Discovery II
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.
