OBD II - fuel trims (adaptives)

[James Boughton]

>If the vehicle was travelling at about 55 mph load and had a long term of
>say -7%,  and from the time it takes to accelerate to 65 mph and return to
>closed loop, if in fact the computer ever left, the long term drops to
>-11%, is this an acceptable amount of drift.  (i.e. should the longterm
>numbers move this rapidly or should it be a slow transition) Based on a
>situation where the vehicle did not undergo a hot soak condition .  

You did not say if you acclerated to 65 at WOT or part throttle, but
assuming part throttle it is possible that you changed the cell you were
in and the long term adaptive in the 55mph cruise cell is -7% and the 
adaptive in the 65mph cruise cell is -11%.  If you did not change
cells and the long term actually changed I would say that is quite
rapid, however, you are not saying that as you cruise the long term
is updating all along which is what I would expect, especially with
the short term adaptive hovering about +5%.

The other thing that gets thrown into the mix is that it is going to depend
on the guy who calibrated the package.  He may have the long terms 
updating quickly, or very slowly, depending on what he likes to see
happen.  Calibrators are a strange breed and they all tend to have
opinions on how things should be calibrated so you never know what
might be causing what you are seeing.

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.


>I realize that at WOT the computer is in open loop and on most of the
>chryslers that I have seen there is no feedback what so ever, concerning
>fuel trims (i.e. diagnostics tool outputs 0% for both.)  Now the fact that
>the F-150 did report fuel trims @ WOT confused me.  You "hint" that these
>are probably bogus numbers and I guess I will have to agree and take that
>into account.  However I have investigated what the fuel trims are in
>closed loop just after letting off WOT. My long term is around -14%, which
>brings me back to the question above which is how much drift is acceptable
>and in what period of time.  The still more confusing factor is that the
>short term is +9%.  It seems that as the pw increase the short and lon term
>grow apart from each other.  I swapped in new O2 sensors to see if that
>would change anything, it brought them closer but significantly there was
>no change.

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.

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.

Jim Boughton
boughton at bignet.net