Fuel injection components
7.11.3 Fuel injection components
Battery voltage
Many of the sensors and actuators associated with fuel injection are covered in Chapter 6. Figure 7.29
Figure 7.27 Fuel injection system block diagram
shows those associated with the LH-Jetronic injec- tion system. Starting at the top left and working clockwise the components are as follows.
the system are speed and load. The basic fuelling requirement is determined from these inputs in a
Air flow meter
similar way to the determination of ignition timing. An engine’s fuelling requirements are stored as
The type shown is a hot-wire meter. This allows part of a ROM chip in the ECU. When the ECU has
direct measurement of air mass as temperature determined the ‘look up value’ of the fuel required
compensation is built in. The air quantity helps (injector open time), corrections to this figure can
to determine the fuel required.
be added for battery voltage, temperature, throttle
Electronic control unit (ECU)
change or position and fuel cut off. Figure 7.28 shows an injection system ECU.
This is also referred to as the electronic control Idle speed and fast idle are also generally con-
module (ECM). The circuitry to react to the sen- trolled by the ECU and a suitable actuator. It is
sor signals by controlling the actuators is in the also possible to have a form of closed loop control
ECU. The data is stored in ROM. with electronic fuel injection. This involves a
lambda sensor to monitor exhaust gas oxygen con-
Fuel pump
tent. This allows very accurate control of the mix- Pressurised fuel is supplied to the injectors. Most ture strength, as the oxygen content of the exhaust
pumps work on the centrifugal roller principle. is proportional to the air fuel ratio. The signal
The pump ensures a constant supply of fuel to from the lambda sensor is used to adjust the injec-
the fuel rail. The volume in the rail acts as a swamp
Engine systems 125
Figure 7.29 Injection components in position
operate. The pump must be able to maintain a
Fuel pressure regulator
pressure of about 3 bar. This device is to ensure a constant differential pressure across the injectors. It is a mechanical
Fuel filter
device and has a connection to the inlet manifold. The fuel supplied to the injectors must be free from any contamination or else the injector noz-
Throttle position switch
zle will be damaged or blocked. This is used to supply information as to whether the throttle is at idle, full load or somewhere in
Lambda sensor
between.
The quantity of oxygen in the exhaust, when accurately measured, ensures that the fuel air mix- ture is kept within the lambda window (0.97 to
7.11.4 Fuel mixture calculation
1.03). The quantity of fuel to be injected is determined primarily by the quantity of air drawn into the
Temperature sensor
engine. This is dependent on two factors:
A simple thermistor is used to determine the ● engine speed (rev/min); engine coolant temperature.
● engine load (inlet manifold pressure).
Fuel injectors
This speed load characteristic is held in the ECU memory in ROM lookup tables.
These are simple solenoid operated valves
A sensor connected to the manifold by a pipe designed to operate very quickly and produce a
senses manifold absolute pressure. The sensor finely atomised spray pattern.
is fed with a stabilised 5 V supply and transmits an output voltage according to the pressure. The
Idle or fast idle control actuator
sensor is fitted away from the manifold and hence The rotary actuator is shown here. It is used to
a pipe is required to connect it. The output signal provide extra air for cold fast idle conditions as
varies between about 0.25 V at 0.17 bar to about well as controlling idle speed. It is supplied with
4.75 V at 1.05 bar. The density of air varies with
126 Advanced automotive fault diagnosis MAP sensor on air quantity will be incorrect over
7.12 Diagnostics – fuel
wide temperature variations. An air temperature sensor is used to inform the ECU of the inlet air
injection systems
temperature such that the quantity of fuel injected may be corrected. As the temperature of air
7.12.1 Testing procedure
decreases its density increases and hence the quantity of fuel injected must also be increased. Warning
The other method of sensing engine load is direct Caution/Achtung/Attention – burning fuel can measurement of air intake quantity using a hot-
seriously damage your health! wire meter or a flap type air flow meter.
The following procedure is generic and with a In order to operate the injectors the ECU needs
little adaptation can be applied to any fuel injec- to know, in addition to air pressure, the engine
tion system. Refer to manufacturer’s recommen- speed to determine the injection quantity. The
dations if in any doubt. It is assumed that the same flywheel sensor used by the ignition system
ignition system is operating correctly. Most tests provides this information. All four injectors
are carried out while cranking the engine. operate simultaneously once per engine revolution,
injecting half of the required fuel. This helps to ensure balanced combustion. The start of injec-
7.12.2 Fuel injection fault
tion varies according to ignition timing.
diagnosis table
A basic open period for the injectors is deter- mined by using the ROM information relating to
Symptom
Possible fault
manifold pressure and engine speed. Two correc-
tions are then made, one relative to air tempera- No fuel in the tank!
Engine rotates but
does not start
Air filter dirty or blocked
ture and another depending on whether the engine
Fuel pump not running
is idling, at full or partial load.
No fuel being injected
The ECU then carries out another group of
Difficult to start
Air filter dirty or blocked
corrections, if applicable:
when cold
Fuel system wiring fault Enrichment device not working
● after start enrichment;
Coolant temperature sensor
● operational enrichment;
short circuit
● acceleration enrichment;
Difficult to start
Air filter dirty or blocked
● weakening on deceleration;
when hot
Fuel system wiring fault
● cut off on overrun;
Coolant temperature sensor open ●
reinstatement of injection after cut off; circuit
correction for battery voltage variation. Fuel system contamination
Engine starts but then
stops immediately
Fuel pump or circuit fault (relay)
Under starting conditions the injection period is cal-
Intake system air leak
culated differently. This is determined mostly from
Erratic idle
Air filter blocked
a set figure varied as a function of temperature.
Inlet system air leak
The coolant temperature sensor is a thermistor Incorrect CO setting
Idle air control valve not operating
and is used to provide a signal to the ECU relat-
Fuel injectors not spraying correctly
ing to engine coolant temperature. The ECU can
Misfire through all
Fuel filter blocked
then calculate any corrections to fuel injection
speeds
Fuel pump delivery low
and ignition timing. The operation of this sensor
Fuel tank ventilation system blocked
is the same as the air temperature sensor.
Engine stalls
Idle speed incorrect
The throttle potentiometer is fixed on the
CO setting incorrect
throttle butterfly spindle and informs the ECU of
Fuel filter blocked
throttle position and rate of change of throttle Air filter blocked
Intake air leak
position. The sensor provides information on accel-
Idle control system not working
eration, deceleration and whether the throttle is
Lack of power
Fuel filter blocked
in the full load or idle position. It comprises a
Air filter blocked
variable resistance and a fixed resistance. As is
Low fuel pump delivery
common with many sensors a fixed supply of 5 V
Fuel injectors blocked
is provided and the return signal will vary
Backfires
Fuel system fault (air flow sensor on
approximately between 0 and 5 V. The voltage
some cars)
increases as the throttle is opened. Ignition timing
Engine systems 127
Fuel injection system diagnostic chart Start
Hand and eye checks (loose wires, loose switches and other faults
No
MIL off and
such as filters) – all
system ok? No
connections clean and
Yes
tight. Check battery – must be 70% charged
Yes
Follow route identified by
DTC available?
the DTC – check sensor, actuator or wiring as
No
appropriate
Check fuel pressure at rail (multipoint systems
it will be about 2.5
Yes
No
bar but check)?
Is injector operation ok – (suitable Supply voltage to pump (within
spray pattern or dwell reading across 0.5 V battery – pump or regulator fault
injector supply) if supply is ok)
Check supply circuits from main Check pump relay and circuit
relay (battery volts minimum) (note in mostcases the ECU closes the relay but this may be by passed on cranking)
Continuity of injector wiring (0 to 0.2) Check continuity of windings – (note that many injectors are connected
Ensure all connections (electrical and in parallel) fuel are remade correctly) – return
to earlier stage if necessary Sensor readings and continuity of wiring (0 ohms for the wiring sensors will vary with type)
If no fuel is being injected and all tests so far are ok then suspect ECU
End
7.13 Diesel injection
The timing of a diesel fuel injection pump to an engine is usually done using start of delivery