OBDII cars, they don't always set a code when there's a
>problem.  It sets a counter and if the problem occurs again within X ignition
>cycles, the light goes on.  Anyway, I'll get another one in to try out.
>Technician across the shop was working on government fire truck today with
a 96
>Vortech 454.  The truck was driven 50 miles to get to the shop, and the
problem
>turned out to be a blown fuse to the MAF.  These new vehicles can't run
too badly
>without it.

>Shannen

I've had a little experience with the GM OBD II MAF/MAP system, but it's
been a while so I might be a bit rusty.  The MAF is the primary sensor used
for measuring the air flow introduced to the system.  The MAP sensor along
with crank, IAT, etc.. are used just as they would in a normal speed
density configuration.  The difference being, in an OBD II vehicle, the MAF
reading is compared with the calculated speed density value to determine if
the MAF sensor is operating within and expected range.  If a delta of an
unacceptable amount is measured then an occurence count is incremented.
Every time I've seen it, the light has come on immediately, but who
knows... Your vehicle should run fine with the MAF sensor unplugged AS LONG
AS you have not increased the volumetric efficency very much at all above
the stock configuration.  If a fault is detected with the MAF sensor then
the speed density values are used.  Speed Density relies on look up tables
used in the ECU to determine the fuel delivery required for that
MAP,IAT,and RPM (there are some other variables in here also).  A severe
case would be if you add a turbo or a supercharger.  The system will detect
a delta that is too large, revert to speed density, and use volumetric
efficiency tables developed for the stock system and a 1 bar MAP sensor!
Things will be a little LEAN!!!  I have seen this happen in prototype
supercharger kits for GM OBD II systems developed by Vortech (A 27:1 AFR
resulted), but they were quick to solve this problem before they went into
production by reprogramming the ECU.  

Jennifer