Understanding K-Jetronic Fuel System Pressure Performance

The K-Jetronic fuel system, also known as Continuous Injection System (CIS), is a mechanical fuel injection system developed by Robert Bosch GmbH in the 1960s. It works by continuously flowing fuel from all injectors while the fuel pump pressurises the fuel up to approximately 5 bar (73.5 psi). The K-Jetronic system has no lambda loop or lambda control and was first introduced in the 1973 Porsche 911T.

To ensure the system is working correctly, it is important to test the fuel flow and pressure. The fuel flow can be tested by disconnecting the fuel return line off the main regulator at the fuel distributor and measuring the amount of fuel in a container after running the fuel pump for 30 seconds. The pressure can be tested using a K-Jet specific pressure tester between the fuel distributor and the WUR (Warm Up Regulator).

The typical system pressure for a K-Jetronic system is around 4.8-5.4 bar, while the control pressure varies depending on the ambient temperature. For example, at 20 degrees Celsius, the control pressure should be between 1.6-2.0 bar.

K-Jetronic fuel system pressure testing

Overview

It is important to carry out diagnostic tests on K-Jetronic fuel systems to fix problems efficiently and avoid unnecessary replacements. A fuel pressure gauge with a shut-off tap in the line is useful for this process. The following sections outline the steps for testing the K-Jetronic fuel system pressure, including the return volume test, fuel pressure measurements, and accumulator and injector checks.

Return Volume Test

The first test is the return volume test, which involves measuring the volume of fuel returned to the tank. Here are the steps:

• Locate the fuel return line on the fuel distributor, which consists of two larger hoses and up to four smaller hoses.
• Disconnect the flexible fuel line from the solid pipe and insert it into a measuring container.
• Start the engine or bridge the fuel pump relay and measure the volume of fuel returned. It should be at least 1 litre in 30 seconds.
• If the volume is less than 1 litre, there may be an issue with the fuel pump, accumulator, fuel filter, or other components.

Fuel Pressures – K-Jet

To measure fuel pressures in the K-Jetronic system, follow these steps:

• Remove the fitting from the centre of the fuel distributor and the large fitting on the warm-up regulator.
• Install the fuel pressure gauge with the tap closest to the warm-up regulator.
• With a cold engine, open the gauge tap, crank the engine, and record the "Cold Control Pressure."
• Close the tap and record the "System Pressure."
• Open the tap, reconnect the warm-up regulator electrical terminal, and start the engine. Observe the changing pressure as the engine warms up.

Accumulator and Injector Checks

After ensuring the fuel flow is adequate, proceed with testing the accumulator and injectors:

• With the fuel pump running and the test gauge shut-off valve open, turn off the fuel pump. The gauge should read approximately 2.5 BAR.
• After 10 minutes, the pressure should remain at 2.5 BAR, and after 20 minutes, it should drop to 2.0 BAR.
• If the pressure drops more than expected, run the pump again, shut it off, and close the test gauge shut-off valve. If the pressure still drops, the accumulator may be leaking.
• To test the injectors, remove them from the manifold and place them into containers. Turn on the fuel pump and check for leaks. Then, lift the air sensor plate to trigger the injectors and inspect the flow pattern.

By following these steps and referring to the relevant manuals for specific measurements, you can effectively test the K-Jetronic fuel system pressure and identify any issues.

K-Jetronic fuel flow testing

Return Volume Test

Before you begin, ensure you have a good K-Jet specific pressure tester. If you cannot test the pressures, you will not be able to diagnose or set the system correctly.

The first test to carry out is the return volume test. If you have a low return volume, every other test will give you false information.

Locate the fuel return line. On fuel distributors, there will be two larger hoses and up to four smaller hoses. The large hose on its own is the supply, and the other large hose, beside the system pressure valve, is the return hose.

On K-Jet fuel distributors, there is usually a flexible hose about 30cm long from the fuel distributor fitting beside the pressure regulator, which joins a solid line back to the tank. On KE-Jet fuel distributors, the flexible hose goes from the separate fuel pressure regulator to the solid fuel return line.

With the engine off, disconnect the flexible fuel line from the solid pipe and insert the flexible hose into a measuring container. Start the engine or bridge the fuel pump relay and start a timer. The volume of fuel must be at least 1L in 30 seconds. Anything less means there is a fault, usually with the fuel pump, but there could also be a blocked fuel filter, a blockage in the pick-up in the tank, or a restriction in the fuel pipe.

Fuel Pressures – K-Jet

For the K-Jet pressure gauge set-up:

• Remove the fitting from the centre of the fuel distributor and the large fitting on the warm-up regulator.
• Install the gauge with the tap closest to the warm-up regulator.
• With a cold engine and the gauge tap open, start the engine and record the pressure. This is the “Cold Control Pressure”.
• Close the tap and record the pressure. With the tap closed, you will record the “System pressure”.
• Open the tap, re-fit the Warm-up Regulator electrical terminal, and watch/record the changing pressure as the engine warms up.

Typical Control Pressure readings:

• 10 degrees C: 1.3 – 1.6 bar
• 20 degrees C: 1.6 – 2.0 bar
• 30 degrees C: 2.0 – 2.4 bar

Typical System Pressure reading: 4.8 – 5.4 bar.

Testing the Accumulator and Injectors

With the fuel pump running, have the test gauge shut-off valve open. Now turn off the fuel pump and the gauge should drop to about 2.5BAR. This is the accumulator pressure. After 10 minutes, it should still be 2.5BAR. After 20 minutes, it should drop to 2.0BAR. If it's lower than this, either the accumulator is leaking internally, the injectors are leaking, or the cold start injector is leaking.

Testing the Injectors

Remove the injectors from the manifold and place them into a series of jars or containers of similar size. Turn the fuel pump on. Without lifting the sensor plate, check to see if any fuel is leaking from any injectors. Any leaking is bad. Now lift the Air Sensor Plate to trigger the injectors. Inspect the flow pattern of each. After a few seconds, shut off the pump and measure how much fuel came from each injector. It should be very similar between injectors. If they are leaking, the flow pattern isn’t a nice cone, or there is a variation in flow, the injectors will need to be replaced.

Additional Testing

Other tests include testing the accumulator, the fuel distributor, the auxiliary components (the Cold Start Injector, Thermotime Switch, Auxiliary Air Device and WUR Warmup Circuit), and the electrical side of the Warm Up Regulator.

K-Jetronic system and control pressures

The K-Jetronic fuel injection system is a mechanical fuel injection system developed by Bosch. It is a continuous injection system that uses the engine's vacuum to regulate fuel flow to the engine. The system consists of a fuel distributor, a warm-up regulator, and a series of injectors.

To ensure the K-Jetronic system is functioning optimally, it is essential to test the system's pressures. The two main pressures to be aware of are the System Pressure and the Control Pressure.

The System Pressure is the overall pressure that the injection system operates at. It is important to ensure that the system pressure is set correctly, as an incorrect setting will impact the Control Pressure. The typical System Pressure for a K-Jetronic system is around 4.8 to 5.4 bar.

The Control Pressure is what controls the enrichment of the fuel mixture. It is influenced by the ambient temperature, with higher temperatures requiring lower control pressures. For example, at 20 degrees Celsius, the Control Pressure should be between 0.6 and 0.9 bar, while at 30 degrees Celsius, it should be between 1 and 1.3 bar.

To test the System and Control Pressures, a K-Jet specific pressure tester is required. The tester is connected between the fuel distributor and the warm-up regulator (WUR). The shut-off valve on the tester must be on the side connected to the WUR. By running the fuel pump and opening and closing the shut-off valve, both the System Pressure and the Cold Control Pressure can be measured.

Additionally, the Warm Control Pressure can be tested by warming up the engine or leaving the ignition and pump running to heat up the WUR. The Warm Control Pressure should be around 3 bar, depending on the specifications of the vehicle.

It is important to note that the K-Jetronic system is sensitive to fuel flow and pressure, and incorrect pressures can lead to performance issues. Therefore, it is crucial to have accurate measurements and make adjustments as needed to ensure the system operates optimally.

K-Jetronic fuel pump issues

K-Jetronic is a mechanical system used in vehicles from 1973 to 1995 that meters fuel proportionally to air intake. The fuel pump is located at the rear of the car on the driver's side and is sealed. It cannot be repaired, only replaced. If the pump fails, the car will not start. Other signs of a failing fuel pump include unusual noises, jerky running when under heavy load, and misfiring when revving up while stationary.

The fuel pump is connected to the fuel accumulator, which in turn connects to the fuel filter. Fuel then passes to the metering head or fuel distributor. The metering head is finely machined and should not be stripped down. However, it can be damaged by corrosion caused by water in the fuel. This can be remedied by adding a good fuel system cleaner to the tank.

The return volume test is the first test that should be carried out when troubleshooting K-Jetronic issues. This test checks for a low return volume, which can result in low power and/or misfires. If the return volume is low, the fault usually lies with the fuel pump, but it could also be due to a faulty accumulator, blocked fuel filter, or restriction in the fuel pipe.

Another test that can be performed is the fuel flow test. This test involves disconnecting the fuel return line off the main regulator at the fuel distributor and directing it into a container. The fuel pump is then run for 30 seconds, and the volume of fuel collected is measured. A volume of less than 750ml indicates a restriction in the system or a failing fuel pump.

It is important to note that when working on any fuel system, care should be taken to avoid petrol coming into contact with any source of ignition, including hot engine components, High Tension (HT) sparks, and smoking.

K-Jetronic fuel distributor issues

The next test is the fuel pressure test, which requires a K-Jet specific pressure tester. This test involves measuring the cold control pressure, system pressure, and warm control pressure. If the control pressure is within the correct range, the system is functioning correctly. If not, there may be an issue with the warm-up regulator.

The accumulator and injectors can then be tested by observing the pressure drop over time with the fuel pump turned off. A significant pressure drop could indicate a leaking accumulator or injectors.

Other issues with the K-Jetronic fuel distributor can include a sticking control plunger, which can be identified by observing the movement of the plunger with the engine off and the fuel pump on. Restricted or leaking injectors can also cause problems and should be tested and cleaned or replaced if necessary.

In addition, a faulty thermo-time switch, which controls the cold start injector, can cause issues with cold starts. The thermo-time switch can be tested by checking the resistance between its terminals at different temperatures.

Finally, issues with the auxiliary air device (AAD) can be identified by observing whether the sliding plate inside the unit is functioning correctly at different temperatures. A blocked AAD can be identified by pinching off the hose and observing the engine idle speed.

Frequently asked questions