Mitsubishi Eclipse. Technical Information Manual (1994) - part 16

1-42

ENGINE <NON-TURBO>   Control System

Fuel System Monitor
Background

To control the level of undesirable emissions, the
fuel system must be able to maintain strict control
of the air/fuel ratio. Stoichiometry is the optimum

air/fuel ratio, which is 14.7 to 1. At this point the
best balance between the production of 

 and

CO’s (which drop as the mixture becomes leaner)
and 

 (which increases as the air/fuel mix 

comes leaner) can be found. This is also the point
where the catalytic converter is most efficient at
converting all three gases to less harmful 
pounds. The goal of the PCM is to examine input
information and control outputs to produce a
constant stoichiometric ratio.

Air/fuel ratio

ENGINE <NON-TURBO>

 - Control System

I-43

Operation

The Power-train Control Module (PCM) varies the

pulse width of the fuel injectors to provide precise

control of the air/fuel mixture. Wider pulse widths
increase the volume of fuel delivered to the cylinders.
The PCM uses the input from a number of sensors

in its attempt to reach and maintain this air/fuel
ratio. Manifold Absolute Pressure (MAP), and the

 sensor have the greatest influence (authority)

over injector pulse width. Other inputs such as the
Throttle Position Sensor (TPS), engine RPM, Engine

Upstream 
sensor

Coolant Temperature (ECT) Sensor, Intake Air Tem-
perature 

 Sensor. Vehicle Speed Sensor (VSS)

and battery voltage all have varying levels of influ-
ence on pulse width, depending on the circum-
stances.

As noted earlier, the system uses two oxygen sen-
sors both of which monitor the oxygen content of
the combustion byproducts on their way out of the
engine as exhaust. Only the upstream sensor has
authority over fuel injector pulse width.

Downstream 
sensor

Exhaust gas

If a large amount of oxygen remains following the

combustion process, this sensor produces a low
voltage. This indicates a lean condition caused by
a ratio greater than stoichiometric. Little oxygen

in the exhaust allows the sensor to produce a higher

voltage, indicating a rich condition where the air/fuel

ratio is less than 

Upstream 

 sensor feedback to the PCM is used

to fine tune injector pulse width to maintain 

metric and meet emission standards. It can increase
or decrease injector pulse width by as much as

50%. Pulse width is calculated from the data 
plied from the MAP, TPS, ECT, 

 upstream 

sensor, battery voltage, RPM and VSS.

ENGINE <NON-TURBO>   Control Svstem

To control air/fuel ratio feedback, the PCM uses

short term correction and long term memory. Before
the PCM can alter the programmed injector pulse
width, it must enter closed loop operation. The re-
quirements for closed loop operation are listed be-
low:

l

 timers have timed out -following the START

to RUN transfer (The length of these timers

varies with engine temperature)

l 

Engine temperature exceeds 

l 

 sensor is in the 

 mode

   41 sec.

   35 sec.
   22 sec.

13) are used only during idle. Each cell represents

   11 sec.

a manifold pressure and rpm range and can be

Once in the closed loop control, the feedback sys-
tems begin to operate. Short term memory works
with the long term memory, which is broken down

accessed with the MUT-II diagnostic scan tool.

into 14 different cells. Two of these cells (12 and

Long Term Memory Cells

ABOVE

2048

RPM

BELOW

Map Voltage

1

3

5

Purge

Free*

0

2

Purge

Free*

4

7

9

11

10

1.38

2.0

2.64

3.26

3.9

13

Idle

Neutral
Manual

Transaxle

Purge

Free*

*PCM disables the canister purge function in these cells to gather data used for purge control and diagnostics.

For example:
If the PCM were in cell 3, the MAP voltage must
be between 1.38 and 

 and engine rpm must

be greater than 2048. This is the cell that the control
system would update.

ENGINE <NON-TURBO>   Control System

If the oxygen sensor registers a rich or lean condition

while driving in this cell, the cell will require updating
to aid in fuel control. The short term correction is

used first. It starts increasing pulse width quickly
(kick), then ramps up slowly. Each control is in in-
verse relation to the signal sent from the 

 sensor.

For example:

The 

 sensor switches lean to rich. Short term

compensation kicks in lean, then ramps lean until
the 

 sensor switches lean. At this point short

term compensation reverses the process.

 

 ratio 

 .

If the oxygen sensor shows lean, the short term

compensation goes rich and multiplies the pulse
width from long term memory in that cell by an
amount greater than 1. If the sensor shows rich,
the short term compensation drives the pulse width

narrower by multiplying by a number less than 1
(perhaps 0.97). The short term compensation can
multiply pulse width by as much as 1.25 or as little
as 0.75 to compensate for lean or rich conditions.

In this way, the short term compensation can in-

crease pulse width by up to 25% (by multiplying
by 1.25) or decrease pulse width by up to 25%

(by multiplying by 0.75).

Example:
Pulse width 0.05 x 1.25   0.0625 (increase of 25%)
Pulse width 0.05 x 0.75   0.0375 (decrease of 25%)