The Volkswagen fuel pump, except on fuel injected engines, is mechanical, and of the diaphragm type, being pushrod operated from a cam on the distributor drive gear. The fuel flow is regulated automatically as the fuel is used up from the float bowl. The fuel pump consists of the top cover, containing the suction valve and the delivery valve, and the lower half, which contains the rocker mechanism.
As the distributor shaft turns, the cam on the distributor drive gear pushes the pushrod against the rocker arm, which in turn pulls the diaphragm downward against the diaphragm spring. In this way a vacuum is created above the diaphragm, causing the lifting of the suction valve off its seat so that fuel can be drawn in. After the pushrod moves away, the loaded diaphragm spring pushes the diaphragm upward, forcing the fuel in the pump through the delivery valve and to the carburetor. This process is repeated each time the distributor drive gear turns, which is once every two turns of the engine.
The pump pressure is determined by the amount the diaphragm spring is compressed during the suction stroke of the pump. The pressure of the spring is balanced by the upward force of the carburetor float on the needle valve. The higher the fuel level in the carburetor, the greater is the upward force on the needle valve. Under normal engine operation, the diaphragm of the fuel pump is moved only a fraction of an inch.
Other than cleaning the filter of the pump at regular intervals, no other maintenance is necessary. The pushrod and pump rocker arm are lubricated by the lubricant in the lower part of the pump.
REMOVAL & INSTALLATION
The fuel pump is removed by taking off the fuel line, disconnecting the hose from the pump, and removing the retaining nuts from the mounting studs. After the pump has been removed, the intermediate flange, pushrod and gaskets can be removed. Be careful in handling the pushrod, as it could be inconvenient to have to fish it out of the crankcase.
Once removed, the stroke of the fuel pump is adjusted by the insertion or removal of the proper number of flange gaskets. Adjustment is checked after installing the intermediate flange with two gaskets and pushrod, and nuts are tightened to the same tightness as if the entire pump were being installed. Normal full-stroke is approximately 4mm. The length of the pushrod stroke is measured from the pump contact surface on the intermediate flange, including gaskets.
When installing the fuel pump, care must be taken to install the intermediate flange before the pushrod, otherwise the rod may fall through into the crankcase. Before installing the fuel pump, the lower chamber should be filled with universal grease. Tighten nuts to mounting studs, taking care not to overtighten. (Nuts should be retightened when the engine has reached operating temperature.) Connect the fuel line and hose, and check for correct seating of the fuel line rubber grommet in the panel of the engine compartment.
Fuel pump pressure can be checked by the insertion of a suitable gauge between the pump and the carburetor. Correct fuel pump pressure for various models is given in the Capacities and Pressures Chart.Rebuild Kits
Carburetor and fuel pump repair and rebuilding kits are available at authorized Volkswagen dealers and various other sources. These kits contain the critical parts of the units to be rebuilt or repaired, and are well worth the money compared to the trading in of the old unit on a new one. Such kits are also handy to have on hand during long trips through low-population areas, because even the ultra-reliable Volkswagen sometimes (though rarely) becomes incapacitated due to unexpected failures in these two most important elements of the fuel system.