Volkswagen Golf / Golf GTI / Jetta. Manual — part 79

20-39

The pressure retention valve prevents fuel

vapors from being drawn from the tank
when the solenoid valve is open and an
intake manifold vacuum is present. It
therefore ensures that the evacuation of the
EVAP canister has priority.

When no voltage is applied (e.g. wiring open

circuit), the solenoid valve is closed. The
EVAP canister will not purge.

The vacuum pipe from the throttle valve

control module to the EVAP canister has an
additional by-pass which is connected with
the crankcase breather valve. The non-
return valve integrated in the crankcase
breather valve prevents intake air entering
into the crankcase during some partial load
ranges. This improves the overall crankcase
breathing. The by-pass guarantees the
crankcase breathing.

Functional description of leak diagnosis

The EVAP system (including the fuel tank) is

equipped with a leak diagnosis system. The
leak diagnosis system detects leaks in the
breather system.

The diagnosis is based on the pressure

principle and should detect leaks with a
circumference larger than 1 mm.

Evaporative Emissions (EVAP) system

20-40

During the diagnosis the Leak Detection Pump
(LDP) -144- creates a pressure of 30 mbar in the
EVAP system. When the pressure is attained the
pump switches off. If the pressure drops below a
predetermined value the pump switches on
again. The self-diagnosis monitors the switching
periods and sets a code in the Diagnostic
Trouble Code (DTC) memory when the period is
too short.

Observe safety precautions

Page 20-12

.

Observe rules for cleanliness

Page 20-14

Servicing parts of the EVAP system

Page 20

41

.

Assembly overview of EVAP system

Page 20

44

.

Note:

Hose connections are secured with either

spring-type or clamp-type clips.

Always replace clamp-type clips with spring

type clips.

Assembly tool VAS 5024 or hose clip pliers

VAG 1921 are recommended for installing
spring-type clips.

Evaporative Emissions (EVAP) system

20-41

Components
of the EVAP
system,
servicing

Note:

Repair

Manual, 2.8
Liter VR6 4V
Fuel Injection &
Ignition, Engine
Code(s): BDF,
Repair Group
01

Repair

Manual, 2.8
Liter VR6 4V
Fuel Injection &
Ignition, Engine
Code(s): BDF,
Repair Group
01

Checking

EVAP
system for
leaks

Page 20-
47

.

Components

marked with
an * are
checked by
On Board
Diagnostic
(OBD):

Components

marked with
** are
checked by
output
Diagnostic
Test Mode
(DTM):

1 - Breather

line

White

Evaporative Emissions (EVAP) system

Check

seated
securely

Press

together
at front
to
release

To

pressure
retention
valve

Page
20-3

,

item - 7
-

2 - Seal

Replace

if
damaged

Evaporative Emissions (EVAP) system