February 2002

Bosch K-Jetronic (C.I.S.)

Fuel Injection

By Steve Grosekemper

 

Bosch K-jetronic fuel injection was first introduced on the 1973.5 Porsche 911T. It was used until 1983 on all 911's and on the 911 turbo until 1994. 924 and early 928 models got into the act as well. This system differs from all other Bosch fuel injection systems because it delivers a constant supply of fuel to the engine instead of injecting fuel at a specific time. This is where the common name Constant Injection System, or C.I.S. comes from.

A C.I.S. injection system is regulated by two opposing forces. The first force is system pressure which is measured in bar (1 bar = 14.7 p.s.i.). This pressure (about 5 bar) comes from the fuel pump and is regulated by a check valve in the fuel distributor, see figure 1.

The second force is control pressure (about 3 bar), which is regulated by a control pressure regulator. It is the control pressure that regulates the fuel mixture. Lower control pressures will give richer fuel mixtures.

When an engine is cold, it needs a richer fuel mixture and may have a control pressure of 1.8 bar. As the engine warms up and needs less fuel, the control pressure regulator will raise pressures to 3 - 3.5 bar. Checking the system and control pressures is as simple as installing the proper gauge between the fuel distributor and the control pressure regulator, see figure 2.

(See figure #3 for temperature/pressure specifications)

The third check is for residual pressure. Residual pressure is needed to keep the fuel in the injector lines from boiling. Common physics states that a higher pressure will give a higher boiling point. This is good if you want your C.I.S. equipped car to start when hot. If the residual pressure falls too low too quickly, the fuel will boil in the injector lines, creating air bubbles similar to a vapor lock condition. These air bubbles become trapped in the lines, and the engine will need a great deal of cranking before it will start. When it does finally start, the engine will only run on a few cylinders until all the fuel lines have purged themselves of air.

Minimum residual pressures are:
- 2.0 bar after 10 minutes
- 1.7 bar after 20 minutes

The most common reason for residual pressure failure is a leak past the fuel pump check valve. Fortunately, a new check valve can be installed in the fuel pump without replacing the entire unit. For you mathematicians in the group, yes this does save you a considerable amount of money!

The most important thing to remember about C.I.S. fuel injection is that it has an extreme aversion to dirt and moisture. It cannot tolerate any dirt or contaminates in the fuel system. So for proper C.I.S. health, be sure to use quality fuel and change the filter regularly.

For more information on C.I.S. fuel injection and other Bosch fuel injection systems, be sure to attend the PCA-SDR Tech Session at Dieter's on April 17th.

Good Luck!

 

Appointments - Ask a Tech - BMW - Porsche - Detailing - Directions - Events - Feedback - History - Links - Main -

Message Board - Order Form - Parts - PCA - Products - Quotes - Staff - Subscriptions - Warranty