Understanding Bosch K-Jetronic Fuel Pressure Systems

Bosch K-Jetronic is a Continuous Injection System (CIS) that was used from the 1970s to the mid-1990s, primarily in European cars. It is a form of mechanical fuel injection where injectors spray a constant stream of fuel into the cylinders, with the volume depending on various factors. The system consists of several components, including a fuel pump, accumulator, fuel filter, airflow sensor, fuel distribution unit, warm-up regulator, cold start injector, auxiliary air valve, and fuel injectors. The fuel pump delivers fuel at a high pressure to the metering or fuel distribution head, which then supplies the correct amount of fuel to the engine via the injectors. The accumulator helps to smooth out pulses in the fuel flow and maintain pressure when the engine is switched off. The airflow sensor measures the amount of air entering the engine, which, along with engine temperature, determines the amount of fuel delivered to the engine. The warm-up regulator controls the amount of fuel delivered during the warm-up period, while the cold start injector aids with engine starting when cold. Overall, the Bosch K-Jetronic system provides precise fuel metering and injection, optimizing the air-fuel mixture for improved engine performance and reduced emissions.

The role of the accumulator

The accumulator is a critical component of the Bosch K-Jetronic fuel injection system, playing a dual role in ensuring its smooth operation. Firstly, it helps to dampen out pulsation in the fuel coming from the fuel pump, thereby 'deaden [ing]' the noise from the pump. However, its more crucial function is to maintain pressure in the fuel lines when the engine is shut off, which aids in hot starting and reduces the possibility of vapour lock.

The accumulator is positioned immediately after the fuel pump in the fuel system. It achieves its dual functions through the use of a diaphragm and spring mechanism. As fuel passes through the accumulator, it enters a chamber and pushes against a diaphragm, which in turn compresses a spring. This process smooths out the fuel flow and reduces pulsation. When the engine is running, the diaphragm is pushed against the spring, which builds up pressure in the chamber. Once the engine is shut off, the spring pushes against the diaphragm, maintaining residual pressure and preventing fuel seepage. This pressure is critical in the 5 to 20-minute period after a journey when the under-bonnet temperature rises, causing the fuel in the lines to heat up and evaporate. By maintaining pressure, the accumulator ensures a clean start for the vehicle.

The accumulator is divided into two chambers by the diaphragm. One chamber, the spring chamber, accumulates fuel and builds pressure, while the other chamber is connected to the atmosphere. When the engine is running, fuel enters the accumulator, pushing the diaphragm back and compressing the spring until it reaches a maximum volume, with the spring halted in the spring chamber.

The accumulator typically has three fittings: two on the front for fuel inlet and outlet, and one at the rear for venting. If fuel leaks from the vent, it indicates that the diaphragm has failed.

Fuel flow from the pump

The fuel pump is a roller cell pump, with fuel entering and being compressed by rotating cells that force it through the pump at high pressure. The pump is capable of producing a pressure of 8 bar (120 psi) with a delivery rate of approximately 4 to 5 litres per minute. The pump has a pressure relief valve that lifts off its seat at 8 bar to prevent over-pressurisation. The other end of the pump has a non-return valve that maintains pressure within the system when voltage to the pump is removed. The normal operating pressure is approximately 5 bar (75 psi) and the current draw on the pump is 5 to 8 amps.

The fuel pump relay is a tachometric relay, which only sends a voltage to the pump when the engine is cranking or running. It is a safety device that stops the flow of fuel in the event of an accident. The location of the relay varies between manufacturers.

The fuel flow from the pump is directed through an accumulator, which smooths out any pulses in the flow of the fuel and maintains pressure within the system when the engine is switched off. The accumulator is divided into two chambers by a diaphragm. When the engine is running, the fuel enters one chamber and pushes the diaphragm against a spring. When the engine is stopped, the spring pressure against the diaphragm maintains the residual pressure.

The fuel filter ensures excellent filtration without impeding the flow of fuel. A visual inspection of the filter is not possible, but the current draw of the fuel pump can indicate a blockage. The current can be recorded by inserting a multimeter in series with the circuit. A typical current draw is between 5 to 8 amps.

The fuel is then directed through a pressure regulator, which is fitted to one end of the fuel distributor. It maintains the pressure in the fuel system at a constant level of about 5 bar. It consists of a plunger that slides against a spring. When the fuel supplied by the pump exceeds the limit, the plunger moves and opens the exit port, allowing excess fuel to return to the tank. When the fuel delivery quantity is lower, the plunger shifts back and closes the exit port. The constant shifting of the plunger maintains the pressure in the rail.

The fuel pump pushes fuel through the accumulator and filter to the metering head. This acts against the main regulator, which pushes the system pressure to 5.5 bar. Most of the fuel continues on into the metering head, but the excess is returned to the tank.

System and Control pressures

The Bosch K-Jetronic is a Continuous Injection System (CIS) where injectors spray a constant stream of fuel into all cylinders at once. The system pressure is approximately 5 to 5.5 bar, and the pump must be capable of producing more than 6 bar to avoid the system starving.

The System and Control pressures can be tested with a KJet-specific pressure tester. The Cold Control Pressure is dependent on the ambient temperature. For example, at 20°C, the control pressure should be 0.6-0.9 bar, and at 30°C, it should be 1-1.3 bar. The System Pressure should be about 5.1 bar (0.4 bar +/-). The Warm Control Pressure should be about 3 bar.

The accumulator is responsible for maintaining the pressure in the system when the engine is switched off, which helps with restarting the engine, especially when it is hot. The accumulator is divided into two chambers by a diaphragm. One chamber acts as the fuel accumulator, and the other is connected to the atmosphere. When the engine is running, fuel enters the accumulator and pushes the diaphragm against the spring force. When the engine is switched off, the diaphragm moves back, and the spring force pushes the remaining fuel back into the system, maintaining the pressure.

The fuel distributor is also called the metering head, and it contains the primary pressure regulator, which controls the system pressure. When the required pressure is obtained, a plunger within the regulator lifts off its seat, and excess fuel is returned to the tank. The metering head also includes the fuel injectors, which spray the fuel.

Testing the accumulator

The accumulator is a sealed unit that cannot be repaired. It is located at the rear of the car, just above the fuel tank. It is a metal housing with two fuel pipes bolted into one end and a small breather hole at the other. Inside the accumulator is a large spring, a diaphragm, and a steel plate with a fuel feed hole through it.

To test the accumulator, clamp the hose from the outlet, usually the one that goes to the fuel filter, and direct a hose from the vent into a container. Then, run the fuel pump. If the accumulator is functioning correctly, no fuel should come out of the vent. If anything at all comes from the vent, the accumulator has failed.

The accumulator is a common failure point and can cause a bunch of weird issues, including low system pressure.

The fuel distributor

The control plunger resides inside the metering barrel, which has a series of very small slits (0.2 mm wide), one for each injector. As the airflow changes, the control plunger moves up and down, altering the fuel flow through the slits. The warm-up regulator (WUR) control pressure acts on the top of this plunger, altering how far it moves and thus the mixture.

Frequently asked questions