Discovery 2. Manual — part 57

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-23

For systems utilising the vacuum method for determining evaporation leaks, the sensor is used to monitor for a drop
in vacuum pressure. The evaporation system is sealed by the CVS valve and purge valve after a vacuum has been
previously set up from the intake manifold while the purge valve is open and the CVS valve is closed. If any holes or
leaks are present at the evaporation system joints, the vacuum pressure will gradually drop and this change in
pressure will be detected by the fuel tank pressure sensor. This system is capable of determining leaks down to 1 mm
(0.04 in.) in diameter.

The fuel tank pressure sensor is part of the NAS OBD system, a component failure will not be noticed by the driver,
but if the ECM detects a fault, it will be stored in the diagnostic memory and the MIL light will be illuminated on the
instrument pack. Possible failures are listed below:

l

Damaged or blocked sensor

l

Harness / connector faulty

l

Sensor earthing problem

l

Open circuit

l

Short circuit to battery voltage

l

Short circuit to ground

l

ECM fault

Possible failure symptoms of the fuel tank pressure sensor are listed below:

l

Fuel tank pressure sensor poor performance

l

Fuel tank pressure sensor low range fault

l

Fuel tank pressure sensor high range fault

If the fuel tank pressure sensor should malfunction, the following fault codes may be stored in the ECM diagnostic
memory, which can be retrieved using TestBook/T4:

P-code

Description

P0451

Fuel tank pressure signal stuck high within range

P0452

Fuel tank pressure signal short circuit to battery voltage (out of range - High)

P0453

Fuel tank pressure signal short circuit to ground or open circuit (out of range - Low)

EMISSION CONTROL - V8

17-2-24 DESCRIPTION AND OPERATION

Leak Detection Pump (NAS vehicles with positive pressure EVAP system leakage test only)

1 Harness connector
2 Leak detection pump motor
3 Atmosphere connection to/from EVAP canister

4 Atmosphere connection to/from air filter
5 Leak detection pump solenoid valve

The fuel evaporation leak detection pump is mounted forward of the EVAP canister on a bracket fitted beneath the
vehicle on the RH side of the vehicle chassis. The leak detection pump is fixed to the bracket by three screws through
the bottom of the bracket.

A short hose connects between the atmosphere vent port of the EVAP canister and a port at the rear of the fuel
evaporation leak detection pump. The hose is secured to the ports at each end by crimped metal band clips.

An elbowed quick fit connector on the top of the fuel evaporation leak detection pump connects to atmosphere via a
large bore pipe. The pipe is routed along the underside of the vehicle chassis and up into the RH side of the engine
compartment where it connects to an air filter canister.

The leak detection pump incorporates a 3–pin electrical connector. Pin-1 is the earth switched supply to the ECM for
control of the pump solenoid valve. Pin-2 is the earth switched supply to the ECM for the operation of the pump motor.
Pin-3 is the power supply to the pump motor and solenoid valve and is switched on at system start up via the main
relay and fuse 2 in the engine compartment fusebox.

Under normal circumstances (i.e. when the leak detection pump is not operating and the solenoid is not energised),
the EVAP canister vent port is connected to atmosphere via the open solenoid valve.

The pump is operated at the end of a drive cycle when the vehicle is stationary and the ignition is switched off.

The leak detection pump module contains an integral air by-pass circuit with restrictor (reference-leak orifice) which
is used for providing a reference value for the leak detection test. The restrictor corresponds to an air leak equivalent
to 0.5 mm (0.02 in) diameter. With the solenoid valve open and the purge valve closed, the pump forces pressurised
air through the orifice while the current drawn by the leak detection pump motor is monitored to obtain the reference
value. The orifice must be kept free from contamination, otherwise the reference restriction may appear less than for
a 0.5 mm leak and consequently adversely affect the diagnostic results.

M17 0213

3

4

5

1

2

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-25

During the leakage test, the solenoid valve is energised, closing the atmosphere vent line between the EVAP canister
and atmosphere and opening a path to the pressurised air supplied from the leak detection pump motor. Air is pumped
into the EVAP system, while the current drawn by the pump motor is monitored. The current drawn during the leakage
test is compared against the value obtained during the reference check, to determine if an EVAP system leak is
present.

The fuel leak detection pump is powered from a 12V supply and operates at a working pressure of 3 kPa.

Air Filter – (NAS vehicles with positive pressure leak detection system only)

1 Air vents through canister lid
2 Air filter canister

3 To fuel leak detection pump

(EVAP canister atmosphere vent)

A paper element air filter (40

µ

m) is located in a plastic canister at the RH side of the engine compartment. The air

filter canister is fixed to the cruise control mounting bracket by a single nut and bolt. A large bore plastic pipe is
connected to a port at the base of the air filter canister and is secured to the port by a short nylon hose and two crimped
metal band clips.

The air filter is used to prevent particulate contaminants down to 40

µ

m from entering the fuel leak detection pump.

A press-fit lid on top of the canister contains slots to allow the passage of air into and out of the EVAP system.

The bottom end of the paper element is sealed to the canister and is non-serviceable (i.e fit for life). If necessary, the
canister and paper filter must be replaced as a single, complete assembly.

M17 0203

2

1

3

EMISSION CONTROL - V8

17-2-26 DESCRIPTION AND OPERATION

Secondary Air Injection System

The secondary air injection (SAI) system comprises the following components:

l

Secondary air injection pump

l

SAI vacuum solenoid valve

l

SAI control valves (2 off, 1 for each bank of cylinders)

l

SAI pump relay

l

Vacuum reservoir

l

Vacuum harness and pipes

The secondary air injection system is used to limit the emission of carbon monoxide (CO) and hydrocarbons (HCs)
that are prevalent in the exhaust during cold starting of a spark ignition engine. The concentration of hydrocarbons
experienced during cold starting at low temperatures are particularly high until the engine and catalytic converter
reach normal operating temperature. The lower the cold start temperature, the greater the prevalence of
hydrocarbons emitted from the engine.

There are several reasons for the increase of HC emissions at low cold start temperatures, including the tendency for
fuel to be deposited on the cylinder walls, which is then displaced during the piston cycle and expunged during the
exhaust stroke. As the engine warms up through operation, the cylinder walls no longer retain a film of fuel and most
of the hydrocarbons will be burnt off during the combustion process.

The SAI pump is used to provide a supply of air into the exhaust ports in the cylinder head, onto the back of the
exhaust valves, during the cold start period. The hot unburnt fuel particles leaving the combustion chamber mix with
the air injected into the exhaust ports and immediately combust. This subsequent combustion of the unburnt and
partially burnt CO and HC particles help to reduce the emission of these pollutants from the exhaust system. The
additional heat generated in the exhaust manifold also provides rapid heating of the exhaust system catalytic
converters. The additional oxygen which is delivered to the catalytic converters also generate an exothermic reaction
which causes the catalytic converters to 'light off' quickly.

The catalytic converters only start to provide effective treatment of emission pollutants when they reach an operating
temperature of approximately 250

°

C (482

°

F) and need to be between temperatures of 400

°

C (752

°

F) and 800

°

C

(1472

°

F) for optimum efficiency. Consequently, the heat produced by the secondary air injection “afterburning”,

reduces the time delay before the catalysts reach an efficient operating temperature.

The ECM checks the engine coolant temperature when the engine is started in addition to the elapsed time since the
engine was last started. The engine coolant temperature must be below 55

°

C (131

°

F) for the SAI pump to run.

NOTE: The ambient air temperature must also be above 8

°

C (46

°

F) for the SAI pump to run.

Also, depending on the long term 'modelled' ambient temperature determined by the ECM, the minimum elapsed time
required since the last engine start can be up to 8.25 hours. The period of time that the SAI pump runs for depends
on the starting temperature of the engine and varies from approximately 96 seconds at 8

°

C (46

°

F) to 30 seconds at

55

°

C (131

°

F).

Air from the SAI pump is supplied to the SAI control valves via pipework and an intermediate T-piece which splits the
air flow evenly to each bank.

At the same time the secondary air pump is started, the ECM operates a SAI vacuum solenoid valve, which opens to
allow vacuum from the reservoir to be applied to the vacuum operated SAI control valves on each side of the engine.
When the vacuum is applied to the SAI control valves, they open simultaneously to allow the air from the SAI pump
through to the exhaust ports. Secondary air is injected into the inner most exhaust ports on each bank.

When the ECM breaks the ground circuit to de-energise the SAI vacuum solenoid valve, the vacuum supply to the
SAI control valves is cut off and the valves close to prevent further air being injected into the exhaust manifold. At the
same time as the SAI vacuum solenoid valve is closed, the ECM opens the ground circuit to the SAI pump relay, to
stop the SAI pump.

A vacuum reservoir is included in the vacuum line between the intake manifold and the SAI vacuum solenoid valve.
This prevents changes in vacuum pressure from the intake manifold being passed on to cause fluctuations of the
secondary air injection solenoid valve. The vacuum reservoir contains a one way valve and ensures a constant
vacuum is available for the SAI vacuum solenoid valve operation. This is particularly important when the vehicle is at
high altitude.