Engine Controls - Theory & Operation Land Rover Discovery L318

INTRODUCTION

Note. All diagnostic information, specifications and test procedures for current models is contained in Land Rover proprietary TestBook system.

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

COMPUTERIZED ENGINE CONTROLS

Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the ECM. The second category covers OUTPUT SIGNALS, which are components controlled by the ECM. For component location (Scheme 1), (Scheme 2), (Scheme 3) and (Scheme 4).

Discovery Series I

Generic Engine Management System (GEMS) is an electronic ignition and fuel injection system which includes idle speed control, engine load management and security functions. Fuel is provided by full sequential fuel injection. GEMS is controlled by an Electronic Control Module (ECM) which receives data from sensors located on and around the engine. From this information, the ECM provides the correct fuel requirements and ignition timing at all engine loads and speeds.

An on-board diagnostic system detects any faults that may occur within the system. Fault diagnosis include failure of any system sensor or actuator, emission related items, fuel supply and exhaust systems.

The system incorporates default strategies for some sensor failures which will enable the engine to operate in the event of a sensor failure. See suspect component listed under INPUT DEVICES for description of default strategies, if available. A fault is indicated by illumination of the Maintenance Indicator Light (MIL).

Discovery Series II & Range Rover

The Engine Control Module (ECM) is a Bosch Motronic 5.2.1 which is mounted in a plastic E-box located on the left side of engine compartment, under cruise control actuator. The ECM is cooled by a dedicated fan, which supplies cabin air into the plastic E-box. The operating temperature of the ECM is monitored by an internal temperature sensor.

ECM uses a flash, electronic erasable programmable read only memory (EEPROM). This enables ECM to be externally configured, ensuring that ECM can be updated as necessary and configured with market specific data. Land Rover TestBook must be used to configure replacement ECM. ECM can be reconfigured as many times as is necessary to meet changing specifications and legislation.

ECM stores crankshaft and camshaft positions when the engine has stopped based on CKP and CMP sensors. This allows immediate sequential fuel injection and ignition timing during engine cranking. This information is lost if battery voltage is too low or disconnected. This function will be disabled during the first engine start.

ECM uses sensor data, engine operating data and information stored in an internal memory map to control fuel, ignition and some emissions functions. The ECM monitors performance of the engine for misfires, catalyst efficiency, exhaust leaks and EVAP control loss. If a fault is detected, the ECM stores a Diagnostic Trouble Code (DTC) and illuminates the Malfunction Indicator Light (MIL). The system incorporates default strategies for some sensor failures which will enable the engine to operate in the event of a sensor failure. See suspect component listed under INPUT DEVICES for description of default strategies, if available. A fault is indicated by illumination of the Maintenance Indicator Light (MIL).

The ECM also communicates with the

1. With TestBook via diagnostic connector.
2. Electronic Automatic Transmission (EAT) ECU using a Controller Area Network (CAN) data link.
3. Body Electrical Control Module (BECM).
4. Self-Leveling and Anti-Lock Braking System (SLABS) ECU on Discovery Series II.
5. Transfer Box ECU.
6. Anti-Lock Braking System (ABS) ECU on Range Rover.

Main Relay

Main relay is located in underhood fuse/relay box located at right front corner of engine compartment. On all models, the main relay supplies power to the ECM, fuel injectors (8 amps) and Mass Airflow (MAF) sensor (4 amps). Relay is controlled by the ECM which has a second power feed. This enables the ECM to remain powered up after the ignition is turned off. During ECM power down, the ECM records all temperature readings and powers the Idle Air Control (IAC) valve to the cold start position. Failure of this relay will result in the ECM not being powered up, resulting in engine not starting due to absence of fuel and ignition.

INPUT DEVICES

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input device usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input signals include the following

Camshaft Position (CMP) Sensor

Note. It is physically possible to interchange camshaft gear wheel between early 1999 year models with GEMS systems and late 1999 year models with Motronic systems. Because GEMS and Motronic systems are incompatible, an incorrect camshaft signal will be received by the ECM and a P0340 DTC will set.

CMP sensor is a Hall Effect sensor located in the engine front cover. CMP sensor produces 4 pulses for every 2 revolutions of the engine (one pulse is slightly longer than the others). CMP sensor signals are generated from 4 gaps on cam gear (one gap is smaller than the others). CMP sensor signals are used by ECM to correct fuel injector timing for fully sequential operation and for active knock control.

If CMP sensor fails, default strategy is to continue normal ignition timing. Fuel injectors will be actuated sequentially based on top dead center timing. Injection timing will either be correct or one revolution out of synchronization. A fault is indicated by illumination of the MIL.

Crankshaft Position (CKP) Sensor

CKP sensor is located below cylinder No. 7 on the left side of flywheel housing. CKP sensor uses different thicknesses of mounting spacers for manual and automatic transmissions. CKP sensor provides ECM with information indicating that engine is turning, engine speed and crankshaft position. ECM controls fuel injection and coil firing based on signal from CKP sensor. Engine overspeed protection is set to 5000-5500 RPM and is based on CKP sensor signal. There is no default strategy for the CKP sensor. A failure will result in an engine no start condition and/or the tachometer fails to operate. If CKP sensor is installed incorrectly engine will misfire, run rough or stall. A fault is indicated by illumination of the MIL. If CKP sensor input signal wire and ground wire are inadvertently reversed, the ECM will advance ignition timing 3 degrees.

Engine Coolant Temperature (ECT) Sensor

The ECT sensor is located on intake manifold, near thermostat housing. ECT sensor consists of a temperature dependent resistive metal strip. ECT sensor output voltage changes as engine coolant temperature changes. See ECT SENSOR OUTPUT VOLTAGE table. ECT works as a Negative Temperature Coefficient (NTC) sensor. As coolant temperature increases, sensor resistance decreases, as coolant temperature decreases, sensor resistance increases. See ECT SENSOR RESISTANCE table. ECM monitors ECT sensor resistance values and triggers enrichment circuits to extend injector on-time during cold starting and warm-up (a richer mixture at low coolant temperatures). ECM also uses ECT sensor values for Secondary Air Injection activation. See SECONDARY AIR INDUCTION under AIR INDUCTION SYSTEM.

Default strategy for ECT sensor is to select a substitute value based on intake air temperature. Initial ECT sensor default value selected will increase to a pre-warm up value over a time period programed for each default. An ECT sensor fault may result in a fast idle condition on initial start-up until fully warm value is reached. There may be a hot engine restart problem. A fault is indicated by illumination of the MIL.

ECT SENSOR OUTPUT VOLTAGE (1)

Engine Fuel Temperature (EFT) Sensor (Discovery Series I)

The EFT sensor is located on the fuel rail and measures the fuel rail temperature rather than the fuel. EFT sensor informs ECM when fuel rail is hot. Because of fuel vaporization and possible bubbles in the fuel rail, ECM will increase injector pulse width during hot engine restarts to clear bubbles. During engine operation fuel circulation keeps fuel rail cooled. There is no default strategy for the EFT sensor. An EFT sensor fault may result in a hot engine restart problem. A fault is indicated by illumination of the MIL.

ENGINE FUEL TEMPERATURE SENSOR RESISTANCE

Heated Oxygen Sensor (HO2S)

There are 4 HO2S located in the exhaust system. One in front of each catalytic converter and one after. Each HO2S consists of a titanium metal sensor surrounded by a gas permeable ceramic coating. Oxygen in exhaust gas defuses through ceramic coating and reacts with the titanium wire, altering resistance of wire. ECM monitors front HO2S resistance changes and calculates amount of oxygen in exhaust gas, which indicates a rich or lean mixture. ECM adjusts on-time of each injector to maintain correct air/fuel ratio. ECM monitors 2 rear HO2S to determine catalytic converter operating efficiency.

All HO2S are electrically heated to ensure they achieve operating temperature as quickly as possible after start-up. Both front and both rear sensor heaters are connected in parallel. If front HO2S wiring is crossed, vehicle will operate properly until sensors reach operating temperature. ECM will then cause one bank of cylinders to run very rich and the other bank to run very lean. This will cause engine to misfire, idle rough and emit black smoke, with possible catalytic converter damage.

If there is a HO2S failure, system will default to an open-loop operation and ECM will base fuel requirements on other sensor inputs. A fault is indicated by illumination of the MIL.

Intake Air Temperature (IAT) Sensor (Discovery Series I)

Note. For IAT Sensor on Discovery Series II and Range Rover, see MASS AIRFLOW (MAF)/INTAKE AIR TEMPERATURE (IAT) SENSORS (DISCOVERY SERIES II & RANGE ROVER) .

IAT sensor is located in the air cleaner housing. IAT sensor consists of a temperature dependent resistive metal strip. IAT sensor resistance changes as intake air temperature changes. ECM uses IAT sensor signal to retard ignition timing when intake air temperature is greater than 131°F (55°C).

Default strategy for IAT sensor is to select a substitute value based on nominal operating conditions, approximately 122°F (50°C). An IAT sensor fault may result in slight loss of power in high ambient air temperatures. A fault is indicated by illumination of the MIL.

Knock Sensor (KS)

KS are located on each side of engine block between cylinders No. 2 and No. 4, and between cylinders No. 3 and No. 5. KS contains a piezoelectric ceramic element which produces a voltage proportional to engine vibration. ECM uses KS signals and signals from CMP sensor and CKP sensor to verify engine knock based on positions of camshaft and crankshaft.

ECM will calculate which cylinder is knocking and retard ignition timing until knock disappears. Ignition timing will then be advanced until optimum ignition timing for suspect cylinder is achieved. For this reason it is possible for all 8 cylinders to have different ignition timing simultaneously.

If CMP sensor fails engine may run one revolution out of synchronization and ECM may retard timing of wrong cylinder in a pair. There is no default strategy for KS. If KS fails there may be increased engine noise under load conditions. A fault is indicated by illumination of the MIL.

Mass Airflow (MAF) Sensor (Discovery Series I)

MAF sensor is mounted rigidly to air cleaner housing and is connected by flexible hose to plenum chamber inlet. MAF sensor is a hot wire anemometer which uses 2 wires. A sensing wire which is heated using battery voltage and a compensating wire which is not heated. Intake air passing through MAF sensor causes heated wire temperature to change. As heated wire temperature changes, resistance also changes. ECM measures changes in heated wire resistance and calculates amount of air flowing into engine.

As a default strategy for MAF sensor ECM will calculate a MAF value based on throttle position, engine speed and intake air temperature. A MAF sensor failure will result in hard engine starts, engine may not idle, poor throttle response and driveability, incorrect emissions, and/or a high long term fuel correction. A fault is indicated by illumination of the MIL.

Mass Airflow (MAF)/Intake Air Temperature (IAT) Sensors (Discovery Series II & Range Rover)

MAF/IAT sensor is located in the intake air duct between air filter and intake manifold, at the right side of the engine compartment. Ensure airflow directional arrow embossed in the top of the MAF/IAT sensor tube is pointed in the proper direction. MAF/IAT sensor is connected to ECM through a 5-pin connector and the engine harness.

MAF sensor uses a hot-film element contained in intake air tube to monitor the amount of air being drawn into engine. MAF sensor contains two sensing elements, one element is controlled at ambient air temperature, while the other is heated 360°F (200°C) more than ambient temperature using battery voltage. As intake air passes over it, the heated element cools, which reduces the element resistance. In order to maintain the same relative temperature, more current is supplied to the heated element. This voltage increase change is supplied to ECM as a 0.0-5.0 volts signal. Using the MAF signal, the ECM calculates the amount of intake air entering engine. This in turn is used by the ECM to determine injected fuel quantity necessary for optimum engine performance and low emissions.

MAF sensor receives battery voltage from engine compartment fusebox, and a 5.0 volt reference signal from the ECM. MAF sensor and IAT sensor share a common ground connection and each provide a separate signal input to the ECM.

As a default strategy for MAF sensor failure ECM will calculate a MAF value based on throttle position, engine speed and intake air temperature. A MAF sensor failure may result in hard engine starts, engine may not idle, poor throttle response and driveability, incorrect emissions, and/or a high long term fuel correction. A fault is indicated by illumination of the MIL.

IAT sensor uses a thermistor with a negative temperature co-efficient (as temperature increases, thermistor resistance decreases). Changes in resistance cause a change in input voltage at the ECM. ECM converts received voltage value to provide an indication of intake air temperature.

As a default strategy for IAT sensor failure ECM will substitute a default value for intake air temperature of 113°F (45°C). An IAT sensor failure may result in the catalyst monitoring being affected due to exhaust temperature model, warm-up ignition angle affected, ISC speed adaptation disabled, ISC actuator blocked test disabled, fueling adaptations disabled and/or condenser fan hot restart inhibited.

Throttle Position (TP) Sensor

TP sensor is a variable resistor mounted on the throttle body and connected directly to the throttle shaft. TP sensor signal informs ECM of actual throttle position and rate of change in throttle position. ECM cross checks TP sensor output with Mass Airflow (MAF) sensor output. If values from these two sensors do not agree and fuel injection feedback indicates correct air/fuel mixture, ECM assumes MAF sensor is correct and TP sensor has failed.

On Discovery Series I, TP sensor sends a throttle position signal to Transmission Control Module (TCM). Loss of TP signal will cause poor gear change quality, loss of kickdown or cause TCM to select default transmission control. The TP sensor signal is also used by the Electronic Automatic Transmission (EAT) ECU to determine the correct point for gear shifts and acceleration kickdown. If a failure occurs in the closed position, ECM will initiate over-run fuel cut off when engine speed is 1750 RPM or more.

On Discovery Series II and Range Rover, TP signal is also supplied to the Electronic Automatic Transmission Electronic Control Unit (EAT ECU) from the ECM using Controller Area Network (CAN) communication link. EAT ECU uses throttle position data to determine the correct point for gear shifts and acceleration kickdown. The ECM also supplies the Self-Leveling and Anti-Lock Brake System (SLABS) ECU, and Active Cornering Enhancement (ACE) ECU with TP sensor information as a Pulse Width Modulation (PWM) signal. If TP sensor signal fails, ECM uses a default value derived from engine load and speed. On all models, a fault is indicated by illumination of the MIL. A TP sensor failure may result in a poor throttle response and degraded engine performance, emission control failure, closed loop idle speed control inoperative, automatic gearbox kickdown inoperative and/or incorrect altitude adaptation.

Scheme 1

Scheme 2

Scheme 3

Scheme 4

OUTPUT SIGNALS

Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to the system indicated after component.

Malfunction Indicator Light

See SELF-DIAGNOSTIC SYSTEM .

Fuel Injector

See FUEL CONTROL .

Fuel Pump

See FUEL DELIVERY .

Fuel Pump Relay

See FUEL DELIVERY .

Fuel Pressure Regulator

See FUEL DELIVERY .

Inertia Fuel Shutoff Switch

See FUEL DELIVERY .

Idle Air Control Valve

See IDLE SPEED .

Ignition Relay

See IGNITION SYSTEM .

Secondary Air Induction

See AIR INDUCTION SYSTEM .

The Secondary Air Injection (SAI) system includes a SAI pump, SAI vacuum solenoid valve, 2 SAI control valves (2 off, 1 for each bank of cylinders), SAI pump relay, vacuum reservoir, vacuum harness and pipes. (Scheme 5)or (Scheme 6). SAI pump is protected against overheating by an integral thermal cut-out switch. SAI pump automatically enters a soak period between operations to prevent pump overheating. ECM compares engine coolant temperatures when engine is turned off and when it is turned on to determine if pump operation is necessary.

The SAI system is used to limit Carbon Monoxide (CO) and Hydrocarbon (HC) exhaust emissions during cold starts. The concentration of HCs during cold starts are particularly high until the engine and catalytic converter reach normal operating temperature. The lower the cold start temperature, the greater amount of hydrocarbons that are emitted from the engine.

The SAI pump supplies air to the exhaust ports in the cylinder head, onto the back of the exhaust valves, during cold starts. Hot, unburned fuel particles leaving the combustion chamber mix with the injected air and immediately combust. This combustion of unburned and partially burned CO and HC particles help to reduce the emission of these pollutants. The additional heat generated in the exhaust manifold also provides rapid heating of the catalytic converters. The additional oxygen which is delivered to the catalytic converters also generate an exothermic reaction that causes the catalytic converters to light off more quickly.

The effective operating temperature of the catalytic converters is approximately 482°F (250°C) and needs to be between 752-1472°F (400-800°C) for optimum efficiency. The heat produced by secondary air injection afterburning, reduces the amount of time before the catalysts reach an efficient operating temperature. When engine is started the ECM checks engine coolant temperature. If engine coolant temperature is less than 131°F (55°C), ECM will activate SAI pump. SAI pump will operate for approximately 95 seconds when engine coolant temperature is 46°F (8°C) and approximately 30 seconds when engine coolant temperature is 131°F (55°C). ECM can cancel SAI pump operation if engine speed or load is excessive.

Air from the SAI pump is routed to SAI control valves and intermediate "T"fittings which split airflow evenly to each bank. When secondary air pump is activated, ECM operates a SAI vacuum solenoid valve, which allows reservoir vacuum to be applied to SAI control valves. When vacuum is applied to SAI control valves, they open simultaneously to allow air from SAI pump to be injected into the inner exhaust ports on each bank.

Scheme 5

Scheme 6

The fuel pump is an electrically driven, self-priming wet-type fuel pump that is mounted inside the fuel tank. Fuel pump operates at all times when the ignition switch is in RUN position. However, if the engine does not start within 30 seconds the ECM will de-energize the fuel pump relay to protect fuel pump. The fuel pump incorporates a fuel tank pressure sensor, fuel pressure regulator and fuel return line, and a fuel gauge sending unit.

On Discovery Series I, fuel pump relay is part of relay module located at right side of engine compartment. The fuel pump relay is fed from the ignition relay and is controlled by the ECM. The relay is activated when the ignition switch is first turned to RUN position to prime the fuel system for a period of time controlled by ECM. Failure of the fuel pump relay will result in no fuel pressure. The fuel pump receives battery voltage from fuse (20-amp) No. 7 located in engine compartment fusebox and fuel pump relay.

On Discovery Series II and Range Rover, fuel pump relay is located in underhood fuse/relay box located at right front corner of engine compartment. On Discovery Series II, the fuel pump receives battery voltage from fuse (20-amp) No. 10 located in the right side of engine compartment fusebox. On Range Rover, the fuel pump receives battery voltage from fuse (30-amp) No. 10 in engine compartment fusebox.

Fuel Pressure Regulator (Discovery Series I)

Fuel pressure regulator is located in fuel rail at rear of the engine. It consists of a fuel inlet, outlet, vacuum port and internal diaphragm. Regulated fuel pressure varies depending on intake manifold vacuum acting on fuel pressure regulator diaphragm. As vacuum increases, regulated fuel pressure is reduced. Fuel pump pressure should be 35-38 psi (2.5-2.7 kg/cm 2 ). By maintaining constant fuel pressure, quantity of fuel injected for a given duration of injector on-time, remains constant.

Excess fuel is returned to fuel tank. Fuel pressure regulator failure will cause a rich mixture at idle and a normal mixture at full load, or engine flooding, or a leak mixture. Fuel pressure regulator failure will not illuminate MIL, but other faults caused by failure may illuminate MIL.

Fuel Pressure Regulator (Discovery Series II & Range Rover)

Fuel pressure regulator is located on top of fuel pump. Fuel pressure should be 52 psi (3.6 kg/cm 2 ). A Schraeder valve is provided at fuel rail for measuring fuel pressure. By maintaining constant fuel pressure, quantity of fuel injected for a given duration of injector on-time, remains constant.

Inertia Fuel Shutoff Switch (IFSS)

On Discovery, IFSS is located on firewall in engine compartment. On Range Rover, IFSS is located in right side kick panel, behind an access flap. On all models, IFSS isolates the power supply to fuel pump in the event of sudden deceleration, such as during an accident. IFSS only operates when ignition is in RUN position. When IFSS is activated it will unlock all doors and activate HAZARD warning flashers for at least 30 seconds until IFSS has been reset. To reset IFSS, depress the central plunger at top of switch.

Multiport Sequential Fuel Injection (SFI) system uses one fuel injector per cylinder. Fuel injectors are fitted between pressurized fuel rail and intake manifold. Each injector contains a solenoid controlled by ECM. All 8 fuel injectors are supplied with battery voltage. ECM controls injector operation through ground path for each injector. When injector solenoid is energized, a plunger is attracted off its seat and allows pressurized fuel to spray into the intake manifold. Fuel injector total failure or a leak that causes a rich mixture will cause a misfire in effected cylinder.

Idle Air Control (IAC) Valve

Note. DO NOT try to forcibly set IAC valve position by mechanical means. IAC can be damaged. IAC valve actuator can not be serviced. If defective, the entire IAC valve must be replaced as a unit.

On Discovery Series I, IAC valve contains a 2-coil stepper motor and is mounted to throttle housing, next to Throttle Position Sensor (TPS). When energized in correct sequence, IAC coils move a plunger a specified distance or step, which opens or closes the throttle by-pass valve controlling idle air. On Discovery Series I, IAC valve fully open is zero steps and fully closed is 180 steps. Failure of IAC valve will cause low or high idle speed, poor idle, engine stall or engine no start. An IAC valve fault is indicated by illumination of the MIL.

On Discovery Series I, IAC valve position can be checked using a Land Rover TestBook and adjusted if necessary using a by-pass screw in the plenum chamber. The by-pass screw is covered by a tamper proof plug which can be extracted using a self tapping screw.

On Discovery Series II and Range Rover, IAC valve is located on the side of the intake air pipe located on top of the engine. IAC valve acts as an air by-pass valve. ECM calculations regulate the amount of air flow into the engine at idle, compensating for any internal or external loads affecting idle speed. IAC valve uses a 2-coil stepper motor that uses opposing Pulse Width Modulation (PWM) signals to control the opening and closing positions of a rotary valve. If one PWM signal supply circuit fails, ECM closes down remaining signal preventing IAC valve from working at its maximum or minimum setting. IAC valve will automatically resume a default idle position and engine idle speed is set at approximately 1200 RPM with no load placed on engine. Idle speed in cold start condition is set to approximately 1200 RPM in neutral for 20 seconds and ignition timing is retarded as a catalyst heating strategy. Even though engine idle speed is approximately 1200 RPM during cold start and default idle position during IAC valve failure, they must not be confused with each other as they are set separately by the ECM.

DIRECT IGNITION SYSTEM (DIS)

Ignition and spark distribution are controlled by DIS. On Discovery Series I, DIS uses 4 double-ended ignition coils located at the rear of the engine. On Discovery Series II and Range Rover, DIS uses 2 quad-ended ignition coils located at the rear of the engine. On all models, ECM provides internal switching of ground circuit for each coil, which operate on a waste spark principle. A spark is delivered simultaneously to 2 cylinders, one on its compression stroke and the other on its exhaust stroke. Coil No. 1 feeds cylinders No. 1 and 6. Coil No. 2 feeds cylinders No. 5 and 8. Coil No. 3 feeds cylinders No. 4 and 7. Coil No. 4 feeds cylinders No. 2 and 3. Failure of any coil will cause a misfire in 2 cylinders.

Ignition timing is controlled primarily as a function of engine speed, temperature, load, knock and idle speed controls, and A/T shift control. Engine load is sensed by Mass Airflow (MAF) sensor. Engine speed is sensed by a Crankshaft Position (CKP) sensor. ECM determines correct firing sequence and timing of ignition coils from input provided by Camshaft Position (CMP) sensor. An ignition coil fault is indicated by illumination of the MIL.

The ignition relay supplies power to coils (6.5 amps), purge valve (1 amp, non-continuous) and HO2S heating elements (8 amps, non-continuous). Ignition relay is only energized when ignition switch is in ON position. When ignition is in OFF position, ignition relay is not energized. Failure of ignition relay will result in a no-start condition.

EVAP Control System

EVAP system minimizes fuel vapor escaping into atmosphere. EVAP control system consists of rollover valves, liquid/vapor separator, an anti-trickle fill valve, EVAP control canister, EVAP canister purge valve, EVAP canister vent valve, EVAP canister vent valve control solenoid and fuel tank pressure sensor.

Fuel vapor inside fuel tank is directed to liquid/vapor separator. There are 2 rollover valves in EVAP system to prevent liquid fuel from entering separator if vehicle rolls over. Liquid/vapor separator tank and anti-trickle fill valve are located in fuel filler neck at right rear of vehicle. Fuel filler cap contains a relief valve to prevent excessive pressure or vacuum build-up in tank.

When ECM detects desired engine operation it opens EVAP control canister purge valve, allowing a regulated flow of fuel vapor from EVAP control canister. Fuel vapor is drawn into intake manifold and burned in combustion chamber. During canister purging, fresh air is drawn into canister through EVAP control vent solenoid valve.

Anti-Trickle Fill Valve

Anti-trickle fill valve or vent line flow restrictor is located at fuel filler neck. This valve is designed to prevent overfilling fuel tank to preserve fuel vapor expansion space in fuel tank. Anti-trickle fill valve is fitted to fuel filler pipe in EVAP system line between fuel tank and EVAP canister. During vehicle fueling anti-trickle fill valve is closed by insertion of fuel filling nozzle. When valve is closed a blockage is created in EVAP system vent line. When fill valve is closed, air displaced during fuel filling is routed from fuel tank through an internal fill breather. When fuel level reaches level of fill breather, fuel filler neck fills with fuel and shuts filling nozzle off.

EVAP Control Canister

EVAP control canister contains activated charcoal which absorbs fuel vapor and temporarily stores it. ECM will open a canister purge valve to purge fuel vapor and draw it into engine to be burned in combustion chamber. On Discovery, EVAP control canister is located in right front of engine compartment. On Range Rover, EVAP control canister is located in left rear of engine compartment.

EVAP Canister Vent Solenoid Valve

EVAP canister vent solenoid valve is use by ECM to control amount of fresh air drawn into canister. EVAP control canister vent solenoid valve is normally open, allowing fuel tank to breath through EVAP control canister. Valve is a located next to EVAP control canister. ECM will close EVAP control canister vent solenoid valve during EVAP system leak testing.

Fuel Leak Detection System (Vacuum Type)

Note. EVAP system leakage detection is dependent on differential pressure between fuel tank and ambient air pressure. EVAP system leak detection is disabled at altitudes of 9500 feet or more to avoid false detection of fuel leaks due to changes in atmospheric pressure at altitude.

The advanced EVAP control system equipped with a vacuum type, fuel leak detection system includes an EVAP canister and purge valve, a canister vent solenoid (CVS) valve and a fuel tank pressure sensor. The ECM controlled CVS valve is used to seal the EVAP system from atmospheric pressure during leak checks. The test is carried out when the vehicle is stationary and the engine is operating at idle. The system test uses the natural rate of fuel evaporation and engine vacuum. If a leak is detected that has a diameter greater than 0.04" (1.0 mm.), the ECM will illuminate the MIL. Fuel leak detection is part of the On-Board Diagnostics (OBD) strategy. Any fuel system leak that occurs between the purge valve output and the inlet manifold connection cannot be determined using this test. These will be detected through the fueling adaptation diagnostics.

Diagnostic EVAP system leak testing is repeated at regular intervals during current drive cycle, while engine is idling. Diagnostic leak testing will is not performed during EVAP canister purging, during fueling adaptation, or if excess slosh in fuel tank is detected (excess fuel vapor will be generated, invalidating the result). Following leak testing, EVAP system returns to normal purge operation when canister vent solenoid opens.

Possible reasons for an EVAP system leak test failure are

1. Fuel filler cap not properly tightened or missing.
2. Sensor or actuator circuit open.
3. Short circuit to battery voltage or ground.
4. Either purge or CVS valve stuck open.
5. Either purge or CVS valve stuck closed or blocked pipe.
6. Fuel line broken, not properly connected or a leaking fuel line connection.

SELF-DIAGNOSTIC SYSTEM

ECM monitors performance of engine for misfires, catalyst efficiency, exhaust leaks and evaporative control loss. If a fault occurs, the ECM stores a relevant fault code and notifies driver of component failure by illuminating Malfunction Indicator Light (MIL) located in instrument cluster. On vehicles equipped with A/T, ECM combines with Electronic Automatic Transmission (EAT) ECU to provide an OBD/powertrain operating strategy. If ECM detects a fault during its engine operation, it will be caused by one of the following

1. Minimum value of a component signal has been exceeded.
2. Maximum value of a component signal has been exceeded.
3. Component signal is not present or has not been detected.
4. An improbable condition has been diagnosed.

By analyzing various input signals system sensors, ECM detects system failures related to ignition, fueling or exhaust systems. ECM stores Diagnostic Trouble Codes (DTCs) associated with a detected failure until diagnostic system is cleared. A 16-pin Data Link Connector (DLC) is used for vehicle diagnostics. On Discovery models, DLC is located under steering wheel. On Range Rover models, DLC is located in front passenger footwell. When a relevant fault is detected, the fault condition is stored in the ECM memory.

MALFUNCTION INDICATOR LIGHT (MIL)

All models are equipped with a MIL in instrument cluster. MIL will illuminate when ignition switch is turned to ON position (bulb check) and when an engine management fault is detected. MIL indicator light will be illuminated when the fault is confirmed. Confirmation of a fault condition occurs if the fault is found to be present during 2 simultaneous driving cycles.

See also:
• INPUT DEVICES
• ECT SENSOR OUTPUT VOLTAGE
• MASS AIRFLOW (MAF)/INTAKE AIR TEMPERATURE (IAT) SENSORS (DISCOVERY SERIES II & RANGE ROVER)
• SELF-DIAGNOSTIC SYSTEM