OBD II - fuel trims (adaptives)

[Joe Chiasson]

> Basically, you weren't too far off to begin with.  The short term
> adaptives are typically used for two purposes.  First, to account
> for unusually rapid changes in fuel/air.  A full vapor canister after
> a hot soak can cause this type of behavior.  Second, this term
> is usually used to create the small oscillations in fuel/air required
> to make a catalyst work well.
> 
> The long term adaptives are typically much more stable, relating
> to such things as injector drift.  But the long term adaptives can 
> move around due to conditions such as the hot soak condition
> mentioned above.

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 .  

> For both long term and short term adaptives there are usually speed/load
> cells where the response characteristics for the short term and the
> adaptive value for the long term are used.  In other words, it is
possible
> that the long term adaptive at idle is different than the one for 30mph
> cruise and different for the one at 50mph cruise, etc.  In your first
> example (50mph cruise) you had -7% long term.  This is not terrible,
> since depending on who calibrated it there might be a typical -5% built
> in.  This way if you unplug the engine controller the engine will run
about
> 5% rich until the adaptives catch up.  Also, since these are typically
> multipliers your 1.05*0.93=~0.98.  So this is not very far off.
> 
> Under WOT conditions the engine controller should, typically, run
> open loop.  Whether or not any of the adaptives are used will vary from
> company to company, since it is built into the software, which changes
> every year.  The only thing I might be concerned about is why your
> long term is -21%.  This could be caused by the above mentioned
> hot soak condition where the vapors in the charcoal canister build
> up while the car is off, and when the car is running again these vapors
> come out of the canister causing a rich condition.  This can eventually
> drive the long term adaptive to large negative numbers.  Later when
> the canister is empty you will end up with the short term adaptive trying
> to correct for the long term which is what looks like is going on. 
However,
> there is definitely no feedback at WOT, so you would want to back out
> of the throttle slightly until the short terms starts to update to be
sure you
> are using the adaptives that are being displayed.

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.
 
> Oh, BTW,  stoich at WOT is bad.  Even though the EPA is trying
> to mandate it nobody currently operates stoich at WOT.  When
> they do expect vehicle prices to rise.  The engines should have
> some great components in them, however.
> 
> Jim Boughton
> boughton at bignet.net
> 

Thanks for the response Jim, although I am still a bit confused, I now know
I'm on the right track of thinking.  Much appreciated.

J.