Originally Posted by
fsm
In order to maintain proper fuel control, the PCM strategy needs to know the stoichiometric Air/Fuel Ratio for use in
the fuel pulse width equation. On pre-2000 MY flex fuel vehicles, the percent alcohol in the fuel was determined by
reading the output of the Flex fuel Sensor. The percent alcohol was stored in a register called Percent Methanol
(PM). Although current alcohol-blended fuels only include ethanol, the percent methanol nomenclature has
persisted. On 2000 MY and later vehicles, the Flex Fuel Sensor has been deleted and PM is inferred. The strategy
to infer the correct A/F Ratio (AFR) relies on the oxygen sensor input to maintain stoichiometry after vehicle
refueling occurs.
[...]
The fuel level input is used to determine if a refueling event has occurred, either after the initial start or while the
engine is running. If refueling event is detected (typically calibrated as a 10% increase in fuel level), the PCM tracks
the "old" fuel being consumed by the engine. After a calibrated amount of "old" fuel has been consumed from the
fuel lines, fuel rail, etc., the "new" fuel is assumed to have reached the engine. Normal long term fuel trim learning
and purge control are temporarily disabled along with the evaporative system monitor and fuel system monitor to
allow the composition of the fuel to be determined. The filtered value of short-term fuel trim is used during closed
loop to adjust AFR in order to maintain stoichiometry. During learning, all changes in AFR are stored into the
AFRMOD register. As updates are made to the AFRMOD register, the fuel composition register (PM) is updated
and stored in Keep Alive Memory. Learning continues until the inference stabilizes with stabilized engine operating
conditions. The PM inference and engine operating conditions are considered to have stabilized when all of the
following conditions are satisfied:
• ECT indicates the engine has warmed up (typically 170 °F) or an ECT related fault is present.
• Enough "new" fuel has been consumed (typically 0.5 lb - vehicle dependant) to insure fuel is adequately
mixed.
• The filtered value of short term fuel trim is in tight fuel control around stoichiometry, (typically +/- 2%) for at
least 5 O2 sensor switches or AFRMOD is at a clip.
• The engine has been operated for a calibratable length of time, based on ECT temperature at start
(typically 200 sec. at 40 °F and 30 sec at 200 °F) or an ECT related fault is present.
• The engine has been operating in closed loop fuel, with the brake off, within a calibratable (off-idle) air
mass region (typically 2.4 to 8 lb/min) for 5 seconds, to minimize the effect of errors such as vacuum
leaks.
Once the value of PM has stabilized (usually about 7 miles of driving), AFRMOD and PM are locked and deemed
to be "mature." After PM is deemed "mature," normal fuel trim learning and purge control are re-enabled along with
the fuel system monitor and evaporative system monitor. Any observed fueling errors from that point on are rolled
into normal long term fuel trim (via adaptive fuel learning).
All remaining OBD-II monitors remain enabled unless AFR is observed to be changing. If AFR is changing, all
monitors (except CCM and EGR) are disabled until the AFR stabilizes. This logic is same as was used for FFV
applications that used a sensor. The AFR rate of change required to disable OBD-II monitor operation is typically
0.1 A/F (rate is based on the difference between a filtered value and the current value). For a fuel change from
gasoline to E85 or vice versa, AFR typically stabilizes after 2 to 3 minutes on an FTP cycle.
If a large refueling event is detected (typically calibrated as a 40% to 50% increase in fuel level), the PCM strategy
tries to assign the "new" fuel as gasoline or ethanol (E85) on the assumption that the only fuels available are either
gasoline or E85. The strategy performs this fuel assignment to gasoline or ethanol (E85) only if the "old" and the
"new" stabilized inferred fuel composition values are within a specified amount of each other (typically 5-10%),
indicating that the fuel in the tank is the same as the fuel that was added and therefore must be either gasoline or
ethanol (E85). If the "old" and "new" stabilized inferred fuel composition values are not near each other, the fuel
added must be different from what was in the tank and the strategy retains the current inferred value of PM until the
next refuel. By assigning the fuel to gasoline or ethanol (E85) in this manner, normal fuel system errors can be
learned into normal long term fuel trip for proper fuel system error diagnosis.
After a battery disconnect or loss of Keep Alive Memory, the strategy will infer AFR immediately after going into
closed loop fuel operation. A vehicle that previously had fuel system errors learned into long term fuel trim will infer
incorrect values of AFR. After the value of AFR is determined, it is fixed until the next refueling event. If the next
refueling event is performed with the same fuel (either E85 or gasoline), the value of AFR will not change. The fuel
is then assigned to be E85 or gasoline as explained above. The long term fuel trim will again be a reliable
indication of normal fuel system errors.
Only one large tank fill is required to assign the fuel as being either gasoline or ethanol, if the inferred AFR did not
change significantly. If AFR did change significantly, several tank fills with the same fuel may be necessary to
assign the fuel as gasoline or ethanol.
As the vast majority of vehicles are expected to be operated with gasoline, the initial value of AFR is set to
gasoline. This is the starting point for the AFR after a battery disconnect and will allow for normal starting. Some
vehicles may have E85 in the fuel tank after having a battery disconnect, and may not have a good start or drive
away. The startability of alcohol-blended fuels at extreme cold temperatures (< 0 °F) is difficult under normal
conditions; these vehicles may be required to be towed to a garage for starting if a battery disconnect occurs.
E 85 fuel question
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