This pump had it's prime-time, at the time when VW still favoured the pump-jet injection. It was mounted high on the cylinder head behind the vacuum pump and driven by the camshaft. It transports fuel from the tank directly to the units in the cylinder head.
A three-excenter cam rotates in a low cylinder. Two blocking-vanes at an angle of exactly 180° are pressed against it by springs, thus sealing off one half of the remaining cylinder area from the other half.
During the rotation of the three-excenter cam the remaining space on the respective side of the blocking-vane, which creates a suction effect when a recess in the cam passes the blocking-vane and a pressure effect, when it nears the other blocking-vane.
Exactly at these two points is where the respective suction- and pressure lines enter. The first is connected by a long line with the tank, the latter with a drilling in the cylinder head. In addition, if the pressure is too high a flow-back pipe to the tank is opened by a spring-loaded check-valve.
The advantage of this type of of the fuel pumping is that it begins already at low revs., in principle, after just half a rotation, this guarantees the lubrication of the friction surfaces, also when the tank is empty. Although one should prime this type of system, the pump is, in fact, self-priming. 02/13