Understanding K-Jetronic Fuel Pressure Control Mechanisms

The Bosch K-Jetronic is a simple mechanical fuel injection system, but it requires all parts to be in good working order to run well. The fuel pressure is controlled by a few different components, and issues with any of them can lead to problems with the overall fuel pressure. The main components that control fuel pressure include the fuel pump, fuel filter, fuel accumulator, and the main fuel pressure regulator.

The fuel pump is responsible for providing the required fuel pressure, and if it is unable to do so, it may need to be replaced. Similarly, if the fuel filter is blocked, it can restrict fuel flow and affect fuel pressure. The fuel accumulator can also impact fuel pressure if it is leaking, as this can cause fuel to bypass the system and result in low system pressure. Lastly, the main fuel pressure regulator can get stuck, leading to low system pressure.

Fuel flow test

The fuel flow test involves testing the fuel flow from the pump. To do this, disconnect the fuel return line from the main regulator at the fuel distributor and extend it by about 700mm into a suitable container of 1L or larger. Run the fuel pump for exactly 30 seconds and measure the amount of fuel in the container. A result of 750ml or more indicates a healthy fuel flow. A result of less than 750ml suggests a restriction in the system, such as a blocked fuel filter, or a failing fuel pump.

Return Volume Test

The return volume test is the first test that should be carried out when experiencing problems with K-Jetronic or KE-Jetronic. The fuel pump may be able to provide the required pressures but be very low on volume, resulting in low power and/or misfires.

To perform the test, locate the fuel return line. On fuel distributors, there will be two larger hoses and up to four smaller hoses in the lower half. The lone large hose is the supply, and the other large hose, located next to the system pressure valve, is the return hose.

On K-Jet fuel distributors, there is typically a flexible hose of about 30cm from the 17mm fuel distributor fitting next to 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. The fuel pressure regulator is usually found next to the fuel distributor, with a metal pipe of about 75mm between them.

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. A result of less than 1L indicates a fault, usually with the fuel pump, but it could also be a faulty accumulator leaking fuel back to the tank, a blocked fuel filter, a blockage in the pick-up in the tank, a restriction in the fuel pipe, or a blocked strainer in the fuel inlet fitting of the distributor.

Fuel Pressure Test

The fuel pressure test involves measuring the "Cold Control Pressure", "System Pressure", and "Warm Control Pressure".

To measure the Cold Control Pressure, install a fuel pressure gauge with the tap closest to the Warm-up Regulator. With a cold engine or cold warm-up regulator and the gauge tap open, start the engine and record the pressure.

To measure the System Pressure, close the tap and record the pressure.

To measure the Warm Control Pressure, open the tap, refit the Warm-up Regulator electrical terminal, and start the engine. Watch and record the changing pressure as the engine warms up.

Typical Control Pressure readings are as follows:

• 10 degrees C: 1.3 – 1.6 bar
• 20 degrees C: 1.6 – 2.0 bar
• 30 degrees C: 2.0 – 2.4 bar
• Warm engine: 3.4 – 3.8 bar

The typical System Pressure reading is 4.8 – 5.4 bar.

If the fuel flow and return volume tests are successful, but the system pressure is low, the accumulator, fuel filter, and pump should be checked. Start by replacing the regulator seals. If this doesn't fix the issue, try shimming the regulator.

If the system pressure is high, and the rest of the fuel system before the regulator is functioning correctly, try removing or substituting a thinner shim to lower the pressure.

Troubleshooting

If the flow test fails, the fuel filter may be blocked, particularly in older cars with steel tanks where rust may be present in the system. Alternatively, the fuel pump may be failing to output the required amount of fuel, which will be evident in a System Pressure test. This issue could also be caused by fuel bypassing the system via a leaking Fuel Accumulator.

If the fuel flow and return volume tests are successful, but the Cold Control Pressure is incorrect, the Warm-Up Regulator (WUR) may need to be cleaned and reset.

If the fuel distributor plunger is sticking, the distributor should be removed from the airbox, being careful not to let the plunger drop out. The plunger and bore should be cleaned with carb or brake cleaner until the plunger moves freely in the bore.

If the accumulator is leaking from the vent, it should be replaced or removed.

If the cold start injector is leaking, it should be replaced. If it is not triggering, it can be soaked in carb or injector cleaner, or cleaned with an ultrasonic cleaner.

If the Thermotime Switch is faulty, it should be replaced.

If the Auxiliary Air Device is jammed open or closed, it can be cleaned and freed using brake cleaner and a heat gun to warm it up and cool it off.

If the injectors are leaking, have a bad spray pattern, or uneven flow, they should be replaced.

If the engine runs but idles poorly, the idle mixture adjustment on the airbox should be checked. The idle speed should also be adjusted.

If all the above tests and settings are correct, the issue may lie elsewhere, such as ignition or compression.

System pressure test

To perform a system pressure test on a K-Jetronic, you will need a fuel pressure gauge with a shut-off tap in the line. This is to ensure that you get accurate results and do not have to replace unnecessary parts.

Firstly, locate the fuel return line. On K-Jet fuel distributors, there is usually a flexible hose about 30cm (12") long from the 17mm fuel distributor fitting beside the pressure regulator, which joins a solid line back to the tank. Disconnect the flexible fuel line from the solid pipe and insert the flexible hose into a measuring container.

Now, run the fuel pump for 30 seconds and measure the volume of fuel in the container. You should have at least 750ml. If you have less, there may be a restriction in the system or the fuel pump could be failing.

If the fuel flow is normal, reconnect the return line and proceed to test the system and control pressures. You will need to connect the pressure tester between the fuel distributor and the WUR (Warm-Up Regulator). Make sure the shut-off valve is on the side of the gauge that connects to the WUR.

With the shut-off valve open, you should be reading the Cold Control Pressure from the WUR. The Cold Control Pressure is on a scale depending on the ambient temperature. For example, at 20°C, the control pressure should be 0.6-0.9 bar. If the control pressure is within the correct range, you can proceed to the next step.

Next, close the shut-off valve to read the System Pressure. The System Pressure is the overall pressure the injection system is running at and should read about 5.1 bar. If it is not within the correct range, you may need to shim the regulator or replace the fuel pump.

The final test is to test the accumulator and check for leaking injectors. With the fuel pump running, open the test gauge shut-off valve. Now, turn off the fuel pump and the gauge should drop to about 2.5 bar. After 10 minutes, the pressure should still be 2.5 bar, and after 20 minutes, it should drop to 2.0 bar. If the pressure is lower than this, either the accumulator is leaking, the injectors are leaking, or the cold start injector is leaking.

Control pressure test

To perform a control pressure test on a K-Jetronic system, you will need a fuel pressure gauge with a shut-off tap in the line.

First, locate the fuel return line. On K-Jet fuel distributors, there is usually a flexible hose about 30cm (12") long from the 17mm fuel distributor fitting beside the pressure regulator, which joins a solid line back to the tank. Disconnect the flexible fuel line from the solid pipe at the joiner. Insert the flexible hose into a measuring container, such as an old 5L oil container.

Next, run the fuel pump for 30 seconds and measure the volume of fuel in the container. If the volume is less than 1L, there may be a fault in the fuel pump, accumulator, fuel filter, or another component.

Now, connect the pressure tester between the fuel distributor and the WUR (Warm-Up Regulator). Ensure the shut-off valve is on the side of the gauge that connects to the WUR. With the shut-off valve open, you should be reading the Cold Control Pressure from the WUR. Close the tap, and you will now be reading the System Pressure.

Finally, to test the Warm Control Pressure, either warm up the engine or leave the ignition and pump running. The heater in the WUR should warm up and cause the pressure to rise.

It is important to refer to the correct specifications for your vehicle when interpreting the results of the control pressure test.

Accumulator test

The accumulator is a common point of failure in the K-Jetronic fuel system and can cause issues such as low system pressure. It is usually located down by the fuel pump and should be in the fuel system post-pump.

The accumulator has three fittings: two on the front for fuel in and out, and one on the rear, which is a vent (and failure overflow). The vent is on the dry side of the diaphragm, so if any fuel comes out of it, the diaphragm has failed.

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

If the accumulator is leaking internally, the injectors are leaking, or the cold start injector is leaking, the pressure in the system will drop more than it should.

If the pressure drops more than it should, you can run the pump again to build pressure, shut it off and shut the test gauge shut-off valve. If the pressure still drops, this indicates the leak is before the gauge, so likely to be the accumulator.

Injector test

Return Volume Test

The return volume test is the first test you should perform when encountering issues with the K-Jetronic system. This test helps identify if there is a problem with the fuel pump's volume output, which can result in low power and misfires. To perform this test:

• Locate the fuel return line, which consists of two larger hoses and up to four smaller hoses. The larger hose next to the system pressure valve is the return hose.
• Disconnect the flexible fuel line from the solid pipe at the joiner.
• Place the disconnected flexible hose into a measuring container, such as an old oil container.
• Start the engine or bridge the fuel pump relay and start a timer.
• Measure the volume of fuel returned in a given time, typically 30 seconds or one minute. A volume of less than 1 litre in 30 seconds indicates a fault, which could be a faulty fuel pump, leaking accumulator, blocked fuel filter, or other restrictions in the fuel system.

Fuel Pressure Tests

After ensuring sufficient return volume, the next step is to test the fuel pressures in the system.

K-Jet Fuel Pressure Test

To test the fuel pressures in the K-Jet system:

• Remove the fitting from the centre of the fuel distributor and the large fitting on the warm-up regulator (WUR).
• Install the fuel pressure gauge with the tap closest to the WUR.
• With a cold engine and the gauge tap open, crank the engine and record the "Cold Control Pressure."
• Close the tap and record the "System Pressure."
• Open the tap, reconnect the WUR electrical terminal, and start the engine. Observe and record the changing pressure as the engine warms up, which indicates the "Warm Control Pressure."
• Typical control pressure readings vary depending on the temperature: at 10°C, it should be 1.3-1.6 bar; at 20°C, 1.6-2.0 bar; and at 30°C, 2.0-2.4 bar. The warm engine control pressure should be around 3.4-3.8 bar.
• The typical system pressure reading is 4.8-5.4 bar.

KE-Jet Fuel Pressure Test

The fuel pressure testing procedure for the KE-Jet system is slightly different:

• Remove the blanking plugs (usually 12mm) from the centre of the fuel distributor and the bottom half.
• Install the fuel pressure gauge with the tap closest to the top half.
• With the tap open, you will measure the system pressure.
• Close the tap to measure the control pressures or lower chamber pressures, which vary according to the current supplied to the Electro-Hydraulic Actuator (EHA).
• The typical KE-Jet system pressure is around 5.5-6.0 bar, and the control pressure is usually 0.4 bar less than the system pressure at operating temperature.

Accumulator and Injector Leak Test

To check for leaks in the accumulator and injectors:

• With the fuel pump running and the test gauge shut-off valve open, turn off the fuel pump.
• Observe the pressure drop over time. After 10 minutes, the pressure should still be around 2.5 bar, and after 20 minutes, it should drop to 2.0 bar.
• If the pressure drops faster than expected, run the pump again to build pressure, then shut off both the pump and the test gauge shut-off valve.
• If the pressure continues to drop, it indicates a leak before the gauge, likely in the accumulator.

Cold Start Injector Test

To test the cold start injector:

• Remove the injector from the manifold and direct its nozzle into a container.
• Connect the power connector from the Auxiliary Air Device (AAD) to the cold start injector.
• Turn on the ignition and run the fuel pump, which should open the injector and spray fuel.
• Disconnect the power from the injector, wipe the nozzle dry, and hold it in the container for 60 seconds to check for leaks.
• If the injector remains dry, it has passed the test.

Thermotime Switch Test

The thermotime switch is responsible for triggering the cold start injector based on temperature and time. To test this component:

• Measure the resistance of Terminal A when cold and warm. When cold, the resistance should be lower than when warm.
• Check for continuity between Terminal D and the body of the sensor when cold. When warm, Terminal D should show an open circuit.

Auxiliary Air Device (AAD) Test

The AAD can cause issues if the sliding plate inside sticks, leading to the unit staying open or closed when it shouldn't. To test the AAD:

• Inspect the unit when cold and warm. When cold, you should be able to see through the unit, and when warm, it should show a solid plate blocking the airflow.
• Pinch off the hose on either side of the unit when the engine is at operating temperature. If the idle speed drops, it indicates that air is still flowing through the AAD.
• Check for power at the connector when the ignition and fuel pump are on. If one of the terminals has power, the plate may not be closing properly.

Warm-Up Regulator (WUR) Electrical Test

The WUR plays a crucial role in leaning out the fuel pressures when the engine warms up. To test the electrical side of the WUR:

• Check for power at the plug by turning on the ignition and running the fuel pump. Use a test light to see if one of the terminals has power.
• With the ignition off, test the resistance of the heater coil in the WUR. The specific resistance value depends on the unit's specification, but generally, you should not have an open circuit. Test the resistance between the two terminals on the WUR.

Injector Flow Pattern and Leakage Test

The injectors are known to leak and develop erratic spray patterns over time. To test the injectors:

• Remove all injectors from the manifold and place them into containers of similar size, being cautious of raw fuel.
• Turn on the fuel pump and check for any leaking injectors without lifting the sensor plate.
• Lift the air sensor plate to trigger the injectors and inspect the flow pattern of each.
• After a few seconds, shut off the pump and measure the fuel volume dispensed by each injector. The volume should be similar between injectors.
• If there is leakage, an erratic spray pattern, or a significant variation in flow, the injectors should be replaced.

Frequently asked questions