Mitsubishi Eclipse. Technical Information Manual (1994) — part 17
Long term memory also has control over pulse width
by being able to increase or decrease the pulse
width stored in the cell by up to 25%. Long term
memory is retained by the battery in the PCM, while
short term correction is lost whenever the ignition
is turned off.
The long term memory works to bring the short
term correction to the point where the average
1-46
ENGINE
Control System
cent of pulse width compensation it provides in this
memory cell is 0%. The long term memory returns
to this level of pulse width compensation the next
the vehicle enters this cell. It is in this way
that the PCM is continually relearning the most ap-
propriate level of control, even as the vehicle ages,
internal engine components wear, and operating
conditions change.
Short term fuel
compensation
A/F ratio
Long term fuel
compensation
There are several “purge free” cells. These cells
contain information on how much effect the canister
has on the air/fuel ratio. The purge solenoid is turned
on to shut off the purge flow and the cell is allowed
to register any purge corruption. The monitor looks
at the combination of short and long term fuel control
values to see if the system is in control.
The fuel system is continuously monitored during
each trip once the enabling conditions have been
AFUO094
met. Short term and long term values are multiplied
together. The test fails if the fuel control system
reduces pulse width by 25% long term memory and
7% short term compensation due to a rich condition
or increases pulse width by 25% long term memory
and 12% short term compensation due to a lean
condition.
ENGINE <NON-TURBO> Control System
Misfire Monitor
Background
Misfire is defined by the California Air Resources
Operation
Board as the lack of combustion in a cylinder due
The OBD II misfire monitor uses information
to absence of spark, poor fueling, compression,
vided by the crank position sensor to determine
or any other cause. As a result, the air/fuel mixture
engine rpm and detect slight variations due to engine
will not burn, and during the exhaust stroke, it enters
misfire. Crankshaft rpm is calculated between the
the exhaust system. The raw fuel and excess oxygen
and falling edges of the crankshaft position
adversely affect the fuel system’s feedback
sensor signal.
nism (the oxygen sensor) and can cause permanent
damage to the catalytic convertor.
2 revolutions
120”
60” reference notch
Crank signal
ENGINE <NON-TURBO> Control System
The threshold for determining what amount of rpm
change indicates misfire varies with engine speed
and load. This is required because as engine speed
increases or load decreases, the overall effect of
a single cylinder misfire diminishes due to the mo-
mentum of the crankshaft.
The misfire monitor contains an adaptive feature
that can take into account component wear, sensor
fatigue, and machining tolerances. The PCM notes
rpm variance between cylinders during normal ve-
hicle operation and then uses these figures as a
foundation for calculating the threshold at which
rpm variance is considered to indicate misfire.
NOTE
The PCM requires normal vehicle operation to up-
date its memory. Because the PCM has no reference
for “normal operation”, the PCM considers a misfire
condition to be normal if installed in a vehicle that
is misfiring. The same situation can occur anytime
battery power to a PCM is lost and a vehicle is
misfiring when the controller is reconnected,
The misfire monitor triggers operation of the MIL
in two different operating situations:
1,000 Rev Misfire
Misfire is continuously monitored (in 200 revolution
segments) once the enabling conditions have been
met. If the monitor detects misfire in more than
1.6% of the engine cycles in a 1,000 revolution
period, a temporary fault is set. Freeze frame data
of the operating conditions during the last 200 rpm
of that 1,000 revolution period are stored. As with
other monitors, failure of the misfire monitor on the
next trip matures the code, the MIL illuminates, and
a DTC is stored. It is important to note that two
trips are required for the code to mature. Continued
misfiring during the initial trip does not illuminate
the MIL.
200 Rev Misfire
If the monitor detects misfire in more than 15%
of cylinder firing opportunities during any 200 revolu-
tion monitored segment, the MIL immediately begins
flashing, a DTC is started, and the freeze frame
is stored with information regarding the conditions
at which the misfire occurred. This indicates that
engine misfire has reached the point where damage
to the catalytic converter is likely to occur. The ve-
hicle defaults to open loop operation to prevent the
adaptive fuel controls from dumping additional fuel
into the cylinders and accelerating damage to the
catalyst. Driving the vehicle with the MIL flashing
is not recommended.
Once out of the operating condition where the 15%
misfire is occurring, the MIL will stop flashing but
remain illuminated. The vehicle may be driven, but
should be serviced immediately.
Because some misfire is a common occurrence
in a number of driving conditions, MIL illumination
due to misfire may occur when no component has
failed. Try to identify conditions that could trigger
the misfire monitor that are not related to component
failure. Example of these are included below:
l
Damp ignition system components
l
Low fuel/running out of gas (monitor triggers
as engine sputters)
l
“Lugging” the engine
l
Pulling heavy loads
l
Low quality gasoline
In addition, the following situations could mistakenly
trigger the MIL:
l
Large potholes
l
Extended rough road operation
ENGINE <NON-TURBO> Control System
Exhaust Gas Recirculation Monitor
Background
Exhaust Gas Recirculation (EGR) is a method of
reduce the high cylinder operating temperatures
reducing oxides of nitrogen
emissions by
where
is most likely to occur. Lower combustion
introducing non-combustible exhaust gases into the
chamber temperatures result in lower
combustion chamber. These gases absorb heat and
sions.
Upstream (primary)
sensor
.
Exhaust gas
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