| General information |
The fuel system consists of a fuel tank, situated at the rear of the car, with an electric fuel pump immersed in it, a fuel filter, fuel feed and return lines, and the throttle body assembly (which incorporates the single fuel injector and the fuel pressure regulator), as well as the engine management electronic control unit (ECU) and the various sensors, electrical components and related wiring. The air cleaner contains a disposable paper filter element, and incorporates a flap valve air temperature control system, which allows cold air (from the outside of the car) and warm air (heated by the exhaust manifold) to enter the air cleaner in the correct proportions. To reduce emissions and to improve driveability when the engine is cold, the inlet manifold is heated by the cooling system coolant and by an electric pre-heater system. Mixture enrichment for cold starting is a pre-programmed function of the system. The ECU fully controls both the ignition and fuel injection systems, integrating the two in a complete engine management system; refer to Chapter 5B for information on the ignition side of me system. The Rover/Motorola Modular Engine Management System uses ECU controlled single-point injection (MEMS-Spi) and the speed/density method of airflcw measurement. The whole system is best explained if considered as three sub systems; the fuel delivery, air metering and electrical control systems. The fuel delivery system incorporates the fuel tank with an electric fuel pump (immersed in a swirl pot to prevent aeration of the fuel) inside it. When the ignition is switched on, the pump is supplied with current via the fuel pump relay, under the control of the ECU; the pump feeds petrol via a non-retum valve (to prevent fuel draining out of the system components and back to the tank when the pump is not working) to the fuel filter, and from the filter to the injector. Fuel pressure is controlled by the pressure regulator, which lifts to allow excess fuel to return to the tank swirl pot, where a venturi causes the returning fuel to draw cool fuel from the tank into the swirl pot. In the event of sudden decelleration (ie, an accident) an inertia switch cuts off the power to the pump, so that the risk of fire from fuel spraying out of broken fuel lines under pressure is minimised. The air metering system includes the inlet air temperature control system and the air cleaner, but the main components are in the throttle body assembly. This incorporates the injector (which sprays fuel onto the back of the throttle disc). the throttle potentiometer (which is linked to the throttle disc spindle,and sends the ECU information on the rate of throttle opening by transmitting a varying voltage), and the stepper motor, (which is controlled by the ECU, and operates the throttle die cspindle lever via a cam and pushrod to provide idle speed control). The electrical control system consists of the ECU, with all the sensors that provide it with information, and the actuators by which it controls the whole system's operation. The ECU's manifold absolute pressure (MAP) sensor is connected, by hoses and a fuel (vapour) trap mounted on the bulkhead to the inlet manifold; variations in manifold pressure are converted into graduated electrical signals, which are used by the ECU to determine the load on the engine. The inlet air temperature sensor is self-explanatory; the cranckshaft sensor provides the engine speed and crankshaft position: the coolant temperature sensor supplies the engine temperature, the accelerator pedal switch tells the ECU when the accelerator is closed; the throttle potentiometer is explained above, and the function of the lambda sensor is explained in Part C of this Chapter. The ECU also senses battery voltage, and can adjust the injector pulses width and use the stepper motor to increase the idle speed and, the alternator output it if is too low. Short-circuit protection and diagnostic capabilities are incorporated; the ECU can both receive and transmit information via the diagnostic connector, thus permitting engine diagnosis and tuning by Rover diagnostic equipment. If either the coolant temperature sensor, the inlet air temperature sensor or the manifold absoIute pressure sensor circuits should fail, the ECU has a back up facility which assumes a valve corresponding to a coolant temperature of 60 degG, an inlet air temperature of 35 degC and an engine load based on the engine speed and throttle position; these are used to implement a back-up air/fuel mixture ratio. All these signals are compared by the ECU, using digital techniques, with set values pre-programmed (mapped) into its memory. Based on this information, the ECU selects fuel and ignition settings appropriate to these values, and controls the ignition HT coil (varying the ignition timing as required). the fuel injector (varying its pulse width - the length of time the injector is held open - to provide a richer or weaker mixture, as appropriate), the stepper motor (controlling the idle and fast idle speeds), the fuel pump relay (controlling the fuel delivery), the manifold heater relay (controlling the inlet manifold pre-heater system) and the main relay, the purge control vaIve, and the lambda sensor and relay, accordingly. The mixture, idle speed and ignition timing are constantly varied by the ECU to provide the best settings for cranking, starting and engine warm up (with either a hot or cold engine), idle, cruising, and acceleration. A rev-limiter circuit is built into the ECU, which switches off the injector earth (ie, the fuel supply) if the engine speed exceeds the recommended limit. The injector earth is also switched off on the overrun, to improve fuel economy and reduce exhaust emissions. The ECU idle control is an adaptive system; it learns the engine load and wear characteristics over a period of time, and adjusts the idle speed to suit. If the ECU is renewed, or one from another car is fitted, it will take a short period of normal driving for the new ECU to learn the engine's characeristics and restore full idle control. |
| Precautions |
Waming. Petrol is extremely flammable - great care must be taken when working on any part of the fuel system. Do not smoke or allow any naked flames or uncovered light bulbs near the work area. Note that gas powered domestic appliances with pilot flames such as heaters, boilers and tumble dryers, also present a fire hazard - bear this in mind if you are working in an area where such appliances are present. Always keep a suitable fire extinguisher close to the work area and familiarise yourself with its operation before starting work. Wear eye protection when working on fuel systems and wash off any fuel spilt on bare skin immediately with soap and water. Note that fuel vapour is just as dangerous as liquid fuel, a vessel that has just been emptied of liquid fuel will still contain vapour and can be potentially explosive. Petrol is a highly dangerous and volatile liquid, and the precautions necessary when handling it cannot be overstressed. Many of the operations described in this Chapter involve the disconnection of fuel lines, which may cause an amount of fuel spillage. Before commencing work, refer to the above Warning and the information in 'safety first' at the beginning of this manual. When working with fuel system components, pay particular attention to cleanliness - dirt entering the fuel system may cause blockages which will lead to poor running. Note. Residual pressure will remain in the fuel lines long after the vehicle was last used, when disconnecting any fuel line, it will be necessary to depressurize the fuel system as described in Section 6. |