One advantage of the Volkswagen fuel injection system is the reduction of unburnt fuel, carbon monoxide and hydrocarbons in the exhaust gas. The engine receives the proper air/fuel mixture under all conditions as the result of the sophisticated controls designed into the system. The fuel injection system is made up of three sub-systems-the fuel system, the air system and the control unit.
Fuel is drawn from the front-mounted tank through a filter by an electric fuel pump. The pump pushes the fuel through the pressure line into the fuel loop line, to which the electro-magnetic injectors are connected by distributor pipes. A pressure regulator is present to keep the fuel at a constant pressure of 28 psi. Excess fuel flows back to the tank via a return line.
Four pipes are used to transfer air into the four cylinders. Connected to the pipes is the intake air distributor, which is in turn connected to the pressure switch and pressure sensor. The intake side of the air distributor contains a throttle valve attached to the accelerator pedal via a cable. When the engine is idling, the throttle valve is fully closed and the air must pass through an idling air circuit. An idling air screw controls the amount of air for idling. An auxiliary air regulator is used to vary the quantity of air to suit the engine temperature when the engine is idling.
The central brain of the fuel injection system is the control unit which controls the amount of fuel injected according to various inputs, such as the engine speed, the intake manifold pressure, and the engine temperature. The main relay supplies current to the control unit when the ignition is turned ON. The control unit controls the fuel pump via the pump relay.
The control unit "masterminds" the operation by electrically opening the injector valves in pairs. Nos. 1 and 4, and 2 and 3 injector valves open simultaneously. It keeps the injector valves open for exactly the right length of time to suit the requirements of the engine. Because the fuel is under constant pressure (28 psi), the amount of fuel injected into the cylinders is directly proportional to the length of time that the injectors are held open by the control unit. In "deciding" how long to keep the injectors open, the control unit processes information received from the following sources: pressure sensor, pressure switch, two temperature sensors, trigger contacts in the distributor, and the throttle valve switch. The pressure sensor senses the load condition of the engine by measuring the pressure in the intake manifold. The pressure switch controls fuel enriching at full load, and is actuated by the difference in pressure between the air in the intake manifold and the air in the surrounding atmosphere. Two temperature sensors in the crankcase and cylinder head control the mixture enrichment both during starting and warming up of the engine. On later units, one sensor is fitted in the intake air distributor and the other to the cylinder head. Trigger contacts in the distributor "tell" the control unit into which set of cylinders to inject the fuel. The contacts also regulate injection timing. The function of the throttle valve switch is to cut off the supply of fuel when the engine is decelerating. In later models, the throttle valve switch also controls mixture enrichment on acceleration.
A cold starting device is present in later units. This device consists of a fuel jet fitted to the intake air distributor and a thermostatic switch. The switch actuates at 55°F. The cold starting device is available as a kit for installation in earlier models. In current production, a new cold starting device is used, consisting of a thermostatic switch, which operates between 32°F and 50°F and a fuel jet combined into a single unit, fitted under and between the intake manifolds on the right side. The new cold starting device cannot be used to replace the earlier unit.
TESTING & TROUBLESHOOTING
Testing and troubleshooting of the fuel injection system requires special Bosch electronic testing apparatus. For this reason, these operations are best left to qualified personnel at an authorized dealer's shop. Removal and replacement of components, and adjustments that can be made without special equipment are covered in this section. Refer to for ignition timing and idle speed adjustments.