Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Description of System


See Figures 1 and 2

The Continuous Injection System (CIS) is an independent mechanical system. The basic operating principle is to continuously inject fuel into the intake side of the engine by means of an electric pump. The amount of fuel delivered is metered by an air flow measuring device. Since CIS is a mechanical system, there is no electronic "brain" deciding when or how much fuel to inject.

The primary fuel circuit consists of an electric pump, which pulls fuel from the tank. Fuel then passes through an accumulator. The accumulator is basically a container in the fuel line. It houses a spring-loaded diaphragm that provides fuel damping and delays pressure build-up when the engine is first started. When the engine is shut down, the expanded chamber in the accumulator keeps the system under enough pressure for good hot restarts with no vapor locking. Fuel flows through a large, paper element filter to the mixture control assembly.

The mixture control assembly is the heart of the CIS system. It houses the airflow sensor and the fuel distributor. The air sensor is a round plate attached to a counterbalanced lever. The plate and lever are free to move up-and-down on a fulcrum. Accelerator pedal linkage connects to a throttle butterfly, which is upstream (closer to the manifold and intake valves) of the air sensor. Stepping on the accelerator pedal opens the throttle valve. Increased air, demanded by the engine, is sucked through the air cleaner and around the air sensor plate.

In the air funnel, where the air sensor plate is located, the quantity of intake air lifts the plate until an equilibrium is reached between air flow and hydraulic counter-pressure acting on the lever through a plunger. This is the control plunger. In this balanced position, the plunger stays at a level in the fuel distributor to open small metering slits, one for each cylinder in the engine. Fuel under controlled pressure from the pump goes through the slits to the injectors' supply opening. The slit meters the right amount of fuel.

In order to maintain a precise fuel pressure, a pressure regulator, or pressure relief valve, is located in the primary fuel circuit of the fuel distributor. Excess fuel is diverted back to the tank through a return line. To make sure the amount of fuel going through the control plunger slits depends only on their area, an exact pressure differential must always be maintained at the openings. This pressure is controlled by a differential-pressure valve. There's one valve for each cylinder. The valve consists of a spring loaded steel diaphragm and an outlet to the injectors. The diaphragm separates the upper and lower chambers.

The valve keeps an exact pressure differential of 1.42 psi between upper chamber pressure and lower chamber pressure. Both pressures act on the spring loaded steel diaphragm which opens the outlet to the injectors. The size of the outlet opening is always just enough to maintain that 1.42 psi pressure differential at the metering slit. The diaphragm opens more if a larger amount of fuel flows. If less fuel enters the upper chamber, the diaphragm opens less and less fuel goes to the injectors. An exact pressure differential between upper and lower chamber is kept constant. Diaphragm movement is actually only a thousandths of an inch (few hundreths of a millimeter).

The control pressure regulator can alter the pressure on the control plunger according to engine and outside air temperature. For warm-up running, it lowers the pressure so that the air sensor plate can go higher for the same air flow. This exposes more metering slit area, and more fuel flows for a richer mixture. For cold starts, a separate injector is used to squirt fuel into the intake manifold. This injector is electronically controlled. A thermo-time switch, screwed into the engine, limits the amount of time the valve is open and at higher temperatures, cuts it off.

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