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