The purpose of the information on these pages is to provide answers to some of the basic questions which owners of 911 CIS cars (73 1/2 through 83 911s) typically have about their fuel systems, provide a reference point for troubleshooting, and for identification of components for those who are attempting to acquire basic knowledge.
For the 911, the changeover to CIS (Continuous Injection System) from MFI (Mechanical Fuel injection) started in late the '73 model year with the 2.4 liter 911 T. It was a relatively simple system when compared to that of the end-of-the -line CIS on the '83 SC. By the '84 model year, the CIS was history, being replaced by the Motronic Digital Motor Electronics.
1973 CIS Engine
1974 though 1983 chronology
In '74, with the upgrade to the 2.7 liter engine, several changes were made to the fuel system.
Intake pipes changed from bent tubing to cast intake runners.
Documentation of the CIS from '73 up through the '75 models is somewhat confusing, as there is just enough left out of the various books to make a hazy picture. The little factory spec book doesn't mention the specs for a vacuum controlled WUR for '74 or '75, but the factory manual does. So if one doesn't have personal knowledge of what the factory did, it's a little hard to figure out from reading the various books. Each of the books is vague in different areas, and there is not a book I have found yet that states they were definitely both used or that one was used and definitely not the other, for any of the years in question.
Some time in '74, there was a change to a vacuum assisted WUR, which eventually led to the discontinuance of the throttle operated Control Pressure Regulator for the '76 models. This component adjusted the fuel mixture depending on throttle position, leaning out the mixture at mid-throttle position. The vacuum assisted WUR in '76 took over the job of mixture adjustment based on engine load determined from intake manifold vacuum.
'76 saw other changes in the CIS system. The hand throttle was replaced by the auxiliary air valve, and the auxiliary air regulator. The fuel cutoff switch, located on the airflow sensor, was added as a safety feature. In '77, a vacuum switch, or thermo valve, was included to further enrich the mixture during cold starts.
The internal diameter of the intake runners was increased in '78, lasting through '79, and in '80 was reduced again for the US engines, while the RoW engines retained the large runners.
This was the last change until 1980 when the Lambda or O2 sensor was added. These components included the O2 sensor, the control box, a microswitch on the throttle body, and the frequency valve, which is plumbed into the fuel distributor to adjust the fuel mixture. The thermovalve was eliminated in this year. An internal manifold was also added to the air box to better distribute the cold start mixture. An additional throttle switch, an acceleration enrichement relay, and an updated control box were added in 1981.