Mercedes-Benz OM 471 Introduction Into Service Manual page 97

Functions
The fuel pump (3), which has been designed as a gear pump,
supplies fuel to the fuel low pressure circuit.
When the engine is started, the fuel pump (3) draws in the fuel in
the fuel tank (1) and delivers it into the fuel filter module (2) via
the shutoff valve in the fuel feed (15).
The shutoff valve in the fuel feed (15) prevents fuel from escaping
when the fuel line between the fuel tank (1) and the fuel filter
module (2) is disconnected.
The fuel in the fuel filter module (2) first passes into the fuel
prefilter (7), in which large dirt particles (> 100 om) are filtered
out of the fuel that is flowing through it.
A shutoff valve (17) which is located at the fuel inlet of the fuel
prefilter (7) prevents the suction line in the fuel tank (1) from
emptying during long vehicle stop times.
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The water from the water collector can be drained by means of a
mechanical drain valve.
After leaving the water separator (8), the fuel flows through the
fuel filter (9).
Extremely small contamination (> 2 om) is filtered out in the fuel
filter (9), similarly to the water separator (8).
If air has gathered in the water separator (8) housing or the fuel
filter (9) due to replacement of a filter insert, for example, this is
led into the fuel collection chamber (13) via one ventilation bore
each with vent lines (39, 40). The air is then dissipated from there
into the fuel tank (1).
The fuel system must be bled occasionally after changing an air
filter element with the aid of the hand>operated delivery pump
(14). In the case of extensive repair work one can also use the
filling valve (22).
Once the contamination has been filtered out by the fuel filter (9),
the fuel is pumped out of the fuel filter module (2) to the fuel
system high pressure pump (5).
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i If the fuel prefilter (7) is contaminated and the flow of fuel is
therefore inhibited, the fuel is drawn in via the bypass valve (7.1).
The fuel which has been cleaned by the fuel prefilter (7) is led out
of the fuel filter module (2) and enters the fuel pump (3) which
delivers it back into the fuel filter module (2) and into the water
separator (8) that it contains.
i The system pressure in the fuel feed thrust side (B) which can
already be achieved at idle speed is determined by the opening
pressure of the pressure limiting valve (3.1).
The bypass valve (3.2) is installed for the case of external filling of
the fuel system.
The water inside the fuel is separated out in the water separator
(8) and led to the water accumulator. Small dirt particles are also
filtered out due to the construction.
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The fuel is compressed in the fuel system high pressure pump (5)
and then led to the rail (6).
When this occurs, the quantity control valve (Y642) restricts the
amount of fuel to be delivered by the fuel system high pressure
pump (5) to exactly the right quantity.
The remaining fuel is led away via the 2>stage valve (19). The first
stage of the 2>stage valve (19) opens at around 2 bar and provides
the lubrication of the camshaft plain bearings in the fuel system
high pressure pump (5).
The second stage opens at around 4.5 bar and regulates the
system pressure in the thrust side fuel feed (B) to a range between
4.5 and 6.5 bar by leading excess fuel from the 2>stage valve (35)
to the fuel collection chamber (13) via the fuel return line.
If the maximum rail pressure is exceeded, the pressure limiting
valve (20) opens and the excess fuel is led from the pressure
limiting valve (38) to the fuel collection chamber (13) via the fuel
return line.
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