Fuel Pump; Fuel Pump Electrical C I R C U I T - Pontiac Fiero 1988 Service Manual

8E3-C2-8 DRIVEABILITY AND
E M I S S I O N S
- FUEL
I N J E C T I O N ( P O R T )
THROTTLE
VALVE
ENGINE CONTROL MODULE (ECM)
THROTTLE POSITION SENSOR (TPS)
8P0322-SY
Figure C2-11 - Throttle Position Sensor (Typical)
As the throttle valve rotates in response to
movement of the accelerator pedal, the throttle shaft
transfers this rotational movement to the throttle
position sensor (TPS). A potentiometer (variable
resistor) within the TPS assembly changes its
resistance (voltage drop) In proportion to throttle
movement. By applying a reference voltage (5.0 volts)
to the TPS input, a varying voltage (reflecting throttle
position) is available at the TPS output. For example,
the TPS output at engine Idle should be between 0.45
and 0.65 volts. (For further Information on the TPS,
refer to Section "6E3-C1").
F U E L P U M P
The fuel pump Is a roller vane type, high pressure
electric pump, mounted In the fuel tank. Fuel Is
pumped at a positive pressure (above 62 kPa or 9 psi)
from the fuel pump through the in-line filter to the
pressure regulator on the fuel rail assembly. Excess
fuel is returned to the fuel tank through the fuel
return line.
The fuel pump is attached to the fuel gage sender
assembly. A fuel strainer is attached to the fuel pump
inlet line and prevents dirt particles from entering the
fuel line and tends to separate water from the fuel.
Vapor lock problems are reduced when using an
electric pump because the fuel is pushed from the tank
under pressure rather than pulled under vacuum, a
condition that produces vapor.
An inoperative fuel pump would cause a no start
condition. A fuel pump which does not provide enough
pressure can result in poor performance. (See "Fuel
System Pressure Test" procedure).
F U E L P U M P E L E C T R I C A L C I R C U I T
When the key is first turned "ON" without the
engine running, the ECM will turn the fuel pump
relay "ON" for two seconds. This builds up the fuel
pressure quickly. If the engine is not started within
two seconds, the ECM will shut the fuel pump "OFF"
and wait until the engine starts. As soon as the
engine is cranked, the ECM will turn the relay "ON"
and run the fuel pump.
As a backup system to the fuel pump relay, the
fuel pump can also be turned "ON" by the oil pressure
switch. The oil pressure switch is a normally open
switch which closes when oil pressure reaches about
28 kPa (4 psi). If the fuel pump relay fails, the oil
pressure switch will run the fuel pump.
An inoperative fuel pump relay can result in long
cranking times, particularly if the engine is cold. The
oil pressure switch acts as a back-up to the relay and
will turn "ON" the fuel pump as soon as oil pressure
reaches about 4 psi.
An Inoperative fuel pump would cause a no start
condition. A fuel pump which does not provide enough
pressure can result in poor performance.
DIAGNOSIS
F U E L C O N T R O L S Y S T E M
Some failures of this system will result in an
"Engine Cranks But Won't Run." If this condition
exists, See CHART A-3. This chart will determine If
the problem is caused by the ignition system, ECM or
fuel pump circuit. If it's determined to be a fuel
problem, CHART A-7 will be used. This includes the
Injectors, pressure regulator, fuel pump and fuel pump
relay. The fuel system wiring schematic is covered on
the facing page of Code CHART 54.
If a malfunction occurs in the fuel control system,
it usually results In either a rich or lean exhaust
condition. This condition is sensed by the oxygen
sensor and the ECM will change the fuel calculation
(Injector pulse width) based on O2 sensor reading. The
change made to the fuel calculation will be Indicated
by a change in the block learn values which can be
monitored by a "Scan" tool. The normal block learn
values are around 128 and if the O2 sensor is sensing a
lean condition the ECM will add fuel and this will
result In a block learn value above 128. Some
variations In block learn values are normal because
all engines are not exactly the same. However, if the
block learn values are ± 10 counts from 128 a system
problem exists. If the block learn values are greater
than 138 see Code 44 for Items which can cause a lean
system.