Diagram of the Lambda CIS

The fuel flow diagram for the Lambda CIS is basically the same as for mid-year CIS. There are two notable differences. One is that the Lambda system for the Porsche 911 does not have a vacuum connection for the WUR (5) , and the other is the addition of the Frequency Valve (11) connected between the fuel distributor lower chamber and the fuel return line.



(Although the diagram illustrates the system as installed on a water-cooled engine, the components are the same.)

Lambda General Description

In 1980, the US CIS system was modified to include an O2 sensor system, also called the Lambda system.  It consists of  the sensor, the control box, the frequency valve which adjusts the mixture at the fuel distributor, and several input devices to the control box.  The input devices include an engine temperature sensor, a throttle switch, and the O2 sensor itself, mounted in the exhaust flow.  The basic function of the  Lambda system is to adjust the fuel mixture at or near stoichiometric (air-to-fuel mixture of 14.7:1).  This particular air/fuel ratio enhances the operation of the Catalytic convertor, with which the Lambda system was designed to operate.  The Cat operates more efficiently in reducing emissions when the fuel mixture is at stoichiometric. 

The Lambda system does its job by adjusting the duty cycle of the frequency valve, which is plumbed into the lower portion of the fuel distributor control pressure circuit. A new version of the Fuel Distributor was required for this change. If the O2 sensor determines the mixture is too rich, it signals the frequency valve to reduce the duty cycle, which leans the mixture from the fuel distributor.  If it is too lean, it increases the duty cycle.  If the mixture is spot on (14.7:1 air-to-fuel), the duty cycle to the frequency valve is 50%.  Note that the default duty cycle mode for a disconnected or non-operational O2 sensor is also 50%.

The O2 electronics box also has an enrichment function controlled through the throttle switch.  This enrichment task is taken over from the vacuum operated WUR in the earlier CIS engines.  The O2 sensor was installed with the '80 cars, and the vacuum operated WUR was discontinued. In '81, the O2 electronics were slightly changed from '80 and provided additional enrichment features. The electronics box also provides cold starting warm up enrichment in addition to the cold engine enrichment provided by the WUR.

The cold start enrichment by the Lambda system sets the frequency valve at 65% , overriding the input from the O2 sensor, until the engine temperature reaches 15˚C.  At temperatures in excess of 15˚C, closed loop operation begins and the oxygen sensor takes over.  The throttle valve switch also provides enrichment at WOT.

Lambda operation - 1980

[Space reserved for tables describing the 2 temperature based modes of operation of the '80 Lambda system.]

Lambda operation - 1981 to 1983

In 1981, an "Enrichment relay" was added to the Lambda control. An additional temperature switch was added at the engine as an input to the Enrichment Relay. The addition of this switch neccesitated a change in the engine's wiring harness to the ECU from a 6-pin connector to a 12-pin connector. The additional switch is open until 35˚C, and closed above this temperature. The Enrichment Relay operates as a functional part of the ECU to provide transient enrichment stages according to the positions of the throttle switches and engine temperature.

The 3 engine temperature based modes of operation of the '81 - '83 Lambda system are:

Mode 1: Temperature less than 15˚C

Mode 2: Temperature greater than 15˚C but less than 35˚C

Mode 3: Temperature greater than 35˚C