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FUEL INJECTION SYSTEM - BOSCH AFC
1988 Toyota Celica
1988 FUEL INJECTION
Bosch AFC
Toyota; Camry, Celica, Corolla, Cressida, Land Cruiser, MR2,
Pickup, Supra, Van, 4Runner
DESCRIPTION
NOTE: For specifications on Throttle Position Sensor (TPS), idle
speed and mixture, and fuel pump output volume, see
appropriate article in TUNE-UP PROCEDURES section.
The Bosch AFC (L-Jetronic) fuel injection system is used on
all models. However, variations may exist between model applications.
This article covers the Bosch AFC system in general, with
manufacturers' differences noted under SPECIAL FEATURES. For computer
control information, see the appropriate article in 1988 COMPUTERIZED
ENGINE CONTROLS section.
The Bosch Airflow Controlled (AFC) fuel injection system is
an electronically controlled system operated by incoming airflow. Some
vehicles are equipped with a potentiometer to measure incoming air
flow, while other vehicles use a hot wire type airflow sensor.
The AFC fuel injection system also contains a feedback system
which measures oxygen content of exhaust gases and maintains the
air/fuel ratio at about 14.7:1.
The fuel injection system consists of an electric fuel pump,
fuel pressure regulator, fuel damper, fuel injectors, cold start
injector, Electronic Control Unit (ECU), and airflow meter. In
addition, an air temperature sensor, throttle position sensor, coolant
temperature sensor, oxygen sensor, catalytic converter, auxiliary air
valve, idle speed control valve, throttle body, and electrical relays
are used.
NOTE: Not all models use all components.
OPERATION
ELECTRIC FUEL PUMP(S)
Fuel under pressure from electric fuel pump flows through a
fuel damper, fuel filter, injector fuel rail and fuel pressure
regulator. Fuel pump(s) may be located on frame rail, in fuel tank or
both. Electrical power for fuel pump operation during cranking mode is
provided from starter relay via the fuel pump relay (if equipped) and
ECU.
FUEL PRESSURE REGULATOR
The pressure regulator is a sealed unit which is divided by a
diaphragm into 2 chambers (fuel and spring chambers). The fuel chamber
receives fuel through the inlet side (both sides on Subaru 1.8L) from
the injector fuel rail. The spring chamber is connected to intake
manifold vacuum.
At idle, intake manifold vacuum is high. The diaphragm is
pulled down by intake manifold vacuum. Any excessive fuel is returned
to the fuel tank. As the throttle is depressed, intake manifold vacuum
decreases. The regulator spring overcomes manifold vacuum increasing
fuel pressure.
FUEL INJECTORS
A fuel rail links the fuel pressure regulator with the fuel
injectors. Each cylinder is provided with a solenoid-operated injector
which sprays fuel toward the back of each intake valve.
ELECTRONIC CONTROL UNIT (ECU)
All components of the control system are electrically
connected to the ECU. The ECU is a pre-programmed computer which
receives and interprets data from various sensors to calculate the
amount of fuel required by the engine to maintain efficiency with
minimum exhaust emissions. The oxygen sensor informs the ECU of oxygen
content of exhaust gases and the ECU constantly adjusts the air/fuel
ratio by controlling the injector "on" time.
An automatic function of the ECU is to provide fuel
enrichment whenever engine is cranked, regardless of engine
temperature. This is activated by a direct electrical connection from
the starter circuit to the ECU (most models). The ECU is a sealed
unit, and no service is required.
AIRFLOW METER
Hot Wire Type
The airflow meter continually measures temperature, amount,
density, and speed of air entering engine intake system. The meter
consists of a platinum wire filament located within intake air stream.
The wire filament is kept at a constant temperature above
that of air entering engine regardless of composition of air entering
engine. The airflow meter sends a temperature related signal to be
processed by the ECU. See Fig. 1.
Fig. 1: Hot Wire Airflow Meter (Typical)
Potentiometer Type
This airflow meter uses a movable vane connected to a
potentiometer. As air entering the engine moves the vane, the
potentiometer is moved informing the ECU on the amount of air entering
the engine. Some potentiometer airflow meters use an air temperature
sensor located inside the airflow meter air passage. See Fig. 2.
Fig. 2: Potentiometer Airflow Meter (Typical)
AIR TEMPERATURE SENSOR
The air temperature sensor is an integral component of the
airflow meter which converts temperature of incoming air into
electrical signals. These signals are received by the ECU and
processed to adjust the amount of fuel delivered by the injectors. The
air temperature sensor is not serviceable.
THROTTLE POSITION SENSOR (TPS)
A contact-type TPS is installed on the throttle chamber. It
converts throttle position into electrical signals to inform ECU of
throttle position. Signals are sent to ECU when throttle is fully open
or at idle. See Fig. 3. Some models send a specific signal to ECU,
depending on throttle angle. The open contacts prevent loss of power
during sudden acceleration/deceleration by signaling ECU of the
required fuel enrichment.
Fig. 3: Contact-Type Throttle Position Sensor (Typical)
COOLANT TEMPERATURE SENSOR
This sensor provides ECU with engine temperature information
relating to warm-up enrichment operation. Some models use a dual-
sensor element which also signals the ignition computer (if equipped).
During warm-up period after a cold engine start, additional
fuel is required to maintain engine performance. As engine temperature
increases, the ECU decreases fuel enrichment until engine reaches
normal operating temperature.
ELECTRICAL RELAYS
The various relays used with the electronic controls of the
AFC injection system control power to injectors, fuel pump, ECU, and
cold start system. The electrical relays may consist of one component
for all relays or a combination of individual relays.
AUXILIARY AIR VALVE
Most models with Bosch AFC fuel injection use an Auxiliary
Air Valve (AAV) to shorten engine warm-up time. The AAV supplies
additional air into the intake system which increases engine RPM
during a cold start.
The AAV consists of an electrically heated bi-metallic strip,
movable disc, and air by-pass channel. The heater coil on the bi-
metallic strip is energized by the fuel pump relay. Control of the
valve is based upon engine temperature; the air by-pass channel is
open when engine is cold and gradually closes as temperature rises. At
predetermined temperatures, air by-pass channel is blocked and
additional airflow stops. See Fig. 4.
Fig. 4: Auxiliary Air Valve (Typical)
COLD START INJECTOR
Most models use a cold start injector which delivers
additional fuel, and a start injector time switch which controls
operation of the cold start injector. The start injector time switch
limits cold start injection to 1-12 seconds, depending upon engine
coolant temperature. When engine coolant temperature rises above a
specified point, bi-metallic contact breaks ground circuit of cold
start injector and cold start enrichment is by-passed.
SPECIAL FEATURES
AIRFLOW METER
All models except Supra, use a potentiometer type airflow
meter incorporating an air temperature sensor. Supra models use an
airflow meter which has a lumimus diode and a photo transistor. The
lumimus diode emits light which reflects off of a mirror to the photo
transistor.
The airflow meter causes incoming air to swirl into a vortex.
When air swirls, it vibrates a leaf spring which has the mirror
attached to it. As the mirror moves, the photo transistor picks up the
movement of reflected light. The airflow meter sends this signal to
the ECU.
FUEL PULSATION DAMPER
All models except Celica, Corolla FX-16, MR2 and Supra turbo
use a fuel pulsation damper. The function of the damper is to
eliminate fuel pressure surges during vehicle operation.
TESTING
* PLEASE READ THIS FIRST *
NOTE: For testing of fuel system components not covered in this
article, refer to appropriate article in 1988 COMPUTERIZED
ENGINE CONTROLS section.
ELECTRONIC CONTROL UNIT (ECU)
Do not attempt to test ECU, as permanent damage could result.
It is possible to check wires for continuity. The ECU should only by
judged faulty after compression is checked, ignition system has been
tested and found problem-free, and all other fuel injection components
have been thoroughly tested (including wiring).
NOTE: AFC electrical systems can be checked by using Electronic
Fuel Injection testers prescribed by the manufacturer.
Instructions for use of testers must be followed carefully
to prevent damage to system.
FUEL INJECTORS & RESISTORS
1) Connect tachometer to engine. Start engine and run at
idle. Remove harness connector from injectors one at a time. Engine
idle speed should drop 100-300 RPM as each injector is disconnected.
If engine idle speed does not drop, check the wiring connector,
injector resistance or injection signal from the computer.
2) Disconnect electrical connector from each injector.
Measure injector resistance. See INJECTOR RESISTANCE SPECIFICATIONS
table. If injector is not to specification, replace injector.
INJECTOR RESISTANCE SPECIFICATIONS TABLE
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