LTFT and STFT

blackbird R/T wrote:If you have the means
to log over the OBD-II port you can watch what the short term and
long-term fuel trims (STFT and TLFT) are doing. These are the
corrections that the PCM is applying to add or remove fuel based on its
O2 feedback. There are different cells (spots or ranges of correction)
based on what MAP voltage level you're at and if it's above or below
~2k rpm's (as well as separate corrections for idle/decel). Short term
is on the spot temporary changes of up to 25% correction to add or
remove fuel. If the PCM continually has to add 10% more fuel under
certain operating conditions/ranges (above 16psi boost and above 2k
rpm's, for example), the long term would get adjusted so the short term
can go back towards zero correction. The PCM always likes to keep short
term as close to zero correction as possible. Long term can also
provide up to 25% correction, and both together can make up to a total
of 50% correction. The PCM makes these corrections by altering the
injector pulse width which is the time the fuel injectors are "on" and
flowing fuel.

Both the short and long term fuel being cell based
means they use a matrix based on RPM and load (determined from the MAP
sensor) to determine which cell the engine is operating in. Each cell
can store its own fuel trim correction. Under normal driving with open
throttle the engine management uses 12 cells, referencing MAP voltage
against RPM's. It basically uses low RPM (below around 2k rpm's) and
high (above 2k), referenced against 6 different MAP voltage set points.
There's also a deceleration and idle fuel trim cells for above and
below 2k rpm's.

So as an example, one cell might have a range of
3.6V to 3.9V on the MAP sensor with the engine operating above 2k
rpm's. So if you had a LTFT correction of 10% it would be adding 10%
fuel when the boost is between ~10-12 psi (3.6-3.9V). As soon as the
MAP voltage goes over 3.9V you'd enter the next cell which may have a
different LTFT correction.

When you're looking at fuel trims
remember that there are only two different reference points for rpm's
in regards to the fuel trims. There's one set of fuel trims for above
~2k rpm's and one set of fuel trims for below. For above and below that
2k rpm point there are six different fuel trim corrections stored
(called cells) that is MAP voltage based.

So if you're at 4k
rpm's and 3.5V on the MAP that would fall in a certain cell. At 5k
rpm's and 3.5V it's still in the same cell (because the MAP voltage
didn't change and it's still above 2k rpm's). Once the MAP voltage
changes a certain amount or the engine drops below 2k rpm's you may
drop into a different cell that may or may not have different fuel trim
corrections. A lot of the older PCM's used many more tables and cells
to store different fuel trims. My educated guess is that because the
NGC (Next Generation Controller) used in the SRT-4 is model-based and a
lot more powerful than older PCM's they can rely on it being able to
use O2 and other feedback as well as calculations to make faster, more
accurate "live" decisions on changes to the fuel trims without having
to rely on extensive cells for different rpm and MAP voltage ranges
like the old PCM's.

These short and long term fuels trims can be
viewed on most scan tools or data loggers that can monitor generic
OBD-II information and will be represented as a percentage. A positive
number (such as 15%) means the PCM is adding fuel and a negative number
(-12% for example) means its removing fuel. Some older pre-OBD-II cars
like GM use a different system based off a block learn value between 0
and 255. In those types of systems 128 would be the middle or average
value with zero correction and a higher number would mean the PCM is
adding fuel and a lower number meaning that it's removing fuel.

Blackbird R/T wrote:The
long-term is adjusted by the PCM to keep short-term closer to zero/no
correction. The PCM adjusts long-term based on the short-term
adjustments, and the short-term is based on the O2 feedback. And just
because you have +5% fuel in short-term doesn't mean it will
automatically change long-term to +5% either and short-term back to
zero every time. You can use short -term for drivability issues and
light acceleration/boost to see what your fuel system is doing. And if
you wanted to you could take a look at WOT in open loop to see if the
long-term values are ever changing based on adjustments made by the PCM
in closed loop to the long-term.


The short-term and
long-term can both add and remove up to a total of 25% of the
pulse-width. That's what the numbers you're seeing on a scan tool mean
if it is displaying a percentage value. A positive number is the PCM
adding extra pulse-width (add fuel/richen) and a negative number is
removing pulse-width (remove fuel/lean out). Pulse-width is the time in
milliseconds the injector is on and can also be converted to duty cycle.

After
startup the PCM uses short-term correction (which means O2 feedback) in
open loop warm-up when the engine coolant reaches ~30-35°F. When the
engine goes into a closed loop mode and is above ~170-190°F it will
adjust or allow the long-term adaptive correction values to be updated
based on the short-term direction of movement/correction. It's
important to note again that the long-term adaptive memory and cells
are used in all operating conditions (including open and closed loop
modes) but only change when above that temp, in closed loop, and when
the engine has been running for over two minutes.

The long-term
values are adjusted by the PCM in order to keep the short-term as close
to zero (no correction) as possible. It probably has an algorithm in
the programming that alters the long-term if it continually sees
short-term in a certain cell having to add/remove fuel, how much it
add/removes, and how long the short-term has been making corrections.
The long-term can add/remove up to 25% pulse-width, so if the LTFT
max's out, the short-term can still alter an additional 25% when in
closed loop (O2 feedback) for a total of 50%. But when you got to WOT
(open loop/no O2 feedback) it will only be using LTFT and subsequently
a max of 25% change in pulse-width.

During acceleration the car
is still in closed loop mode with the O2 feedback being used (which
means short-term values are adjusted). This might occur when you roll
onto the throttle while cruising and start building boost but aren't at
WOT. That's a likely scenario of how the short-term and eventually the
long-term get adjusted under boost. When you go to WOT it reverts to
open loop and only reads (not modifies) the LTFT. Since the last cell
range is for 3.9 volts and above, that correlates to about 12 psi and
above on a stock/S1 car.

Also the short-term memory is lost when
you shut the car off. The long-term adaptive memory is held as long as
you don't disconnect power NGC.

Blackbird R/T wrote:Long-term
isn't just some some average, but then again it is, and it's only
adjusted based on the short-term movements. How's this for an example.
The short-term goes to +5% in a certain cell to increase the injector
pulse-width to richen the mixture. The long-term is at 0% (no)
correction but after the NGC sees it keeps having to add 5% in that
cell it eventually adjusts the long-term to 5% and the short-term back
to zero. You're still getting the same 5% extra fuel.

Now the
car starts to run a little lean again when operating in the same cell.
This time it has to add 3% pulse-width. So now you have 3% in
short-term, plus 5% of long-term, for a total of 8% extra pulse-width
being added. If it continues to have to add that 3% it may bump the
long-term to 8% and short-term back to zero. So short-term may never
get to as big of a correction as long-term. It could also go from
long-term adding fuel in a cell and suddenly the short term goes
negative in that cell trying to remove some of that fuel. In that case
both numbers might converge towards the baseline of none/zero
correction (the PCM decreases LTFT, the mixture leans, then the O2
sensor tells the PCM it can increase the STFT from a negative value
back towards zero).