Range Rover P38
19
FUEL SYSTEM
NEW RANGE ROVER
58
DESCRIPTION AND OPERATION
The purge valve is opened and closed in accordance
with a PWM signal supplied from the ECM. Possible
failure modes associated with the purge valve failure
are listed below:
•
Valve drive open circuit
•
Short circuit to vehicle supply or ground
•
Purge valve or pipework blocked or restricted
•
Purge valve stuck open
•
Pipework joints leaking or disconnected.
Possible symptoms associated with purge valve or
associated pipework failure is listed below:
•
Engine may stall on return to idle if purge valve
is stuck open
•
Poor idling quality if the purge valve is stuck
open
•
Fuelling adaptions forced excessively lean if the
EVAP canister is clear and the purge valve is
stuck open.
•
Fuelling adaptions forced excessively rich if the
EVAP canister is saturated and the purge valve
is stuck open.
•
Saturation of the EVAP canister if the purge
valve is stuck closed.
To maintain driveability and effective emission control,
purging control must be closely controlled by the
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 fuel pressure in the system
could increase the likelihood of vapour leaks. Canister
purging is cycled with the fuelling adaption 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 optimum
stoichiometric air:fuel mixture for the engine.
See EMISSION CONTROL, Description and
operation.
Leak Test
The evaporative emission system used on NAS
vehicles includes a fuel pressure sensor and a
canister vent solenoid (CVS) valve. The system is
capable of detecting holes in the fuel system down to
1 mm (0.04 in.).
The test is carried out in three parts:
First the purge valve and the CVS 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 (preferably with the engine idling) and the fuel
tank pressure will decrease due to purge operation. In
part three of the test, the leak measurement test is
performed. The pressure response of the tests
determines the level of the leak, and if greater than
the 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 idle with the vehicle stationary. Following the
test, the system returns to normal purge operation
after the CVS valve opens. The in-tank pressure
sensor monitors the pressure build-up to determine
whether leaks are present.
Possible reasons for a test failure are listed below:
•
Fuel filler not tightened or cap missing
•
Sensor or actuator open circuit
•
Short circuit to vehicle supply or ground
•
Either purge or CVS valve stuck open
•
Either purge or CVS valve stuck closed or
blocked pipe
•
Piping broken or not connected
•
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 test, but by the engine management ECM
detecting that the fuelling adaption is suspended. The
evaluation of the leakage is dependent on the
differential pressure between the fuel tank and the
ambient atmospheric pressure. The diagnostic test is
disabled at altitudes above 2,800 metres (9,500 ft).