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