HOLDEN GEN III V8, Service Training

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GEN III V8 ENGINE MANAGEMENT
Long Term Fuel Trim
LTFT is used to adjust for engine to engine
variation and to adjust for engine ageing LTFT is a
portion of the PCM memory used to adjust fuel
delivery across all operating conditions of the
engine The PCM monitors the STFT and will adjust
the long term trend of the fuel injector pulse width if
the STFT has been at a value for a certain period of
time LTFT is used to change the long term fuel
injector pulse width and is only operational when
the fuel control system is in Closed Loop A normal
LTFT value is 0% and should follow the STFT
value
If an engine has a restricted fuel filter, the low fuel
pressure will result in less fuel being injected and
will cause the STFT value to go higher than 0%,
say 2% If this STFT value change does not
compensate for the restricted fuel filter, the PCM
will continue to increase the STFT value The STFT
may climb as high as its maximum calibrated value
if there is a severe restriction The PCM will
continue to monitor STFT as it climbs, but it will not
make any changes to the fuel injector pulse width
for a specific period of time After a specific period
of time has elapsed and the STFT value has
remained above say +8%, the LTFT will move up to
say 4% and wait again to detect if the STFT has
dropped back down to 0% If not, the STFT will
gradually move toward its maximum calibrated
value limit until it gains control of the fuel injection
system If STFT and LTFT are both set at their
maximum value limit, the fuel control system is out
of the limits of control and will set a Diagnostic
Trouble Code and go into open loop operation
The PCM will keep the latest LTFT values stored in
its LTFT memory cells MAP sensor readings and
engine RPM are used by the LTFT to determine
what cell to read LTFT values are stored in the
PCM's long term memory, for use each time the
engine's RPM and load matches one of the LTFT
cells All LTFT values are reset to 0% when the
PCM's long term memory power supply is
disconnected The Tech 2 scan tool also has the
ability to reset LTFT to 0% with a special command
Long Term Fuel Trim Cell
The LTFT function of the PCM is divided up into
cells 0-22 arranged by MAP sensor readings and
Engine RPM Each cell corresponds to a region on
a MAP vs RPM table Each region is calibrated to a
LTFT value of 0% A value of 0% in a given block
indicates no fuel adjustment is needed for that
engine load condition A higher number, say + 4%,
indicates that the PCM has detected a lean exhaust
indication under those conditions, and is adding fuel
(increasing fuel injector pulse width) to compensate
Conversely, a lower number, say -6%, indicates
that the PCM has detected a rich exhaust indication
under those load conditions, and is subtracting fuel
(decreasing fuel injector pulse width) to
compensate
As the vehicle is driven from a standing start and
accelerated or decelerated from various engine
speeds, the engine's LTFT calibration will change
from one cell to another cell As the LTFT changes
cell so does STFT, however, STFT will only make
short term corrections in whatever LTFT cell the
engine is operating in When the engine is idling, it
can be in one of two cells Depending upon canister
purge, the engine will idle in cell 20 If the engine
was running at idle and the canister purge was on,
we would be in cells 1 6 - 1 9 depending on AC clutch
status and PRNDL position
Whatever cell the engine is operating in, the PCM
will read that cell's particular LTFT value and
electronically adjust the fuel injector base pulse
width to compensate for a rich or lean condition in
the engine If an engine has a restricted fuel filter
and the customer has driven the vehicle like this for
quite some time, the LTFT value would be high, and
the PCM would be compensating for this condition
by adding more fuel Because the STFT value is
above 0%, LTFT will also be greater than 0% in
most of the cells to compensate for the lean
exhaust If you suspect a dnveability problem
associated with an over rich or over lean condition,
then use the STFT value to detect what the fuel
control system is doing at the present time Use the
LTFT to identify what the system has learned over a
greater period of time to compensate for the
condition
Use the LTFT cells to determine if the fuel control
system is commanding rich or lean throughout the
operating range If it is only rich or lean at idle or
part throttle, look for components that would cause
problems in these areas
All LTFT cell values are reset to 0% when long term
memory power to the PCM is removed
The Tech 2 scan tool has the ability to reset all
LTFT cells to 0%
GEN III V8 ENGINE MANAGEMENT Page - 97