The Passat fuel injection systems until 1994 used a CIS-E injection system. This system controls the fuel pressure dependent on a difference of pressures, which activates the fuel injectors. It is still divided into an upper and a lower chamber but the pressure difference between chambers is not constant. A differential pressure regulator, mounted on the side, controls the pressure in the lower chamber, which ultimately controls the fuel flow to the injectors. Control plunger movement controls the amount of fuel supplied to the upper chamber. When the engine is not running, the control plunger rests on an O-ring and there is some free-play between the plunger and airflow sensor arm.
In 1994 VW went to an electronically control fuel system. The injectors are controlled by the ECU. The same system is used also in the Audi A4 vehicles.
This type fuel system complies with OBD-I and OBD-II standards. Fuel is metered via engine speed and the mass of the air entering the intake manifold. The Engine Management System meters this fuel by firing the fuel injectors at a varying rate. Fuel volume is controlled by the duration of time the injector is left on. This "Injector Dwell Time" is usually measured in milliseconds of time.