OBD II - fuel trims (adaptives)

[James Boughton]

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.

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.

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


----------
From: 	Joe Chiasson[SMTP:chiasson at hutchtel.net]
Sent: 	Tuesday, September 09, 1997 9:58 PM
To: 	diy_efi at coulomb.eng.ohio-state.edu
Subject: 	OBD II - fuel trims (adaptives)


If any one could unravel the mystery behind OBD II short and long term fuel
trims (adaptives) it would be greatly appreciated.  My interpretation was
that short term fuel trims dealt with the short term fuel delivery (i.e.
computer increasing or reducing pulsewidth to reach a stoich O2 sensor
reading). My interpretation of long term fuel trims is how out of whack the
fuel delivery to the engine is, either over an OBD II driving cycle or over
the entire driving life of the vehicle, until the ecm is diconnected that
is.  

This is what I thought (now please correct me if I am wrong) untill I ran
into this senario with a 1997 Ford F-150 automatic 4.6 L V-8.  Under
cruising speeds of 55 mph the short term fuel trim was +5% and the long
term fuel -7%.  Under WOT conditions the short term fuel trim was +33% and
the long term -21%.  Now under my intial interpretations on how fuel trims
were calculated I assumed that at WOT the ecm was increasing the fuel
delivery by 33% to reach stoich.  But then why would the long term fuel
trim decrease?  Well this has confused the hell out of me.

If anybody can clue me on on what the fuel trims actually mean, and how
they are determined (i.e. referenced off the front O2, the rear O2, or no
O2 at all)  help would be appreciated.

J.