Each cylinder has its own fuel injector that resides in the fuel rail attached to the inlet manifold. The fuel rail is supplied by the fuel pump via an in-line fuel filter. The pressure is controlled by a fuel pressure regulator (FPR) which permits up to 3bar fuel pressure in a full-load condition. More about the FPR in another section.

The fuel injectors themselves utilise an electromagnetic solenoid to rapidly open and close the jet nozzle at the tip of the injector. This jet nozzle has a single aperture on the 3B engine. On the ABY engine, a slightly different version of the fuel injector is used. This variety has four injector nozzle which provide even finer atomisation of the fuel spray when the injector is activated.

Each fuel injectors is triggered sequentially by a switched ground connection that is controlled by the ECU in accordance with its fuel injection map. The trigger pulses are typically of 1-5ms duration. Each injector is triggered one engine revolution before the ignition firing point on each cylinder i.e All five injectors are pulsed every two engine revolutions - or every 144 degrees of crankshaft rotation (or 54 flywheel teeth).

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Testing the Fuel Injectors

There are several tests which can be performed to verify if the injectors are working correctly. Firstly, with VAG-COM (or an OBD jumper cable) you can run the ECU 'output tests' to individually trigger the injectors in firing sequence order of 1-2-4-5-3.

The resistance measured across the two terminals on the injector should be in the range 15-17 ohms.

The amount of fuel flowing thru the injectors (regardless of 3B or ABY) at nominal fuel rail pressure is specified to be in the range of 150-170ml over a 30s time period. This equates to fuel flow of 320cc per minute. This figure is used a rule of thumb method to provide sufficient fuelling for up to approx 320bhp on the Audi S2 engine. For that reason, even higher performance injectors were used on the RS2 engine to provide sufficient performance headroom.

It is not uncommon for old injectors to require cleaning and recalibration after high mileage. This work should only be carried out by trained agents on a machine such as manufactured by a company called ASNU. This uses ultrasonic technology to clean the injectors and accurate flow measurin equipment that compares the spray patterns and verifies the fuel throughput over time. Such injector servicing is an inexpensive way to extend the length of fuel injectors. The serviced injector also has filter baskets and O-ring seals renewed as a matter of course.