General information and precautions

Fuel system: single-point injection models / General information and precautions

General information
1 The Bosch Mono-Motronic system is a selfcontained engine management system, which controls both the fuel injection and ignition.

This Chapter deals with the fuel injection system components only - refer to Chapter 5A for details of the ignition system components.

2 The fuel injection system comprises a fuel tank, an electric fuel lift pump an electric fuel line pump, a fuel filter, fuel supply and return lines, a throttle body with an integral electronic fuel injector, and an Electronic Control Unit (ECU) together with its associated sensors, actuators and wiring.

3 The fuel lift and line pumps deliver a constant supply of fuel through an accumulator/coarse mesh filter and a fine mesh cartridge filter to the throttle body, at a slightly higher pressure than required. The fuel pressure regulator (integral with the throttle body) maintains a constant fuel pressure at the fuel injector and returns excess fuel to the tank via the return line. This constant flow system also helps to reduce fuel temperature and prevents vaporisation, aiding hot starting.

4 The fuel injector is an electromagnetically operated pintle valve, which is opened and closed many times per second by an Electronic Control Unit (ECU). When the valve is open, fuel is sprayed in conical pattern onto the back of the throttle disc. The ECU calculates the injection timing and duration according to engine speed, throttle position and rate of opening, inlet air temperature, coolant temperature, road speed and exhaust gas oxygen content information, received from sensors mounted on the engine.

5 Inlet air is drawn into the engine through the air cleaner, which contains a renewable paper filter element. The inlet air temperature is regulated by a vacuum operated valve mounted in the air cleaner, which blends air at ambient temperature with hot air, heated by the exhaust manifold.

6 Idle speed control is achieved as follows.

An electronic throttle positioning module, mounted on the side of the throttle body alters the amount of air entering the engine and hence the idle speed. The ignition system provides idle speed stability by increasing and decreasing the engines torque through making small alterations to the ignition timing.

As a result, manual adjustment of the engine idle speed is not necessary.

7 To improve cold starting and idling, an electric heating element is mounted on the underside of the inlet manifold; this prevents fuel vapour condensation on the manifold walls when the engine is cold. Power is supplied to the heater by a relay, which is in turn controlled by the ECU.

8 The exhaust gas oxygen content is constantly monitored by the ECU via the Lambda sensor, which is mounted in the exhaust pipe. The ECU then uses this information to modify the injection timing and duration to maintain the optimum air:fuel ratio - a result of this is that manual adjustment of the idle exhaust CO content is not necessary.

In addition, certain models are fitted with an exhaust catalyst - see Chapter 4C for details.

9 The ECU also controls the operation of the activated charcoal filter evaporative loss system - refer to Chapter 4C for further details.

10 It should be noted that fault diagnosis of the Bosch Mono-Motronic engine management system is only possible with dedicated electronic test equipment.

Problems with the systems operation should therefore be referred to a VW dealer for assessment. Once the fault has been identified, the removal/refitting sequences detailed in the following Sections will then allow the appropriate component(s) to renewed as required.

Note: Throughout this Chapter, engines are frequent/y referred to by their code number, rather than by capacity - refer to Chapter 2A for engine code number listings.

Precautions
Warning: 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 been emptied of liquid fuel will still contain vapour and can be potentially explosive.

11 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.

12 Residual fuel pressure always remain in the fuel system, long after the engine has been switched off. This pressure must be relieved in a controlled manner before work can commence on any component in the fuel system - refer to Section 9 for details.

13 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.

14 In the interests of personal safety and equipment protection, many of the procedures in this Chapter suggest that the negative cable be removed from the battery terminal. This firstly eliminates the possibility of accidental short circuits being caused as the vehicle is being worked upon, and secondly prevents damage to electronic components (eg sensors, actuators, ECU’s) which are particularly sensitive to the power surges caused by disconnection or reconnection of the wiring harness whilst they are still “live”.

15 It should be noted, however, that certain engine management systems described in this Chapter (and Chapter 5B) have a “learning” capability, that allows the system to adapt to the engine’s running characteristics as it wears with use. This “learnt” information is lost when the battery is disconnected and the system will then take a short period of time to “re-learn” the engines’ characteristics - this may be manifested (temporarily) as rough idling, reduced throttle response and possibly a slight increase in fuel consumption, until the system re-adapts. The re-adaptation time will depend on how often the vehicle is used and the driving conditions encountered.