Fuel System Tune Up
- Published: 24 November 2008 24 November 2008
- Created: 15 November 2008 15 November 2008
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Fuel System Tune Up
Driven primarily by tightening US emission standards, Porsche first used Bosch mechanical fuel injection to replace carburetors on the 1969 911 E and S. From the early 1970's through the early 1990's, Bosch K Jetronic, or continuous injection system (CIS), was used on the 911 (1973 - 1983), 911 Turbo (1976 - 1992), 924 (1976 - 1982), 924 Turbo (1981 - 1982), and 928 (1978 - 1979). Let's see what makes CIS tick.
CIS is predominantly a mechanical system, where the words "continuous injection" mean just that - there is always some level of gas constantly being fed to the injectors. Let's follow what happens when you start your engine. Key in the ignition, foot off the gas, turn key. The starter begins spinning, which allows the fuel pump to pump fuel from the tank towards the fuel accumulator. The accumulator helps buffer the fuel distributor from pulses in the fuel supply and, along with the fuel pump check valve, keeps pressure on the line to help with hot starts. Then it's on to the fuel filter, which is a very important component in a fuel injection system, keeping any fine grit from getting to the fuel injectors. After the fuel filter, the gas flows to the fuel distributor. Here two things happen: One, based on how much air is flowing past the airflow sensor (the plate that moves under the air filter), the fuel distributor determines how much gas to send to the fuel injectors. Second, gas is routed from the fuel distributor to the control pressure or warm up regulator. By varying pressure on the gas being fed to it, the warm up regulator helps the fuel distributor to richen the air-gas mixture when the engine is cold. As the above takes place, there's a cold start fuel injector which injects extra gas into the engine for cold starting purposes. The cold start injector uses a temperature switch (thermotime switch) to determine whether it should pump in extra fuel, which typically takes place when the engine temperature is less than around 100 degrees. To help the engine warm up, there's an auxiliary air valve, which allows air to bypass the airflow sensor, causing the idle speed to increase. Speaking of idle speed, there's an idle speed screw which also allows air to bypass the airflow sensor. The difference is that you can adjust the idle speed screw, unlike the auxiliary air valve. Note that unmetered air (air not passing through the airflow sensor), beyond that controlled by the idle screw or auxiliary air valve, can cause poor running in CIS engines.
Catalytic converters and lambda sensors came into play in 1980 for 911's and 924's, 1981 for the 924 Turbo, and 1986 for the 911 Turbo. The lambda, or oxygen or O2, sensor measures the level of oxygen in the exhaust stream, and causes the fuel injection system to make adjustments so the air-to-fuel ratio is correct for a given load on the engine. This variant is called K Jetronic with Lambda or just K Lambda.
There are several good sources for additional information on CIS systems: The Porsche Workshop Manuals, "Bosch Fuel Injection and Engine Management" by Charles Probst, "Porsche 911SC Service Manual: 1978-1983" by Bentley, "How to Tune and Modify Bosch Fuel Injection" by Ben Watson, and articles in both the PCA "Panorama", and the "Up-Fixin'" series of past PCA "Panorama" articles.
Written by Bill Gregory for the "Challenge", monthly publication of the Connecticut Valley Region, Porsche Club of America.