OBD II - fuel trims (adaptives)

[Joe Chiasson]

James Boughton <boughton at bignet.net>   wrote: 


> I would be interested in what tool you are using to obtain the data you
> see.  It sounds pretty handy.  Does it update quickly enough for you 
> to see the oscillations of the short term adaptive?  This should be in 
> the 1-2 hz range, maybe a little slower at idle.

The diagnostics tool is called the "New Generation Star Tester"
manufactured by Rotunda for Ford.  Basically any Ford dealer's garage will
(or should have one), and possibly other up-to-date garages would.  The
tool updates pretty much as fast as the computer process its input and
outputs.  Using the previous example of closed loop acceleration from 55
mph to 65 mph you will see all the long term oscillations that would take
you from -7% to -11%.  The tool can sense short term fuel trims rapidly
enough that you would probably miss seeing half the data.  The tool is on
the expensive side (I believe around $1500), fortunately I have one
availble through my employer, thus using one of these for free makes it a
whole lot better

> What the long term or short term adaptives do at WOT is strictly up to
the
> software writer, and whether they are used at WOT is usually the result
> of some big meeting where the guy with the most political clout decides
> what he is going to tell everyone else to live with.  Now, with OBDII I
just
> remember something.  If you do repeated WOTs and lifts you will drive
> a lot of hydrocarbons through the catalyst.  If you begin updating your
> adaptives before the catalyst has had a chance to clean up again the
down-
> stream O2 sensor will try to lean out the mixture.  This may be causing
some
> of the problem you are seeing.  If you try an extended cruise I would
expect
> the long terms to stabilize at some value and the short terms to
oscillate around
> 0.

This vehicles eventually does settle down and the short terms do eventually
oscillate around zero (from about -2% to +2%) under normal loads (i.e.
30-60%).  The long term eventually settles around -7% under the above
conditions. Will  take your advice into account and do some more driving
and see if I can not get to the bottom of this.

> More and more this is sounding like piss poor calibrating and the parts
are
> all fine.  Is there a particular drive problem that instigated this
inquisition or
> are you just curious?  Believe me cars from the factory aren't
necessarily
> calibrated perfectly, my truck actually dies out if you go WOT rapidly
while
> it is still warming up.  Remember, the auto makers can sell cars that
drive
> bad, but they can't sell cars that blow high emission numbers.

Basically what instigated this whole situation was I had never driven a
truck with the 4.6L in it before.  Most people I had talked to concerning
the engine told me that it was built to be in the same class as the
Chrysler 5.2L.  So in comparison I though the engine was very weak and
seemed to lag on acceleration.  We ran some compression and leakdown tests
and found some outrageously (atleast I though being that it's a newer
engine design) inconsistent numbers from cylinder to cylinder.  This
prompted me to put the truck on the diagnostics tool and voila fuel trims
did not behave as I thought they should.  Thus the question I posted to
dyi.   SO I GUESS I'M JUST CURIOUS.

Thankyou,

J