Discovery 2. Manual — part 61

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-39

Evaporative Emission Control Operation

Fuel vapour is stored in the activated charcoal (EVAP) canister for retention when the vehicle is not operating. When
the vehicle is operating, fuel vapour is drawn from the canister into the engine via a purge control valve. The vapour
is then delivered to the intake plenum chamber to be supplied to the engine cylinders where it is burned in the
combustion process.

During fuel filling the fuel vapour displaced from the fuel tank is allowed to escape to atmosphere, valves within the
fuel filler prevent any vapour escaping through to the EVAP canister as this can adversely affect the fuel cut-off height.
Only fuel vapour generated whilst driving is prevented from escaping to atmosphere by absorption into the charcoal
canister. The fuel filler shuts off to leave the tank approximately 10% empty to ensure the ROVs are always above
the fuel level and so vapour can escape to the EVAP canister and the tank can breathe. The back pressures normally
generated during fuel filling are too low to open the pressure relief valve, but vapour pressures accumulated during
driving are higher and can open the pressure relief valve. Should the vehicle be overturned, the ROVs shut off to
prevent any fuel spillage.

Fuel vapour generated from within the fuel tank as the fuel heats up is stored in the tank until the pressure exceeds
the operating pressure of the two-way valve. When the two-way valve opens, the fuel vapour passes along the vent
line from the fuel tank (via the fuel tank vapour separator) to the evaporation inlet port of the EVAP canister. The fuel
tank vents between 5.17 and 6.9 kPa.

Fuel vapour evaporating from the fuel tank is routed to the EVAP canister through the fuel vapour separator and vent
line. Liquid fuel must not be allowed to contaminate the charcoal in the EVAP canister. To prevent this, the fuel vapour
separator fitted to the fuel neck allows fuel to drain back into the tank. As the fuel vapour cools, it condenses and is
allowed to flow back into the fuel tank from the vent line by way of the two-way valve.

The EVAP canister contains charcoal which absorbs and stores fuel vapour from the fuel tank while the engine is not
running. When the canister is not being purged, the fuel vapour remains in the canister and clean air exits the canister
via the air inlet port.

The engine management ECM controls the electrical output signal to the purge valve. The system will not work
properly if there is leakage or clogging within the system or if the purge valve cannot be controlled.

+

ENGINE MANAGEMENT SYSTEM - V8, DESCRIPTION AND OPERATION, Description - engine

management.

When the engine is running, the ECM decides when conditions are correct for vapour to be purged from the EVAP
canister and opens the canister purge valve. This connects a manifold vacuum line to the canister and fuel vapour
containing the hydrocarbons is drawn from the canister's charcoal element to be burned in the engine. Clean air is
drawn into the canister through the atmosphere vent port to fill the displaced volume of vapour.

The purge valve remains closed below preset coolant and engine speed values to protect the engine tune and
catalytic converter performance. If the EVAP canister was purged during cold running or at idling speed, the additional
enrichment in the fuel mixture would delay the catalytic converter light off time and cause erratic idle. When the purge
valve is opened, fuel vapour from the EVAP canister is drawn into the plenum chamber downside of the throttle
housing, to be delivered to the combustion chambers for burning.

The purge valve is opened and closed in accordance with a pulse width modulated (PWM) signal supplied from the
engine management ECM. The system will not work properly if the purge valve cannot be controlled. Possible failure
modes associated with the purge valve are listed below:

l

Valve drive open circuit.

l

Short circuit to vehicle supply or ground.

l

Purge valve or pipework blocked or restricted.

l

Purge valve stuck open.

l

Pipework joints leaking or disconnected.

Possible symptoms associated with a purge valve or associated pipework failure is listed below:

l

Engine may stall on return to idle if purge valve is stuck open.

l

Poor idling quality if the purge valve is stuck open

l

Fuelling adaptions forced excessively lean if the EVAP canister is clear and the purge valve is stuck open.

l

Fuelling adaptions forced excessively rich if the EVAP canister is saturated and the purge valve is stuck open.

l

Saturation of the EVAP canister if the purge valve is stuck closed.

EMISSION CONTROL - V8

17-2-40 DESCRIPTION AND OPERATION

To maintain driveability and effective emission control, EVAP canister purging must be closely controlled by the
engine management ECM, as a 1% concentration of fuel vapour from the EVAP canister in the air intake may shift
the air:fuel ratio by as much as 20%. The ECM must purge the fuel vapour from the EVAP canister at regular intervals
as its storage capacity is limited and an excessive build up of evaporated fuel pressure in the system could increase
the likelihood of vapour leaks. Canister purging is cycled with the fuelling adaptation as both cannot be active at the
same time. The ECM alters the PWM signal to the purge valve to control the rate of purging of the canister to maintain
the correct stoichiometric air:fuel mixture for the engine.

Fuel Leak Detection System Operation (vacuum type) – NAS only
The advanced evaporative loss control system used on NAS vehicles is similar to the standard system, but also
includes a CVS valve and fuel tank pressure sensor and is capable of detecting holes in the fuel evaporative system
down to 1 mm (0.04 in.). The test is carried out in three parts. First the purge valve and the canister vent solenoid
valve closes off the storage system and the vent pressure increases due to the fuel vapour pressure level in the tank.
If the pressure level is greater than the acceptable limit, the test will abort because a false leak test response will
result. In part two of the test, the purge valve is opened and the fuel tank pressure will decrease due to the depression
from the intake manifold, evident at the purge port of the EVAP canister during purge operation. In part three of the
test, the leak measurement test is performed. The pressure response of the tests determines the level of leak, and if
this is greater than the acceptable limit on two consecutive tests, the ECM stores the fault in diagnostic memory and
the MIL light on the instrument pack is illuminated. The test is only carried out at engine idle with the vehicle stationary,
and a delay of 15 minutes after engine start is imposed before diagnosis is allowed to commence.

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-41

EVAP System, Leak Detection Diagnostic (vacuum type)
The EVAP system leak detection is performed as follows:

1 The ECM checks that the signal from the fuel tank pressure sensor is within the expected range. If the signal is

not within range, the leakage test will be cancelled.

2 Next the purge valve is held closed and the canister vent solenoid (CVS) valve is opened to atmosphere. If the

ECM detects a rise in pressure with the valves in this condition, it indicates there is a blockage in the fuel
evaporation line between the CVS valve and the EVAP canister, or that the CVS valve is stuck in the closed
position and thus preventing normalisation of pressure in the fuel evaporation system. In this instance, the
leakage test will be cancelled.

3 The CVS valve and the purge valve are both held in the closed position while the ECM checks the fuel tank

pressure sensor. If the fuel tank pressure sensor detects a decline in pressure, it indicates that the purge valve
is not closing properly and vapour is leaking past the valve seat face under the influence of the intake manifold
depression. In this instance, the leakage test will be cancelled.

4 If the preliminary checks are satisfactory, a compensation measurement is determined next. Variations in fuel

level occur within the fuel tank, which will influence the pressure signal detected by the fuel tank pressure
sensor. The pressure detected will also be influenced by the rate of change in the fuel tank pressure, caused by
the rate of fuel evaporation which itself is dependent on the ambient temperature conditions. Because of these
variations, it is necessary for the ECM to evaluate the conditions prevailing at a particular instance when testing,
to ensure that the corresponding compensation factor is included in its calculations.

The CVS valve and purge valves are both closed while the ECM checks the signal from the fuel tank pressure
sensor. The rise in fuel pressure detected over a defined period is used to determine the rate of fuel evaporation
and the consequent compensation factor necessary.

5 With the CVS valve still closed, the purge valve is opened. The inlet manifold depression present while the purge

valve is open, decreases EVAP system pressure and sets up a small vacuum in the fuel tank. The fuel tank
pressure sensor is monitored by the ECM and if the vacuum gradient does not increase as expected, a large
system leak is assumed by the ECM (e.g. missing or leaking fuel filler cap) and the diagnostic test is terminated.

If the EVAP canister is heavily loaded with hydrocarbons, purging may cause the air:fuel mixture to become
excessively rich, resulting in the upstream oxygen sensors requesting a leaner mix from the ECM to bring the
mixture back to the stoichiometric ideal. This may cause instability in the engine idle speed and consequently
the diagnostic test will have to be abandoned. The ECM checks the status of the upstream oxygen sensors
during the remainder of the diagnostic, to ensure the air:fuel mixture does not adversely affect the engine idle
speed.

6 When the fuel tank pressure sensor detects that the required vacuum has been reached (-800 Pa), the purge

valve is closed and the EVAP system is sealed. The ECM then checks the change in the fuel tank pressure
sensor signal (diminishing vacuum) over a period of time, and if it is greater than expected (after taking into
consideration the compensation factor due to fuel evaporation within the tank, determined earlier in the
diagnostic), a leak in the EVAP system is assumed. If the condition remains, the MIL warning light will be turned
on after two drive cycles.

The decrease in vacuum pressure over the defined period must be large enough to correspond to a hole
equivalent to 1 mm (0.04 in.) diameter or greater, to be considered significant enough to warrant the activation
of an emissions system failure warning.

The diagnostic test is repeated at regular intervals during the drive cycle, when the engine is at idle condition. The
diagnostic test will not be able to be performed under the following conditions:

l

During EVAP canister purging

l

During fuelling adaption

l

If excess slosh in the fuel tank is detected (excess fuel vapour will be generated, invalidating the result)

Following the test, the system returns to normal purge operation after the canister vent solenoid opens. Possible
reasons for an EVAP system leak test failure are listed below:

l

Fuel filler not tightened or cap missing.

l

Sensor or actuator open circuit.

l

Short circuit to vehicle supply or ground.

l

Either purge or CVS valve stuck open.

l

Either purge or CVS valve stuck shut or blocked pipe.

l

Piping broken or not connected.

l

Loose or leaking connection.

EMISSION CONTROL - V8

17-2-42 DESCRIPTION AND OPERATION

If the piping is broken forward of the purge valve or is not connected, the engine may run rough and fuelling adaptions
will drift. The fault will not be detected by the leak detection diagnostic, but it will be determined by the engine
management ECM through the fuelling adaption diagnostics.

The evaluation of leakage is dependent on the differential pressure between the fuel tank and ambient atmospheric
pressure, the diagnostic is disabled above altitudes of 9500 ft. (2800 m) to avoid false detection of fuel leaks due to
the change in atmospheric pressure at altitude.

Fuel Leak Detection System Operation (positive pressure leak detection type) – NAS only
The EVAP system with positive pressure leak detection capability used on NAS vehicles is similar to the standard
system, but also includes a fuel evaporation leak detection pump with integral solenoid valve. It is capable of detecting
holes in the EVAP system down to 0.5 mm (0.02 in.). The test is carried out at the end of a drive cycle, when the
vehicle is stationary and the ignition switch has been turned off. The ECM maintains an earth supply to the Main relay
to hold it on, so that power can be supplied to the leak detection pump.

First a reference measurement is established by passing the pressurised air through a by-pass circuit containing a
fixed sized restriction. The restriction assimilates a 0.5 mm (0.02 in) hole and the current drawn by the pump motor
during this procedure is recorded for comparison against the value to be obtained in the system test. The purge valve
is held closed, and the reversing valve in the leak detection pump module is not energised while the leak detection
pump is switched on. The pressurised air from the leak detection pump is forced through an orifice while the current
drawn by the pump motor is monitored.

Next the EVAP system diagnostic is performed; the solenoid valve is energised so that it closes off the EVAP system's
vent line to atmosphere, and opens a path for the pressurised air from the leak detection pump to be applied to the
closed EVAP system.

The current drawn by the leak detection pump is monitored and checked against that obtained during the reference
measurement. If the current is less than the reference value, this infers there is a hole in the EVAP system greater
than 0.5 mm (0.02 in) which is allowing the positive air pressure to leak out. If the current drawn by the pump motor
is greater than the value obtained during the reference check, the system is sealed and free from leaks. If an EVAP
system leak is detected, the ECM stores the fault in diagnostic memory and the MIL light on the instrument pack is
illuminated.

On NAS vehicles, the ECM works on a 2 trip cycle before illuminating the MIL. On EU-3 vehicles, the ECM works on
a 3 trip cycle before illuminating the MIL.

Following the test, the solenoid valve is opened to normalise the EVAP system pressure and the system returns to
normal purge operation at the start of the next drive cycle. Possible reasons for an EVAP system leak test failure are
listed below:

l

Fuel filler not tightened or cap missing.

l

Sensor or actuator open circuit.

l

Short circuit to vehicle supply or ground.

l

Either purge or solenoid valve stuck open.

l

Either purge or solenoid valve stuck shut.

l

Blocked pipe or air filter.

l

Piping broken or not connected.

l

Loose or leaking connection.

If the piping is broken forward of the purge valve or is not connected, the engine may run rough and fuelling adaptions
will drift. The fault will not be detected by the leak detection test, but will be determined by the engine management
ECM through the fuelling adaption diagnostics. This test can be run from TestBook/T4.