Basic Fuel Flow Diagram of 911 CIS


The above is a basic fuel flow diagram of the Porsche 911 CIS fuel system as seen in the CIS Description. It shows the basic components which appear in all the CIS models.

Fuel is drawn by the fuel pump from the tank through the accumulator and fuel filter and into the fuel distributor. The lower chambers (6) are commonly connected to each other, and to the pressure regulator which sets the system pressure, about 70 psi. The regulator is a bypass type, with a spring determining the opening of the bypass port sending fuel to the return line leading back to the fuel tank. The effective spring length and resulting fuel pressure is set through the use of shims.

Through the primary restriction (small hole in the internal diaphragm), fuel flows to the top if the internal piston, where it pushes down with opposing force to the airflow pushing up on the sensor plate, and out the top of the fuel distributor to the Control Pressure Regulator (WUR).dada

The fuel for the cold start valve is also tapped into the lower chamber.

The upper chambers, with their individual valves, supply the fuel to the injectors, with the injector flow rate depending on the position of the narrow slits in the piston allowing fuel into the upper chambers.

Lambda flow adds another path from the bottom chambers of the Fuel Distributor through the Frequescy Valve and back to the FD where it joins the return to the fuel tank. ( See CIS Description > Lambda)

Flow Rates

There are two basic flow rates of importance, both are checked relatively simply.

Fuel Pump:

The fuel pump has to be capable of maintaining pressure and flow simultaneously. At system pressure, the flow rate back through the return line to the tank can be checked by running a temporary hose to a container and measuring the volume of fuel over a 30 second period. The amount varies from 750...1000 ml/30 sec. from early models to late models.

WUR:

The flow rate from the fuel distributor to the WUR can be determined by capturing the fuel normally sent to the WUR over a 1 minute period with the fuel pump running. The pressure line to the WUR is disconnected and with the pump running, the fuel caught in a container and measured. The amount is 160...240 ml, nominally 225 ml.