Discovery 2. Manual — part 459

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-35

Failure of the closed loop control of the exhaust emission system may be attributable to one of the failure modes
indicated below:

l

Mechanical fitting & integrity of the sensor.

l

Sensor open circuit / disconnected.

l

Short circuit to vehicle supply or ground.

l

Lambda ratio outside operating band.

l

Crossed sensors.

l

Contamination from leaded fuel or other sources.

l

Change in sensor characteristic.

l

Harness damage.

l

Air leak into exhaust system (cracked pipe / weld or loose fixings).

System failure will be indicated by the following symptoms:

l

MIL light on (NAS and EU-3 only).

l

Default to open-loop fuelling for the defective cylinder bank.

l

If sensors are crossed, engine will run normally after initial start and then become progressively unstable with
one bank going to its maximum rich clamp and the other bank going to its maximum lean clamp – the system will
then revert to open-loop fuelling.

l

High CO reading

l

Strong smell of H

2

S (rotten eggs)

l

Excessive emissions

Fuel metering
When the engine is cold, additional fuel has to be provided to the air:fuel mixture to assist starting. This supplementary
fuel enrichment continues until the combustion chamber has heated up sufficiently during the warm-up phase.

Under normal part-throttle operating conditions the fuel mixture is adjusted to provide minimum fuel emissions and
the air:fuel mixture is held close to the optimum ratio (

λ

= 1). The engine management system monitors the changing

engine and environmental conditions and uses the data to determine the exact fuelling requirements necessary to
maintain the air:fuel ratio close to the optimum value that is needed to ensure effective exhaust emission treatment
through the three-way catalytic converters.

During full-throttle operation the air:fuel mixture needs to be made rich to provide maximum torque. During
acceleration, the mixture is enriched by an amount according to engine temperature, engine speed, change in throttle
position and change in manifold pressure, to provide good acceleration response.

When the vehicle is braking or travelling downhill the fuel supply can be interrupted to reduce fuel consumption and
eliminate exhaust emissions during this period of operation.

If the vehicle is being used at altitude, a decrease in the air density will be encountered which needs to be
compensated for to prevent a rich mixture being experienced. Without compensation for altitude, there would be an
increase in exhaust emissions and problems starting, poor driveability and black smoke from the exhaust pipe. For
open loop systems, higher fuel consumption may also occur.

EMISSION CONTROL - V8

17-2-36 DESCRIPTION AND OPERATION

Exhaust Emission System Diagnostics
The engine management ECM contains an on-board diagnostics (OBD) system which performs a number of
diagnostic routines for detecting problems associated with the closed loop emission control system. The diagnostic
unit monitors ECM commands and system responses and also checks the individual sensor signals for plausibility,
these include:

l

Lambda ratio outside of operating band

l

Lambda heater diagnostic

l

Lambda period diagnostic

l

Post-catalytic converter lambda adaptation diagnostic (NAS only)

l

Catalyst monitoring diagnostic

Lambda ratio outside operating band
The system checks to ensure that the system is operating in a defined range around the stoichiometric point. If the
system determines that the upper or lower limits for the air:fuel ratio are being exceeded, the error is stored as a fault
code in the ECM diagnostic memory (the MIL light is illuminated on NAS vehicles).

Lambda heater diagnostic
The system determines the heater current and supply voltage so that the heater's resistance can be calculated. After
the engine has been started, the system waits for the heated oxygen sensors to warm up, then calculates the
resistance from the voltage and current measurements. If the value is found to be outside of the upper or lower
threshold values, then the fault is processed (the MIL light is illuminated on NAS vehicles).

Lambda period diagnostic
The pre-catalytic converter sensors are monitored. As the sensors age, the rich to lean and the lean to rich switching
delays increase, leading to increased emissions if the lambda control becomes inaccurate. If the switching period
exceeds a defined limit, the sensor fault is stored in the ECM diagnostic memory (the MIL light is illuminated on NAS
vehicles).

Post-catalytic converter lambda adaptation diagnostic (NAS only)
On NAS vehicles the ageing effects of the pre-catalytic converter sensors are compensated for by an adaptive value
derived from the post-catalytic converter sensors. This is a long term adaption which only changes slowly. For a rich
compensation the additive value is added to the rich delay time. For a lean compensation, the adaptive value is added
to the lean delay time. The adaptive time is monitored against a defined limit, and if the limit is exceeded, the fault is
stored in the ECM's diagnostic memory and the MIL light is illuminated on the instrument pack.

Catalyst monitoring diagnostic
On NAS specification vehicles the catalysts are monitored both individually and simultaneously for emission pollutant
conversion efficiency. The conversion efficiency of a catalyst is monitored by measuring the oxygen storage, since
there is a direct relationship between these two factors. The closed loop lambda control fuelling oscillations produce
pulses of oxygen upstream of the catalyst, as the catalyst efficiency deteriorates its ability to store oxygen is
decreased. The amplitudes of the signals from the pre-catalytic and post-catalytic converter heated oxygen sensors
are compared. As the oxygen storage decreases, the post-catalytic converter sensor begins to follow the oscillations
of the pre-catalytic converter heated oxygen sensors. Under steady state conditions the amplitude ratio is monitored
in different speed / load sites. There are three monitoring areas, and if the amplitude ratio exceeds a threshold in all
three areas the catalyst conversion limit is exceeded; the catalyst fault is stored in the diagnostic memory and the MIL
light is illuminated on the instrument pack. There is a reduced threshold value for both catalysts monitored as a pair.
In either case, a defective catalyst requires replacement of the downpipe assembly.

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-37

In the case of a catalytic converter failure the following failure symptoms may be apparent:

l

MIL light on after 2 driving cycles (NAS market only).

l

High exhaust back pressure if catalyst partly melted.

l

Excessive emissions

l

Strong smell of H

2

S (rotten eggs).

Oxygen sensor voltages can be monitored using 'Testbook', the approximate output voltage from the heated oxygen
sensors with a warm engine at idle and with closed loop fuelling active are shown in the table below:

Mass air flow sensor and air temperature sensor
The engine management ECM uses the mass air flow sensor to measure the mass of air entering the intake and
interprets the data to determine the precise fuel quantity which needs to be injected to maintain the stoichiometric
air:fuel ratio for the exhaust catalysts. If the mass air flow sensor fails, lambda control and idle speed control will be
affected and the emission levels will not be maintained at the optimum level. If the device should fail and the ECM
detects a fault, it invokes a software backup strategy.

+

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

management.

The air temperature sensor is used by the engine management ECM to monitor the temperature of the inlet air. If the
device fails, catalyst monitoring will be affected. The air temperature sensor in integral to the mass air flow sensor.

+

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

management.

Throttle position sensor
If the engine management ECM detects a throttle position sensor failure, it may indicate a blocked or restricted air
intake filter. Failure symptoms may include:

l

Poor engine running and throttle response

l

Emission control failure

l

No closed loop idle speed control

l

Altitude adaption is incorrect

If a signal failure should occur, a default value is derived using data from the engine load and speed.

+

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

management.

Atmospheric pressure will vary with altitude and have a resulting influence on the calculations performed by the ECM
in determining the optimum engine operating conditions to minimise emissions. The following are approximate
atmospheric pressures for the corresponding altitudes:

l

0.96 bar at sea level

l

0.70 bar at 2,750 m (9,000 ft.)

Measurement

Normal catalyst

Defective catalyst

Pre-catalytic heated oxygen sensors

~ 100 to 900 mV switching @ ~ 0.5

Hz

~ 100 to 900 mV switching @ ~ 0.5 Hz

Post-catalytic heated oxygen sensors

~ 200 to 650 mV, static or slowly

changing

~ 200 to 850 mV, changing up to same

frequency as pre-catalytic heated oxygen

sensors

Amplitude ratio (LH HO

2

sensors & RH

HO

2

sensors)

<0.3 seconds

>0.6 seconds (needs to be approximately

0.75 seconds for single catalyst fault)

Number of speed/load monitoring areas
exceeded (LH & RH)

0

>1 (needs to be 3 for fault storage)

EMISSION CONTROL - V8

17-2-38 DESCRIPTION AND OPERATION

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.