Diesel engine management systems common rail STD cg8258sen

Technical Service Training

Diesel Injection and Engine
Management Systems

Common Rail Systems

CG 8258/S en 01/2008
TC304 3 060H


To the best of our knowledge, the illustrations, technical information, data and descriptions in this issue were correct at the time
of going to print. The right to change prices, specifications, equipment and maintenance instructions at any time without notice
is reserved as part of FORD policy of continuous development and improvement for the benefit of our customers.
No part of this publication may be reproduced, stored in a data processing system or transmitted in any form, electronic,
mechanical, photocopy, recording, translation or by any other means without prior permission of Ford-Werke GmbH. No liability
can be accepted for any inaccuracies in this publication, although every possible care has been taken to make it as complete and
accurate as possible.
Copyright ©2008

Ford-Werke GmbH
Service training programs D-F/GT1 (GB)


Preface

More stringent exhaust and noise emission standards and calls for lower fuel consumption continue to place new
demands on the fuel injection and engine management systems of diesel engines.
In order to satisfy these requirements, the injection system must inject the fuel at high pressure into the combustion
chamber to provide good mixture preparation and, at the same time, meter the injected fuel quantity with the highest
possible accuracy. The common rail system offers good potential for development, which is of particular significance

both now and in the future. By separating the pressure generation process from the injection process, the optimum
injection pressure is always available for the injection process, regardless of engine speed.
Modern engine management systems ensure that the fuel injection timing and injected fuel quantity are exactly
calculated and delivered to the engine cylinders by the fuel injectors.
The following common rail systems are currently installed in Ford vehicles:
–

Bosch common rail system,

–

Siemens common rail system,

–

Denso common rail system.

Another big step towards achieving cleanliness in diesel engines is the newly developed diesel particulate filter
system. This system helps reduce micro-fine diesel particulates by up to 99%.
Completion of the eLearning program "Diesel Fuel Injection and Engine Management Systems" is a prerequisite
for the study of this Student Information.
This Student Information is divided into lessons. At the end of each lesson there is a set of test questions that are
designed to monitor the student's progress. The solutions to these test questions can be found at the end of the
Student Information.

Please remember that our training literature has been prepared for FORD TRAINING PURPOSES only. Repairs
and adjustments MUST always be carried out according to the instructions and specifications in the workshop
literature. Please make full use of the training offered by Ford Technical Training Courses to gain extensive
knowledge of both theory and practice.

Service Training (G1008812)

1


Table of Contents

PAGE

Preface...............................................................................................................................

1

Lesson 1 – General Information
Overview of the systems...................................................................................................................................................

6

Introduction.......................................................................................................................................................................

10

Injection characteristics.....................................................................................................................................................

11

Torque................................................................................................................................................................................

13

Emission Standard IV with or without DPF......................................................................................................................

13

Cleanliness when working on the common rail system....................................................................................................

13

Test questions................................................................................................................................................

14

Lesson 2 – Fuel System
Overview...........................................................................................................................................................................

15

Low-pressure system....................................................................................................................................

19

General..............................................................................................................................................................................

19

Bosch common rail system...........................................................................................................................

20

Fuel filter...........................................................................................................................................................................

20

Overview of the high-pressure system..............................................................................................................................

24

Fuel pump..........................................................................................................................................................................

26

Fuel rail (common rail)......................................................................................................................................................

32

High-pressure fuel lines.....................................................................................................................................................

32

Fuel injectors (general)......................................................................................................................................................

32

Solenoid valve-controlled fuel injectors............................................................................................................................

33

Piezo-controlled fuel injectors...........................................................................................................................................

36

Siemens common rail system.......................................................................................................................

41

Fuel filter...........................................................................................................................................................................

41

Overview of the high-pressure system..............................................................................................................................

42

Fuel pump..........................................................................................................................................................................

43

Fuel rail and high-pressure fuel lines................................................................................................................................

46

Fuel injectors.....................................................................................................................................................................

47

Denso common rail system...........................................................................................................................

52

Fuel filter...........................................................................................................................................................................

52

Overview of the high-pressure system..............................................................................................................................

53

Fuel pump..........................................................................................................................................................................

54

Fuel rail and high-pressure fuel lines................................................................................................................................

57

Fuel injectors.....................................................................................................................................................................

58

Test questions................................................................................................................................................

61

2

Service Training


Table of Contents

Lesson 3 – Powertrain Control Module (PCM)
General..............................................................................................................................................................................

62

Input signals......................................................................................................................................................................

62

Output signals....................................................................................................................................................................

62

Diagnosis...........................................................................................................................................................................

63

PCM and peripherals...................................................................................................................................

64

Bosch common rail system................................................................................................................................................

64

Siemens common rail system............................................................................................................................................

68

Denso common rail system...............................................................................................................................................

72

Strategies.......................................................................................................................................................

74

Idle speed control..............................................................................................................................................................

74

Fuel metering calculations.................................................................................................................................................

74

Smooth-running control (cylinder balancing)...................................................................................................................

76

External intervention into the injected fuel quantity.........................................................................................................

76

Controlling fuel injection..................................................................................................................................................

77

Controlling the fuel pressure.............................................................................................................................................

78

EGR system.......................................................................................................................................................................

80

Boost pressure control.......................................................................................................................................................

82

EOBD.............................................................................................................................................................

84

General..............................................................................................................................................................................

84

Fault logging and storing...................................................................................................................................................

85

Test questions................................................................................................................................................

86

Lesson 4 – Sensors
Introduction.......................................................................................................................................................................

87

CKP sensor........................................................................................................................................................................

87

CMP sensor.......................................................................................................................................................................

89

MAP sensor.......................................................................................................................................................................

90

IAT sensor..........................................................................................................................................................................

91

MAPT sensor.....................................................................................................................................................................

91

BARO sensor.....................................................................................................................................................................

92

ECT sensor........................................................................................................................................................................

92

CHT sensor (Kent and Puma diesel engines only)............................................................................................................

94

Combined IAT sensor and MAF sensor............................................................................................................................

96

HO2S.................................................................................................................................................................................

97

Turbocharger position sensor (certain versions only).......................................................................................................

98

Vehicle speed signal..........................................................................................................................................................

99

APP sensor........................................................................................................................................................................

100

Service Training

3


Table of Contents

Fuel temperature sensor.....................................................................................................................................................

101

Fuel pressure sensor..........................................................................................................................................................

102

Engine oil level sensor (2.4L/3.2L Duratorq-TDCi (Puma) diesel engine)......................................................................

103

Engine oil level sensor (2.2L Duratorq-TDCi (DW) diesel engine).................................................................................

105

Oil pressure switch............................................................................................................................................................

107

Stoplamp switch/BPP switch.............................................................................................................................................

107

CPP switch........................................................................................................................................................................

108

Test questions................................................................................................................................................

109

Lesson 5 – Actuators
Fuel metering valve...........................................................................................................................................................

110

Fuel pressure regulator......................................................................................................................................................

112

Fuel injectors (solenoid valve-controlled).........................................................................................................................

114

Fuel injectors (piezo-controlled).......................................................................................................................................

116

EGR valve.........................................................................................................................................................................

117

Wastegate control valve (vacuum-controlled systems).....................................................................................................

119

Intake manifold flap and intake manifold flap solenoid valve (vacuum-controlled systems)...........................................

120

Intake manifold flap actuator motor (1.6L Duratorq-TDCi (DV) diesel engine, Emission Standard IV)........................

121

Turbocharger variable vane electrical actuator..................................................................................................................

123

Electric fuel pump (2.2L Duratorq-TDCi (DW) diesel engine only)................................................................................

126

Test questions................................................................................................................................................

127

Lesson 6 – Engine Emission Control
Introduction...................................................................................................................................................

128

Pollutant emissions reduction............................................................................................................................................

128

DPF (general)....................................................................................................................................................................

128

Regeneration of the DPF (general)....................................................................................................................................

129

DPF with fuel additive system.....................................................................................................................

131

Component overview.........................................................................................................................................................

131

DPF....................................................................................................................................................................................

133

Charge air cooler bypass...................................................................................................................................................

134

Fuel additive system – general..........................................................................................................................................

136

Components of the fuel additive system...........................................................................................................................

137

Component overview – system control.............................................................................................................................

139

PCM...................................................................................................................................................................................

141

Fuel additive control unit...................................................................................................................................................

141

Fuel additive pump unit.....................................................................................................................................................

142

Tank flap switch................................................................................................................................................................

143

Exhaust gas temperature sensor(s)....................................................................................................................................

144

4

Service Training


Table of Contents

DPF differential pressure sensor ......................................................................................................................................

145

Intake manifold flap actuator motors (Bosch system only)..............................................................................................

146

Charge air cooler bypass flap actuator motor (Bosch system only)..................................................................................

146

Intake manifold flap and charge air cooler bypass flap solenoid valves (Siemens system)..............................................

148

Coated diesel particulate filter (DPF).........................................................................................................

149

Overview of the DPF.........................................................................................................................................................

149

Passive regeneration..........................................................................................................................................................

149

Active regeneration............................................................................................................................................................

150

Notes on the oil change interval........................................................................................................................................

150

DPF regeneration indicator (2006.5 Transit only).............................................................................................................

151

Intake manifold flap..........................................................................................................................................................

151

Components of the engine emission control system.........................................................................................................

152

Exhaust gas temperature sensor(s)....................................................................................................................................

153

DPF differential pressure sensor ......................................................................................................................................

153

Intake manifold flap position sensor (vacuum-controlled systems)..................................................................................

154

Intake manifold flap unit...................................................................................................................................................

155

Fuel vaporiser system...................................................................................................................................

156

General..............................................................................................................................................................................

156

Fuel vaporiser system fuel pump.......................................................................................................................................

157

Fuel vaporiser....................................................................................................................................................................

158

Test questions................................................................................................................................................

159

Answers to the test questions...........................................................................................

160

List of Abbreviations........................................................................................................

161

Service Training

5


Lesson 1 – General Information

Overview of the systems
Bosch common rail system with "solenoid valve-controlled" fuel injectors

6

(G1009902)

Service Training


Lesson 1 – General Information

Bosch common rail system with "piezo-controlled" fuel injectors

Service Training (G1009902)

7


Lesson 1 – General Information

Siemens common rail system

8

(G1009902)

Service Training


Lesson 1 – General Information

Denso common rail system

Assignment of the common rail systems to the engines
Engine
1.4L Duratorq-TDCi (DV)
diesel
1.6L Duratorq-TDCi (DV)
diesel
1.8L Duratorq-TDCi (Kent)
diesel
2.0L Duratorq-TDCi (DW)
diesel

Service Training (G1009902)

Bosch

Siemens
X

Denso

X
X*
X

9


Lesson 1 – General Information

Engine
2.2L Duratorq-TDCi (DW)
diesel
2.2L Duratorq-TDCi
(Puma) diesel
2.4L Duratorq-TDCi
(Puma) diesel
3.2L Duratorq-TDCi
(Puma) diesel

Bosch
X

Siemens

Denso

X*
X*
X

* Older versions are equipped with the Delphi common rail system. The Delphi common rail system is not part of this Student Information.

Introduction
Increasingly higher demands are being placed on modern
diesel engines. The focus today is not only on exhaust
emissions but also on increasing environmental
awareness and the demand for increasingly better
economy and enhanced driving comfort.
This requires the use of complex injection systems, high
injection pressures and accurate fuel metering by fully
electronically-controlled systems.
The high injection pressures convert the fuel, via the
injector nozzle, into tiny droplets, which, again due to
the high pressure, can then be optimally distributed in
the combustion chamber. This results in less unburned
HC (Hydrocarbon), less CO (Carbon Monoxide) and
fewer diesel exhaust particulates being produced in the
subsequent combustion stage.
In addition, the optimised mixture formation reduces
fuel consumption.
Diesel knock caused by the combustion process of an
engine with direct injection is significantly reduced by
means of additional pilot injection. NOX (Oxides Of
Nitrogen) emissions can also be reduced by using this
method.

• load-independent idle speed control,
• closed-loop EGR (Exhaust Gas Recirculation),
• low injection timing and injected fuel quantity

tolerances and high degree of precision over the
entire service life,
• possibility of interaction with other systems such as
stability assist, PATS (Passive Anti-Theft System),
• comprehensive diagnostic facilities,
• substitute strategies in the event of faults.
The common rail injection system has a large range
of features to meet these demands.
In common rail injection systems, pressure generation
is separate from the injection process. The injection
pressure is generated independently of engine speed and
injected fuel quantity.
The common rail injection system consists of a
high-pressure pump and a fuel rail (fuel
accumulator/rail). This fuel rail offers constant pressure
for distributing fuel to the electrically-controlled fuel
injectors.

• high injection pressures,

With this type of diesel injection or engine management,
the driver has no direct influence on the injected fuel
quantity. For example, there is no mechanical connection
between the accelerator pedal and the injection pump.
The injected fuel quantity is determined by various
parameters. These include:

• shaping of injection timing characteristics,

• driver demand (accelerator pedal position),

• multiple injections,

• operating condition,

• values for injected fuel quantity, start of injection
and boost pressure adapted to every operating
condition,

• engine temperature,

In particular, the demands placed upon the injection
system and its regulation are as follows for modern
diesel engines:

10

• effects on exhaust emissions,

(G1009902)

Service Training


Lesson 1 – General Information

• prevention of engine and transmission damage,
• faults in the system.
Using these parameters, the injected fuel quantity is
calculated in the PCM (Powertrain Control Module)

and fuel injection timing and injection pressure can be
varied.

Simple main injection
Needle lift of the fuel injector nozzle and pressure curve
in a cylinder without pilot injection

The fuel is metered fully electronically by the PCM.
The fully electronic diesel engine management system
features a comprehensive fail-safe concept (integrated
in the PCM software). It detects any deviations and
malfunctions and initiates corresponding actions
depending on the resulting effects (e.g. limiting the
power output by reducing the quantity of fuel).

Injection characteristics
As already mentioned at the beginning of the lesson,
the exhaust emissions and fuel consumption of an
engine are of great significance. These factors can only
be minimised through precise operation of the injection
system and comprehensive engine management
strategies.

1

Combustion pressure in the cylinder

2

Needle lift

Consequently, the following requirements must be met
by the common rail system:

3

TDC (Top Dead Center)

4

Needle lift with simple main injection

• The injection timing must be exact. Even small
variations have a significant effect on fuel
consumption, exhaust emissions and combustion
noise.

5

Crankshaft angle

• The fuel injection pressure is independently adapted
to all operating conditions.
• Injection must be reliably terminated. Calculation
of the injected fuel quantity and the injection timing
is precisely adapted to the mechanical components
of the injection system. Uncontrolled fuel dribble
(e.g. caused by a defective fuel injector) results in
increased exhaust emissions and increased fuel
consumption.

In the case of diesel engines with a distributor-type
fuel injection pump (e.g. in the Transit 2000.5), fuel
injection takes place via simple main injection.
The fuel is then injected mechanically into the
combustion chamber by the injector nozzles in two
seamlessly integrated stages (two-spring nozzle carrier
principle).
In the pressure curve, the combustion pressure increases
only slightly in the phase before TDC corresponding to
compression, but increases very sharply at the start of
combustion.
The sharp pressure rise intensifies the combustion noise.

Service Training (G1009902)

11


Lesson 1 – General Information

Pilot injection
Needle lift of the fuel injector nozzle and pressure curve
in a cylinder with pilot injection

Note: As pressure generation and injection are separate
in common rail systems, it is possible to considerably
enhance the range for pilot injection. This has led to a
significant improvement in the running smoothness of
the engine.

With modern fuel injectors, it is also possible to work
with multiple pilot injections. The greater the number
of pilot injections, the lower the noise emissions.

Post-injection (vehicles with DPF (Diesel
Particulate Filter) system)
Needle lift of the injector nozzle with pilot and
post-injection

1

Combustion pressure in the cylinder

2

Needle lift

3

TDC

4

Needle lift with pilot injection

5

Needle lift with main injection

6

Crankshaft angle

In the case of vehicles with a common rail injection
system, electrically-controlled pilot injection occurs
after a set time prior to the main injection event.
Pilot injection means that a small amount of fuel is
injected into the cylinder prior to the main injection.
The small pilot-injection fuel quantity is ignited, heats
up the upper part of the cylinder and thereby brings it
into an optimum temperature range (preconditioning of
the combustion chamber).
This means that the main injection mixture ignites more
quickly and the rise in temperature and combustion
pressure is less abrupt as a result.
Advantage:
• Continuous build-up of combustion pressure,
resulting in reduced combustion noise.
• Reduction of oxides of nitrogen in the exhaust gas.

12

1

Needle lift

2

Pilot injection

3

Crankshaft angle

4

Main injection

5

Advanced post-injection

6

Retarded post-injection

For vehicles with a DPF (Diesel Particulate Filter)
system, two post-injections are employed during the
regeneration process, in addition to the pilot and main
injections, depending on the requirements.
Advanced post-injection is initiated in certain
load/speed ranges immediately after main injection.
Fuel is then injected during the on-going combustion.
The main purpose of this advanced post-injection is to
raise the exhaust gas temperature during the regeneration
process of the DPF. In addition, some of the diesel
particulates produced during regeneration are
after-burned.
Retarded post-injection only occurs shortly before
BDC (Bottom Dead Center) and also serves to raise the

exhaust gas temperature.

(G1009902)

Service Training


Lesson 1 – General Information

In contrast to advanced post-injection, during retarded
post-injection the fuel is not burned, but vaporises due
to the residual heat in the exhaust gas. This exhaust/fuel
mixture is delivered to the exhaust system by the exhaust
stroke.

Measures for the reduction of exhaust emissions inside
the engine include, for example:

In the oxidation catalytic converter, the fuel vapour
reacts with the residual oxygen (above a certain
temperature) and burns. This provides sustained heating
of the oxidation catalytic converter, which supports the
regeneration of the DPF.

• optimisation of the combustion chamber design and
the injection characteristics.

Torque
In general, diesel engines generate a high torque across
a wide engine speed range. This is achieved through

uniformly good cylinder charging (working without a
throttle plate) and high combustion pressure.

Overtorque function
On some vehicle versions, an overtorque function (also
called an overboost function) is used. This makes it
possible to briefly exceed the maximum specified torque
during rapid acceleration (by about 15 to 35 Nm
depending on the calibration).
The short-term torque increase is an advantage when
overtaking, for example.
The vehicle acceleration is calculated based on the
vehicle speed signal and the CKP (Crankshaft Position)
sensor. During acceleration, the PCM activates the
overtorque function in an engine speed range between
1,700 and 3,500 rpm.

• further optimised exhaust gas recirculation by means
of an electrically-controlled EGR system with intake
air restriction,

In addition to the internal engine measures, the second
method employs a DPF system.
The use of the DPF reduces diesel particulate emissions
by up to 99%. This reduction far exceeds the
requirements for the European emission limits of
Emission Standard IV.
It can therefore be deduced that the use of the DPF will
be of great importance with regard to future emission
standards, but is not absolutely necessary for meeting

Emission Standard IV.

Cleanliness when working on the
common rail system
NOTE: Because the components of the high-pressure
fuel system are high-precision machined parts, it is
essential that scrupulous cleanliness is observed when
carrying out any work on the system.
In this regard, refer to the instructions in the current
Service Literature.

Emission Standard IV with or without
DPF
At the time of going to press, Emission Standard IV
applies in Europe.
In the diesel sector, Emission Standard IV is achieved
using two different methods.
One method consists of reducing exhaust emissions by
means of internal engine measures to the extent that
the prescribed limits are met.

Service Training (G1009902)

13


Test questions

Lesson 1 – General Information

Tick the correct answer or fill in the gaps.
1. What is the advantage of the common rail system?
a. The high injection pressures reduce combustion temperatures; exhaust gas recirculation is not required.
b. Pressure generation and injection are separated.
c. The injection pressure is generated as a function of engine speed.
d. Combustion noise is substantially reduced as a result of indirect injection.

2. What is the effect of pilot injection?
a. Pilot injection results in an abrupt build-up of combustion pressure and therefore reduced combustion noise.
b. Pilot injection results in an abrupt build-up of combustion pressure and therefore increased combustion
noise.
c. Pilot injection results in a gradual increase in combustion pressure.
d. Pilot injection only results in a reduction of fuel consumption.

3. Where are post-injections utilised?
a. In vehicles with an electric EGR system.
b. In vehicles with an NOX catalytic converter.
c. In vehicles without a diesel particulate filter system.
d. In vehicles with a diesel particulate filter system.

4. The overtorque function
a. prevents abrupt deceleration when the accelerator pedal is suddenly released at high vehicle speeds.
b. makes it possible to briefly exceed the maximum specified torque when starting the vehicle on a gradient.
c. makes it possible to briefly exceed the maximum specified torque during rapid acceleration.
d. is activated in response to certain malfunctions in the engine management system.

14

(G1009903)

Service Training


Lesson 2 – Fuel System

Overview
Bosch common rail system with "solenoid valve-controlled" fuel injectors

A

Fuel line

1

Fuel pump

B

Run-off line for excess delivered fuel

2

Fuel rail (common rail)

C

High-pressure line

3

Fuel injector

D

Fuel injection line

4

Fuel temperature sensor

E

Fuel return from the fuel pump

5

Fuel return collector pipe

F

Leak-off pipe

6

Fuel filter

G

Fuel return to the fuel tank

Service Training (G1009904)

15


Lesson 2 – Fuel System

7

Fuel tank

8

Fuel pump and sender unit

Bosch common rail system with "piezo-controlled" fuel injectors

A

Fuel return from the fuel pump

2

Fuel rail

B

High-pressure line

3

Fuel injector

C

Fuel injection line

4

Back pressure valve

D

Leak-off pipe

5

Return gateway

E

Fuel return to the fuel tank

6

Fuel tank

F

Fuel line

7

Fuel pump and sender unit

1

Fuel pump

8

Fuel filter

16

(G1009904)

Service Training


Lesson 2 – Fuel System

Siemens common rail system

A

Fuel line

2

Fuel rail (common rail)

B

High-pressure line

3

Fuel injector

C

Fuel injection line

4

Fuel return collector pipe

D

Fuel return from the fuel pump

5

Fuel temperature sensor

E

Leak-off pipe

6

Fuel filter

F

Fuel return to the fuel tank

7

Fuel tank

1

Fuel pump

8

Fuel pump and sender unit

Service Training (G1009904)

17


Lesson 2 – Fuel System

Denso common rail system

A

Fuel return from the fuel pump

2

Fuel rail (common rail)

B

High-pressure line

3

Fuel injector

C

Fuel injection line

4

Pressure relief valve

D

Leak-off pipe

5

T-piece

E

Fuel return to the fuel tank

6

Fuel tank

F

Fuel line

7

Fuel pump and sender unit

1

Fuel pump

8

Fuel filter

18

(G1009904)

Service Training

Lesson 2 – Fuel System

Low-pressure system

General
Function
Fuel is drawn from the fuel tank through the fuel filter
by the transfer pump which is integrated in the fuel
pump.
The fuel pump compresses the fuel and forces it into
the fuel rail.
The fuel pressure required for any given situation is
available for the fuel injectors for each injection process.
Leak-off fuel from the fuel injectors and/or returning
fuel from the fuel pump is fed back into the fuel tank.

Possible causes of faults in fuel lines and the fuel
tank
Fuel lines may be blocked due to foreign bodies or
bending.
In addition, blocked parts and lines of the low-pressure
system can cause air to get into the low-pressure system
on account of the increased vacuum in the system.
Air can also enter the low-pressure system through loose
or leaking line connections.
Faulty valves or lines in the tank ventilation system can
impair the flow of fuel through the low-pressure system.

Effects of faults (low-pressure system contains
air or is blocked)
Poor engine starting when warm or cold.
Irregular idling.
Engine will not start.
Engine starts, but cuts out again immediately afterwards.
Engine has insufficient power.
Note: At a certain residual fuel amount, the PCM causes
the engine to judder. The intention is to draw the driver's
attention to the fact that the vehicle must urgently be
refuelled.
Note for vehicles with EOBD: If the PCM causes the
engine to judder because the fuel tank is empty, the
EOBD (European On-Board Diagnostics) are
deactivated during this phase. This prevents apparent
faults from being displayed.

Service Training (G1009904)

19


Bosch common rail system

Lesson 2 – Fuel System

Fuel filter
System with solenoid valve-controlled fuel
injectors

1

Fuel line to the fuel pump

2

Water drain screw

3

Electric fuel preheater

4

Fuel line connection with fuel tank

The fuel filter clipped onto the transaxle end of the
cylinder head is equipped with an electric fuel heater.
There is a water drain screw in the top section of the
filter housing for draining the filter.
The fuel filter must be drained of water regularly in
accordance with the service intervals.

20

1

Battery junction box

2

Fuel preheater relay

3

Fuse (10A)

4

Fuse (15A)

5

Ground

6

Electric fuel preheater in the fuel filter

7

Ground

The electric bimetallically-controlled fuel preheater
works independently of the PCM.
It is actuated via a fuel preheater relay when the ignition
is switched on (ignition ON). However, the activation
of the heating element is dependent on the current
temperature.
Below a fuel temperature of 0 to –4 °C, the circuit is

closed by the bimetal and the heating element thus
energised.
The bimetal opens the circuit at a fuel temperature from
1 to 5 °C and ends the heating phase.

(G1009904)

Service Training


Lesson 2 – Fuel System

System with piezo-controlled fuel injectors

Bosch common rail system

1

Fuel return (to the fuel tank)

2

Fuel return line (from the fuel pump)

3

Fuel temperature sensor

4

Fuel line (to the fuel pump)

5

Fuel filter water drain screw

6

Water drain line

7

Fuel line (from the fuel tank)

The fuel filter is made of plastic. It is installed on the
front side of the engine on the intake side. A security
shield protects the fuel filter from damage in the event
of a frontal impact.
Located on the fuel filter housing is a water drain screw.
The fuel filter must be drained via this screw in
accordance with the service intervals.
Note:
• Before draining the fuel filter, make sure that the
surrounding components do not come into contact
with the fuel that is drained.
There is a thermo valve integrated in the fuel filter for
preheating the fuel.

Service Training (G1009904)

21


Bosch common rail system

Lesson 2 – Fuel System

How fuel preheating works

A

Fuel return temperature < 10 °C

4

Upper part of the fuel filter

B

Fuel return temperature > 20 °C

5

Fuel return line (from the fuel pump)

1

Fuel return line (to the fuel tank)

6

Thermostat open

2

Fuel return outlet

7

Thermostat closed

3

Bypass (to the fuel filter)

The fuel filter is equipped with a mechanical fuel
preheater.

Fuel return temperature < 10 °C:

There is a spring-loaded thermo valve integrated in the
fuel return in the upper part of the fuel filter. The thermo
valve determines the quantity of fuel that is returned to
the fuel tank or flows directly back into the fuel filter.

• The bypass to the fuel filter is wide open in this state.
The cross section of the fuel return outlet is slightly
open.

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• The thermo valve is in compressed state.

• The majority of the returning fuel flows through the
wide open bypass into the fuel filter. Only a small
part of the returning fuel can flow back to the fuel
tank via the slightly open cross section of the fuel
return outlet.

(G1009904)

Service Training


Lesson 2 – Fuel System

Bosch common rail system

Fuel return temperature > 20 °C:
• The thermo valve expands against the spring force.
• The bypass to the fuel filter is only slightly open in
this state. The cross section of the fuel return outlet
is now wide open.
Fuel return temperature
< 10 °C
> 20 °C

Possible causes of faults

• The majority of the returning fuel flows through the

wide open fuel return outlet. Only a small part of the
returning fuel can flow through the slightly open
bypass to the fuel filter.

Percentage of fuel to the fuel tank Percentage of fuel to the fuel filter
5 - 10 %
90 - 95 %
95 - 100 %
0-5%
Irregular idling.

Fuel filters may be blocked by dirt. Air may also enter
the low-pressure system as a result of leaks in the fuel
filter.

Engine will not start.
Engine starts, but cuts out again immediately afterwards.
Engine has insufficient power.

Effects of faults
Poor starting when the engine is warm or cold.

Service Training (G1009904)

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