General information and precautions

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

Genera/ information
1 The Digifant system is a completely self-contained 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, a fuel rail, a fuel pressure regulator, four electronic fuel injectors, and an Electronic Control Unit (ECU) together with its associated sensors, actuators and wiring.

3 The fuel pump delivers a constant supply of fuel through a cartridge filter to the fuel rail, at a slightly higher pressure than required - the fuel pressure regulator maintains constant fuel pressure to the fuel injectors and returns excess fuel to the tank via the return line. This constant flow system also helps to reduce fuel temperature and prevents vaporisation.

4 The fuel injectors are electromagnetically operated pintle valves, which are opened and closed many times per second by an Electronic Control Unit (ECU). The ECU calculates the injection timing and duration according to engine speed, crankshaft position, throttle position and rate of opening, inlet air volume flow rate, inlet air temperature, coolant temperature, road speed and exhaust gas oxygen content information, received from sensors mounted on and around the engine.

5 Inlet air is drawn into the engine through the air cleaner assembly, which contains a renewable paper filter element. An air flow meter is also integrated into the air cleaner assembly. This sensor is part of the engine management system and monitors engine load by measuring the volume flow rate of the inlet air entering the engine. It sends this information to the ECU in the form of a varying electrical signal.

6 On engine code 3F, the inlet air temperature is regulated by a vacuum operated valve mounted in the air cleaner, which blends air at an ambient temperature with hot air, drawn over the exhaust manifold.

7 On engine code PY, inlet air is supplied to the engine at a positive pressure by a belt driven supercharger. The filtered air is drawn into the supercharger, where it is compressed to a maximum pressure of around 0.5 bar, depending on engine speed and load. An intercooler, mounted in the air flow through the engine bay, cools the inlet air, increasing its density and maximising the amount of oxygen entering the combustion chambers.

This helps to keep combustion temperatures down and increases engine power and torque. An air bypass valve directs excess air back to the supercharger inlet when the engine load is low, i.e. on part throttle or overrun.

8 Idle speed control is achieved by means of an auxiliary air valve, mounted in parallel with the throttle body, which allows a variable amount of air to bypass the throttle disc and enter the engine. The ignition system provides idle speed stability by increasing and decreasing the engines torque through small alterations to the ignition timing.

9 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.

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

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

11 It should be noted that comprehensive fault diagnosis of the 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 be renewed as required.

Note: Throughout this Chapter, vehicles are frequently referred to by their engine code, rather than by engine capacity - refer to Chapter 2A for engine code 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 to hand 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.

12 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.

13 Residual fuel pressure always remains 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 8 for details.

14 When working with fuel system components, pay particular attention to cleanliness - dirt entering the fuel system may cause internal blockages which could lead to running problems later.

15 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”.

16 It should be noted, however, that the engine management system described in this Chapter (and Chapter 5B) has a “learning” capability, that allows the system to adapt to the engines running characterteristics as it wears with normal use. This “learnt” information is lost when the battery is disconnected - on reconnection the system will then take a short period of time to “relearn” the engines’ characteristics. This may be manifested (temporarily) as rough idling, reduced throttle response and possibly an 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.