Discovery II
EMISSION CONTROL - V8
17-2-36 DESCRIPTION AND OPERATION
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.
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Sensor open circuit / disconnected.
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Short circuit to vehicle supply or ground.
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Lambda ratio outside operating band.
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Crossed sensors.
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Contamination from leaded fuel or other sources.
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Change in sensor characteristic.
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Harness damage.
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Air leak into exhaust system (cracked pipe / weld or loose fixings).
System failure will be indicated by the following symptoms:
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MIL light on (NAS and EU-3 only).
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Default to open-loop fuelling for the defective cylinder bank.
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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.
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High CO reading
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Strong smell of H
2
S (rotten eggs)
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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.
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:
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Lambda ratio outside of operating band
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Lambda heater diagnostic
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Lambda period diagnostic
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Post-catalytic converter lambda adaptation diagnostic (NAS only)
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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).
