Pontiac Fiero 1988 Service Manual page 533

6E2-C2-2 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI)
ratio for starting. This ranges from 1.5:1 at -36°C
(-33°F) to 14.7:1 at 9 4 ° C (201°F) running temperature.
The ECM controls the amount of fuel delivered in
the Starting Mode by changing how long the injector
is turned "ON" and "OFF". This is done by "pulsing"
the injector for very short times.
Clear Flood Mode
If the engine floods, clear it by pushing the
accelerator pedal down all the way. The ECM then
pulses the injector to deliver a 20:1 air/fuel ratio. The
ECM holds this injector rate as long as the throttle
stays wide open, and the engine is below 600 rpm. If
the throttle position becomes less than 80%, the ECM
returns to the Starting Mode.
Run Mode
flTl
TBI UNIT
G O
F U E L PUMP AND
S E N D I N G U N I T
G O
F U E L S U P P L Y L I N E
G O FUEL F I L T E R
M O
1 F U E L R E T U R N L I N E
05 03 84
5S 17 31-6E
Figure C2-2 - Fuel Control System
The RUN mode has two conditions called "Open
Loop" and "Closed Loop".
M O p e n L o o p f
When the engine is first started, and engine is
above 400 rpm, the system goes into "Open Loop"
operation. In "Open Loop", the ECM will ignore the
signal from the oxygen (O2) sensor, and calculates the
air/fuel ratio based on inputs from the coolant
temperature and MAP sensors.
The system will stay in "Open Loop" until the
following conditions are met:
1. The O2 sensor has varying voltage output,
showing that it is hot enough to operate properly.
(This depends on temperature.)
2. The coolant temperature sensor is above a
specified temperature.
3. A specific amount of time has elapsed after
starting the engine.
"Closed Loop"
The specific values for the above conditions vary
with different engines, and are stored in the PROM.
When these conditions are met, the system goes into
"Closed Loop" operation. In "Closed Loop", the ECM
calculates the air/fuel ratio (injector on-time) based on
the signal from the O2 sensor. This allows the air/fuel
ratio to stay very close to 14.7:1.
Acceleration Mode
The ECM looks at rapid changes in throttle
position and manifold pressure, and provides extra
fuel.
Deceleration Mode
When deceleration occurs, the fuel remaining in
the intake manifold can cause excessive emissions and
backfiring. Again, the ECM looks at changes in
throttle position and manifold pressure and reduces
the amount of fuel. When deceleration is very fast, the
ECM can cut off fuel completely for short periods.
Battery Voltage Correction Mode
When battery voltage is low, the ECM can
compensate by:
© Increasing injector on time of fuel delivered
• Increasing the idle rpm
Fuel Cutoff Mode
No fuel is delivered by the injectors when the
ignition is "OFF". This prevents dieseling. Also, fuel
is not delivered if no reference pulses are present,
which means the engine is not running. Fuel cutoff
also occurs at high engine rpm to protect internal
engine components from damage.
FUEL CONTROL SYSTEM
COMPONENTS
The fuel control system is made up of the following
parts:
w Throttle body injection (TBI) unit
» Fuel pump
• Fuel pump relay.