CIS Fuel Pressures

Both the fuel pressures and the fuel flow at different points in the fuel path, from the pump to the tip of the injectors, are paramount to the proper operation of CIS.

The fuel pump itself does not determine the pressure. However, it must be capable of producing both the pressure and the flow rate required to sustain proper system operation and fuel delivery at maximum horsepower.

System Pressure

System pressure is set by the Fuel Distributor's internal regulator. This pressure is contolled by a piston/spring adjusted by shims which in turn determine the spring pressure. The spring pressure is adjusted at the factory, or by the rebuilder when refurbishment of the FD becomes neccesary. Excess fuel from the pressure regulator flows back through a return line to the fuel tank. System pressure for all normally aspirated 911s is 5 Bar. For the Turbo cars (930) system pressure is higher, around 6.5 Bar.

Control Pressure

The control pressure, which determines the fuel mixture changes from a cold to a warm engine, is set by the warmup regulator (don't forget also the TCPR in '73 and '74) for CIS systems preceding the Lambda components in 1980. This cold-to-warm change is accomplished by the Contol Pressure Regulator, more commonly known as the Warm-Up Regulator, or for short, the WUR.

The WUR is connected to a port on the top of the Fuel Distributor. This port bleeds off a portion of the fuel at system pressure which passes through an internal port in the FD to a chamber on top of the FD piston and on to the WUR. ... See description The WUR pressures at cold engine temperatures are from 1 to 2 Bar, and typically 3 to 3.5 Bar at operating temperature. Excess fuel from the WUR is also returned to the Fuel tank. Each model year or range of model years has it's own specs for cold to warm pressures. Note the table under Testing for these numbers. When performing tests on the CIS, a pressure gauge is a neccesary tool.

Typical Pressure Readings with Pressure Guage Connected

FD regulator

The early cars had an additional fuel flow path through the Throttle Control Pressure Regulator which provided additional mixture adjustment according to throttle position. The TCPR was in parallel with the WUR for 1973 - 1974 US models and 1973 - 1975 for Euro models.

In the Lambda cars, a different flow path is added to that of the 1975 - 1979 US cars. As discussed in the CIS Description for Lambda systems, in addition to the WUR regulating the mixture via the pressure on the FD piston, the Lambda ECU provides an enhanced regulation of the mixture by adjusting the pressure in the bottom chambers of the FD. Excess fuel from bleeding off pressure in the FD bottom chambers by the Frequency Valve is returned to the Fuel tank.

The control pressures for various WURs are shown in the Troubleshooting section.