Discovery 2. Manual — part 55

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-15

Evaporative Emission Control System

The evaporation emission control (EVAP) system is used to reduce the level of hydrocarbons emitted into the
atmosphere from the fuel system. The system comprises an EVAP canister which stores the hydrocarbons from the
fuel tank, pressure valves, vent lines and a purge control solenoid valve.

Fuel vapour is stored in the canister until it is ready to be purged to the inlet manifold under the control of the Engine
Control Module (ECM).

A two-way valve is included in the vent line between the fuel tank and the EVAP canister in all markets except NAS.

A fuel vapour separator is fitted next to the fuel filler neck, the construction is different between NAS and ROW
vehicles; the liquid vapour separator (LVS) on NAS vehicles is an L-shaped metal tube and for all other markets it is
an integral part of the moulded plastic filler neck.

+

FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

NAS vehicles have stainless steel filler necks whilst all other markets use moulded plastic filler necks. On NAS fillers,
a valve closes the roll-over valve (ROV) vent line when the fuel filler cap is removed; for all other markets a pressure
relief valve is fitted into the ROV vent line.

+

FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

Four ROV's are fitted to the fuel tank, for NAS vehicles the valves are fitted inside the fuel tank and for ROW vehicles
the ROV's are welded external to the fuel tank. Nylon vent lines from the ROV's connect to the liquid vapour separator
allowing vapour to pass to the EVAP canister via the LVS. To prevent the canister from being overloaded (particularly
in hot ambient conditions) and to prevent wastage of fuel, the vapour is allowed to condense within the LVS and flow
back through the ROVs into the tank.

+

FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

Pressure / vacuum relief valves are incorporated into the fuel filler cap which operate in the event of an evaporation
system failure (e.g. blockage in the evaporation system line to atmosphere). The cap relieves fuel tank pressure to
atmosphere at approximately 1.8 to 2.0 psi (12 to 14 kPa) and opens in the opposite direction at approximately – 0.7
psi (- 5kPa) vacuum. All plastic bodied fuel fillers are fitted with a tank overpressure relief valve.

A vent line flow restrictor (anti-trickle valve) is fitted to the filler pipe in the line between the tank and the canister on
NAS vehicles. The purpose of the anti-trickle valve is to preserve the vapour space in the tank by blocking the vent
line during the fuel filling process. The valve is operated by the action of inserting the filler gun, so that when the fuel
in the tank reaches the level of the filling breather, flow cut off occurs due to fuel filling the filler pipe.

+

FUEL DELIVERY SYSTEM - V8, DESCRIPTION AND OPERATION, Description.

The breather ports from the EVAP canister are located high up in the engine bay (CVS unit on NAS vehicles with
vacuum type, fuel evaporation leak detection capability; via an air filter on NAS vehicles with positive pressure type,
fuel evaporation leak detection capability; snorkel tubes on ROW vehicles), to prevent water ingress during vehicle
wading.

EMISSION CONTROL - V8

17-2-16 DESCRIPTION AND OPERATION

The ECM connectors and pins which are pertinent to evaporative emission control are listed in the following table:

Fuel Leak Detection System (vacuum type) – NAS only
The advanced evaporative loss control system equipped with a vacuum type, fuel evaporation leak detection
capability is similar to the standard evaporative loss system, but also includes additional components to enable the
engine control module (ECM) to perform a fuel evaporation leak detection test. The system includes an EVAPs
canister and purge valve, and in addition, a canister vent solenoid (CVS) valve and a fuel tank pressure sensor.

The function of the CVS valve is to block the atmospheric vent side of the EVAP canister under the control of the ECM
so that an evaporation system leak check can be performed. The test is carried out when the vehicle is stationary and
the engine is running at idle speed. The system test uses the natural rate of fuel evaporation and engine manifold
depression. Failure of the leak check will result in illumination of the Malfunction Indicator Lamp (MIL).

The fuel evaporation leak detection is part of the On-Board Diagnostics (OBD) strategy and it is able to determine
vapour leaks from holes or breaks greater than 1 mm (0.04 in.) in diameter. Any fuel evaporation system leaks which
occur between the output of the purge valve and the connection to the inlet manifold cannot be determined using this
test, but these will be detected through the fuelling adaption diagnostics.

Connector / Pin No.

Function

Signal type

Control

C0635-23

Main relay output

Output drive

Switch to ground

C0635-24

Leak detection pump motor (NAS vehicles
with positive pressure type EVAP system
leak detection only)

Output drive

Switch to ground

C0636-3

Purge valve drive

Output signal

PWM 12 - 0V

C0636-6

Fuel tank pressure sensor (NAS vehicles
with vacuum type EVAP system leak
detection only)

Ground

0V

C0636-30

Canister vent solenoid (CVS) valve (NAS
vehicles with vacuum type EVAP system
leak detection only) / Fuel leak detection
pump (NAS vehicles with positive pressure
type EVAP system leak detection only)

Output drive

Switch to ground

C0637-9

Fuel tank pressure sensor (NAS vehicles
with vacuum type EVAP system leak
detection only)

Output reference

5V

C0637-12

Analogue fuel level (NAS vehicles with

positive pressure type EVAP system leak

detection only)

Input

Analogue 0 - 5V

C0637-14

Fuel tank pressure sensor (NAS vehicles
with vacuum type EVAP system leak
detection only)

Input signal

Analogue 0 - 5V

C0637-20

MIL "ON"

Output drive

Switch to ground

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-17

Fuel Leak Detection System (positive pressure type) – NAS only
The evaporative loss control system equipped with a positive pressure type, fuel evaporation leak detection capability
is similar to the vacuum type, but it is capable of detecting smaller leaks by placing the evaporation system under the
influence of positive air pressure. The system includes an EVAPs canister and purge valve, and in addition, a leak
detection pump comprising a motor and solenoid valve.

The solenoid valve contained in the leak detection pump assembly performs a similar function to the CVS valve
utilised on the vacuum type pressure test. The solenoid valve is used to block the atmospheric vent side of the EVAP
canister under the control of the ECM so that an EVAP system leak check can be performed. At the same time,
pressurised air from the pump is allowed past the valve into the EVAP system to set up a positive pressure. The test
is carried out at the end of a drive cycle when the vehicle is stationary and the ignition is switched off. The test is
delayed for a brief period (approximately 10 seconds) after the engine is switched off to allow any slosh in the fuel
tank to stabilise. Component validity checks and pressure signal reference checking takes a further 10 seconds before
the pressurised air is introduced into the EVAP system.

During reference checking, the purge valve is closed and the leak detection pump solenoid valve is not energised,
while the leak detection pump is operated. The pressurised air is bypassed through a restrictor which corresponds to
a 0.5 mm (0.02 in) leak while the current consumption of the leak detection pump motor is monitored.

The system test uses the leak detection pump to force air into the EVAP system when the purge valve and solenoid
valves are both closed (solenoid valve energised), to put the evaporation lines, components and fuel tank under the
influence of positive air pressure. Air is drawn into the pump through an air filter which is located in the engine
compartment.

The fuel leak detection pump current consumption is monitored by the ECM while the EVAP system is under pressure,
and compared to the current noted during the reference check. A drop in the current drawn by the leak detection pump
motor, indicates that air is being lost through holes or leaks in the system which are greater than the reference value
of 0.5 mm (0.02 in). An increase in the current drawn by the leak detection pump motor, indicates that the EVAP
system is well sealed and that there are no leaks present which are greater than 0.5 mm (0.02 in).

The presence of leakage points indicates the likelihood of hydrocarbon emissions to atmosphere from the
evaporation system outside of test conditions and the necessity for rectification work to be conducted to seal the
system. Failure of the leak check will result in illumination of the Malfunction Indicator Lamp (MIL).

The fuel evaporation leak detection is part of the On-Board Diagnostics (OBD) strategy and it is able to determine
vapour leaks from holes or breaks down to 0.5 mm (0.02 in.) diameter. Any fuel evaporation leaks which occur
between the output of the purge valve and the connection to the inlet manifold cannot be determined using this test,
but these will be detected through the fuelling adaption diagnostics.

Evaporative Emission Control Components
The evaporative emission control components and the fuel evaporation leak detection test components (NAS only)
are described below:

EMISSION CONTROL - V8

17-2-18 DESCRIPTION AND OPERATION

Fuel Vapour Separator (NAS version illustrated)

1 Filler neck
2 Filler cap
3 Liquid vapour separator (LVS)
4 To fuel tank
5 Vapour from fuel tank to liquid vapour separator

(LVS)

6 Rubber hose
7 Pipe connection to OBD sensor in fuel pump

(NAS vehicles with vacuum type leak detection
system only)

8 Vent pipe to EVAP canister
9 Anti-trickle valve (NAS only)

The fuel vapour separator is located under the rear wheel arch next to the filler neck and protected by the wheel arch
lining. The connections to the separator unit are quick release devices at the end of the flexible hoses which connect
the fuel tank to the inlet side of the separator and the outlet of the separator to the evaporation vent line.

The fuel separator construction is different between NAS and ROW vehicles; the LVS on NAS vehicles is an L-shaped
metal tube and for all other markets is an integral part of the moulded plastic filler neck.