Case Studies

Bosch CIS (Continuous Injection System) or K-Jetronic Demystified

So you bought a vintage Audi, BMW, Mercedes, Saab, Volvo or VW with the Bosch CIS fuel injection system. To which I say, “Cool” I cut my teeth on CIS as a young tech back in the 80s and 90s, and have a strong fondness for the system. The design is typically German, simple, reliable, and very ingenious.

Bosch CIS Is K-Jetronic
Bosch CIS (Continuous Injection System) or K-Jetronic, has earned an unfair reputation as being difficult to service as it is largely a hydraulic system relying on pressures and pressure differentials to function.

No Scan Tools
One can’t just plug into it with a scan tool and find answers; it requires understanding how the system operates to know normal and abnormal parameters. On the other hand, since it is largely a mechanical system, there are far fewer things to go wrong with it, and it is nearly bulletproof.

Theory Of Operation
The Bosch K-Jetronic (K-Jet) system comprises different components, which are discussed below, and their functions. Only the components related to the system are shown below. The K-Jet system belongs to the group of CIS injection systems, which stands for “Continuous Injection System.” This means that the fuel is continuously injected into the inlet manifold. The system is completely mechanical, relying on pressures and pressure differentials to function.

From the end of 1979, this system was modified by adding a lambda control system (oxygen sensor to create a closed-loop mixture control system. This new system was known KA-Jetronic, or K-Jet with Lambda. This system is shown in the figure below; the components 11, 12, 16, and 18 are specific to this lambda control system and are, therefore, not present in a mechanical K-Jet system.

From Cold Start
Let’s take it from a cold start. Cranking the starter triggers the cold start valve to spray into the intake plenum on early models. Due to flooding problems, a thermo-time sensor was added to prevent the cold start valve from spraying fuel for more than 8 seconds and when the engine temp is warm.

The control plunger inside the fuel distributor moves according to how much air deflects the air sensor plate and is also affected by control pressure. Control pressure acts on the top of the control pin; cold control pressure is low, about 1 bar (15 psi), depending on the model, which lessens the resistance of the air sensor plate to rise. As the engine temp comes up, the control pressure rises, and the resistance increases leaning out the mixture (hot control pressure is about 3 to 3.5 bar (50 psi).

The internals of the fuel distributor determines how much fuel is output to the injectors. There are two chambers separated by a stainless steel diaphragm; the lower chamber is system pressure set by a small spring and is adjustable but almost never necessary. When the control plunger rises, this allows system pressure into the upper chamber; once the pressures in both chambers are equal, the spring pressure deflects the diaphragm and let’s fuel out to the injectors. Note the diagram below.

So let’s look at each component of a typically K-Jet system.

1 – Fuel Tank
Unfourtantly its greatest enemy is water, which wreaks havoc on the small metering passages in the fuel distributors and filter screens in injectors and ports.

2 – Fuel Pump
The fuel pump is used to supply the system with the necessary fuel pressure. A non-return valve is installed just after the fuel pump, which is needed to keep the system pressurized after the engine has been switched off; this allows for easy warm starts. The design is an electrical roller type with a check valve to prevent backflow and a relief valve in case of restriction.
system pressure = 80 psi or 5.5 bar (14.7 psi  = 1 bar)

3 – Fuel Acculator 
The fuel accumulator has two functions:
– After the engine is switched off, this accumulator keeps the fuel system under pressure to promote a warm start.
– The accumulator dampens out the fuel pulses generated by the fuel pump. A large spring-loaded diaphragm to keep fuel pressure up with the engine off also dampens pulses from the pump.

4 – Fuel Filter
The purpose of the fuel filter is to filter the fuel so that it does not pollute the system. Keep the filters clean; we now recommend replacement yearly and water out of the tank (which goes for extended sitting with alcohol fuel), and the system lasts nearly forever.

5 – Warm Up Regulator / Control Pressure Regulator
The purpose of the warm-up regulator is to help enrich the fuel mixture during a cold start. The mixture is adjusted to the correct ratio when the engine warms up. This component can be overhauled, and we have had great success in cleaning and servicing these units, many of which are no longer available. An electrically heated bi-metallic strip operates a valve controlling fuel pressure to the distributor. Some units have a vacuum diaphragm that provides enrichment during acceleration.

6 – Fuel Injector
The injector provides a nicely atomized fuel to make the fuel-air mixture as homogeneous as possible. Not an electrical design; think of it as a mechanical valve that “pops” when the fuel pressure reaches approximately 45 psi (3 bar). All injectors in A K-Jet system spray at the same time the same amount; there is no timing as with a mechanical injection system. These are usually quite reliable, as long as water or debris doesn’t enter the fuel system. They do have rubber o-ring seals that need replacement from time to time.

7 – Cold Start Fuel
Discharged from the Cold Start Injector is present only when the engine is cranking over, as the operating voltage is derived from the starter solenoid.

8 – Cold Start Injector
Working in connection with the Thermo-time switch, the cold start injector adds extra fuel at very low ambient temperatures to assist when starting the engine. These only operate when the engine is cranking over, as the operating voltage is derived from the starter solenoid.

9- Fuel Distributor
The fuel distributor is the mechanical heart of the system, supplying the different cylinders with exactly the same amount of fuel. Some versions have an internal fuel pressure regulator to keep the system pressure constant. The main moving part is a central plunger which is controlled by the air sensor plate. When the throttle is opened, the increase of air into the manifold causes the air sensor plate rises and pushes the control plunger which allows fuel out to the injectors. Mixture adjustment is via the 3 mm Allen screw. This part can be serviced or rebuilt only with great care and proper tools. This is very easily damaged by debris or water in the fuel.

10 – (Mechanical) Air Flow Meter
The mechanical air flow meter measures the amount of air drawn in by the engine by employing a “teeter-toter” pivoting lever to access the metering plunger in the fuel distributor. Based on this measurement, the fuel mixture is mechanically adjusted. These are quite robust and rarely require service or adjustment.

11 – Frequency Valve / Timing Valve
This control valve is used with KA-Jetronic systems. Here the fuel mixture is regulated by means of a lambda-probe control system. The control valve changes the pressure between the upper and lower chamber of the fuel distributor, thus changing the fuel mixture.

12 – Lambda (Oxygen) Sensor
The lambda-sensor (only on KA-Jetronic) generates a signal that the control unit uses to adjust the fuel mixture. A special ceramic material that senses the difference between the O2 in the exhaust (or lack thereof) and the ambient O2 and produces a voltage that is sent to the Lambda Control Unit to adjust the CO in a closed loop system. The voltage fluctuates between a few millivolts and just less than 1 volt. The early single-wire (unheated) lambda sensor only generates a signal if the component has a sufficiently high temperature.

13 – Thermo-Time Switch
The thermo time switch controls the injection time of the cold start injector (8).  If a lambda control system is fitted, this sensor is also used as an input for the control device (computer) (18).

14 – Igntion Distribotor
It is not directly connected to the CIS system, noted only for diagram continuity.

15 – Auxiliary Air Valve
The purpose of the auxiliary air valve is to supply more air when the engine is cold. This air slide can be operated electrically or can be directly connected to the coolant system to change the system state.

16 – Throttle Position Switch / Position Sensor
The throttle switch is only used in systems with a lambda control system. This switch is present so that the control unit knows the throttle position for idle speed control.

17 – Engine Control Unit / Computer 
The control unit (ECU) processes the various inputs to provide the desired signal to the control valve (11), thus optimizing the fuel mixture. This unit is only present with KA-Jetronic systems or K-Jet with Lambda.

How We Can Help You
The Service Team here at Atlantic Motorcar is well experienced in this issue and others; with over 35 years of European auto specialization, serving clients from the areas of New England, we are familiar with the needs of the special service of your auto.

As Maine’s leading European auto specialists, we provide expert-quality services at a fair rate than nearby dealerships and specialty shops. If you’re experiencing any of the above-mentioned symptoms in your auto, please call us immediately; we can usually see your car the same day! At Atlantic Motorcar, we’ve developed some very specific procedures and tooling, combined with our expert technicians, to make this otherwise onerous repair a snap.

Questions or if we can help with service on your Audi, BMW, Mercedes, Sprinter, Honda, Infiniti, Lexus, Mini Cooper, Porsche, Volvo, and VW, please contact us. Our team of Service Specialists is here to help, for even the newest autos! (207) 882-9969.

Knowing, not just “doing,” that’s the Atlantic Motorcar Center way.

The Atlantic Motorcar Center Service Team

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