Contents - Volkspage

4B.2 Fuel system: multi-point injection models
1 General information and
precautions
Genera/ information
1 The Digifant system is a completely
self-contained engine management system,
which controls both the fuel injection and
ignition. This Chapter deals with the fuel
injection system components only - refer to
Chapter 5A for details of the ignition system
components.
2 The fuel injection system comprises a fuel
tank, an electric fuel lift pump an electric fuel
line pump, a fuel filter, fuel supply and return
lines, a throttle body, a fuel rail, a fuel
pressure regulator, four electronic fuel
injectors, and an Electronic Control Unit (ECU)
together with its associated sensors,
actuators and wiring.
3 The fuel pump delivers a constant supply of
fuel through a cartridge filter to the fuel rail, at
a slightly higher pressure than required - the
fuel pressure regulator maintains constant fuel
pressure to the fuel injectors and returns
excess fuel to the tank via the return line. This
constant flow system also helps to reduce fuel
temperature and prevents vaporisation.
4 The fuel injectors are electromagnetically
operated pintle valves, which are opened and
closed many times per second by an
Electronic Control Unit (ECU). The ECU
calculates the injection timing and duration
according to engine speed, crankshaft
position, throttle position and rate of opening,
inlet air volume flow rate, inlet air temperature,
coolant temperature, road speed and exhaust
gas oxygen content information, received
from sensors mounted on and around the
engine.
5 Inlet air is drawn into the engine through the
air cleaner assembly, which contains a
renewable paper filter element. An air flow
meter is also integrated into the air cleaner
assembly. This sensor is part of the engine
management system and monitors engine
load by measuring the volume flow rate of the
inlet air entering the engine. It sends this
information to the ECU in the form of a varying
electrical signal.
6 On engine code 3F, the inlet air
temperature is regulated by a vacuum
operated valve mounted in the air cleaner,
which blends air at an ambient temperature
with hot air, drawn over the exhaust manifold.
7 On engine code PY, inlet air is supplied to
the engine at a positive pressure by a belt
driven supercharger. The filtered air is drawn
into the supercharger, where it is compressed
to a maximum pressure of around 0.5 bar,
depending on engine speed and load. An
intercooler, mounted in the air flow through
the engine bay, cools the inlet air, increasing
its density and maximising the amount of
oxygen entering the combustion chambers.
This helps to keep combustion temperatures
down and increases engine power and
torque. An air bypass valve directs excess air
back to the supercharger inlet when the
engine load is low, i.e. on part throttle or
overrun.
8 Idle speed control is achieved by means of
an auxiliary air valve, mounted in parallel with
the throttle body, which allows a variable
amount of air to bypass the throttle disc and
enter the engine. The ignition system provides
idle speed stability by increasing and
decreasing the engines torque through small
alterations to the ignition timing.
9 The exhaust gas oxygen content is
constantly monitored by the ECU via the
Lambda sensor, which is mounted in the
exhaust pipe. The ECU then uses this
information to modify the injection timing and
duration to maintain the optimum air:fuel ratio.
In addition, certain models are fitted with an
exhaust catalyst - see Chapter 4C for details.
10 The ECU also controls the operation of
the activated charcoal filter evaporative loss
system - refer to Chapter 4C for further
details.
11 It should be noted that comprehensive
fault diagnosis of the engine management
system is only possible with dedicated
electronic test equipment. Problems with the
systems operation should therefore be
referred to a VW dealer for assessment. Once
the fault has been identified, the
removal/refitting sequences detailed in the
following Sections will then allow the
appropriate component(s) to be renewed as
required.
Note: Throughout this Chapter, vehicles are
frequently referred to by their engine code,
rather than by engine capacity - refer to
Chapter 2A for engine code listings.
Precautions
A
Warning: Petrol is extremely
flammable - great care must be
! taken when working on any part
of the fuel system. Do not
smoke, or allow any naked flames or
uncovered light bulbs near the work area.
Note that gas powered domestic
appliances with pilot flames, such as
heaters, boilers and tumble-dryers, also
present a fire hazard - bear this in mind if
you are working in an area where such
appliances are present. Always keep a
suitable fire extinguisher to hand and
familiarise yourself with its operation
before starting work. Wear eye protection
when working on fuel systems and wash
off any fuel spilt on bare skin immediately
with soap and water. Note that fuel vapour
is just as dangerous as liquid fuel; a vessel
that has been emptied of liquid fuel will
still contain vapour and can be potentially
explosive.
12 Many of the operations described in this
Chapter involve the disconnection of fuel
lines, which may cause an amount of fuel
spillage. Before commencing work, refer to
the above Warning and the information in
Safety First ! at the beginning of this manual.
13 Residual fuel pressure always remains in
the fuel system, long after the engine has
been switched off. This pressure must be
relieved in a controlled manner before work
can commence on any component in the fuel
system - refer to Section 8 for details.
14 When working with fuel system
components, pay particular attention to
cleanliness - dirt entering the fuel system may
cause internal blockages which could lead to
running problems later.
15 In the interests of personal safety and
equipment protection, many of the
procedures in this Chapter suggest that the
negative cable be removed from the battery
terminal. This firstly eliminates the possibility
of accidental short circuits being caused as
the vehicle is being worked upon, and
secondly prevents damage to electronic
components (eg sensors, actuators, ECU’s)
which are particularly sensitive to the power
surges caused by disconnection or
reconnection of the wiring harness whilst they
are still “live”.
16 It should be noted, however, that the
engine management system described in this
Chapter (and Chapter 5B) has a “learning”
capability, that allows the system to adapt to
the engines running characterteristics as it
wears with normal use. This “learnt”
information is lost when the battery is
disconnected - on reconnection the system
will then take a short period of time to “relearn”
the engines’ characteristics. This may
be manifested (temporarily) as rough idling,
reduced throttle response and possibly an
increase in fuel consumption, until the system
re-adapts. The re-adaptation time will depend
on how often the vehicle is used and the
driving conditions encountered.
Removal
1 With reference to Chapter 1, carry out the
following:
a) Slacken the worm drive clips and
disconnect the air ducting from the air
flow meter.
b) Disconnect the wiring harness from the air
flow meter at the multiway connector.
c) Prise open the retaining clips and lift the
top cover and air flow meter from the air
cleaner.
Caution: The air flow meter is a
delicate component - handle it
carefully.
d) Remove the air cleaner filter element.
2 Detach the cold inlet air ducting from the
cowling above the radiator (see illustration)
3 Detach the warm inlet air ducting from the