Fuel Pump

The fuel pump is mounted in various places on the different VWs that have fuel injection. On the Bus, the fuel pump is located underneath the vehicle, near the left rear wheel. On the Vanagon, it is located on the passenger side middle of the van, mounted to the frame. On the Beetle and Super Beetle it is probably mounted at the front under the gas tank. In all models it is a cylindrical device mounted to the car with shock-absorbing mounts and in line with the fuel line from the gas tank.

The fuel injection system works at a higher pressure than the carbureted engines do. Therefore, the fuel injection system uses a special high pressure electric fuel pump as opposed to the mechanical pump used on the carbureted engines. The mechanical pump put out 3-5 psi of pressure while the fuel injection system operates at 28 psi.

The pump is a roller-cell pump. The fuel enters and is pressurized by the rotating motion of the roller and the elliptical pressure chamber. It's very similar in operation to a rotary engine. The gasoline then flows through the rest of the pump where it is used to cool the pump motor and then flows out of the pump into the fuel line. The pump also has a pressure limiting valve (basically a spring-loaded stopper) that returns fuel to the tank if pressure in the lines goes over a set limit. There is also a check valve on the output side of the pump which closes when the pump stops to keep the fuel lines pressurized when the engine is not running. This allows quicker restarts and serves to prevent vapor lock.

Vapor lock, for the curious, is what happens when the fuel in the lines evaporates to a vapor instead of being a liquid. Vapors don't pump very well, so vapor lock can prevent an engine from being started. This is usually only a problem in hot weather with carbureted vehicles. The problem is largely non-existent in fuel-injected vehicles.

The fuel pump is initially powered through the double relay when the key is turned to the start position. The pump operates as long as the starter operates. When the starter is turned off, the double relay stops supplying 12v to the pump, but by then the air flow meter fuel pump safety switch has kicked in, allowing the fuel pump to continue running. In the event of an accident where the engine stalls, the fuel pump, thanks to the safety switch in the air flow meter, will stop pumping fuel and the check valve in the pump will close, preventing siphoning from the gas tank.