Checking Fuel Pressure In Bosch K-Jetronic Fuel Injection Systems

Bosch K-Jetronic is a form of mechanical fuel injection that was used from the 1970s to the mid-1990s, primarily in European cars. The system consists of several components, including a fuel pump, an accumulator, a fuel filter, a sensor plate, a warm-up regulator, a fuel distributor, and injectors.

To check the fuel pressure in a Bosch K-Jetronic system, a pressure tester is required. The pressure tester is connected between specific parts of the system, such as the fuel distributor and the warm-up regulator, to measure the system and control pressures. The system pressure should be around 5 to 5.5 bar, while the control pressure will vary depending on the ambient temperature.

It is important to note that the measurements and procedures may differ depending on the specific vehicle and its manual should be referred to for accurate information.

The importance of a good pressure tester

• Fuel pressure plays a critical role in the K-Jetronic system's performance. The system relies on precise fuel pressure to deliver the right amount of fuel to the injectors, and any deviations can lead to poor performance or damage.
• A good pressure tester can tell you almost everything you need to know about why the system is acting a certain way. It helps you identify issues with fuel pressure, which is often the root cause of many problems.
• With a quality tester, you can measure the fuel flow from the pump, control pressure, system pressure, and warm control pressure. These measurements will help you pinpoint any restrictions in the system, such as a blocked fuel filter or a failing fuel pump.
• By testing the various pressures in the system, you can ensure they are within the correct range. Deviations from the specified pressure ranges can indicate problems with specific components, such as the fuel pump or regulator.
• A good pressure tester allows you to verify that the accumulator is functioning correctly and check for leaks in the injectors or the accumulator itself. This is crucial for maintaining the overall health of the fuel injection system.
• Without a reliable pressure tester, you won't be able to diagnose or adjust the system correctly. Making adjustments or replacing parts without accurate pressure readings can be futile and may even lead to further complications.
• Investing in a quality pressure tester can save you time, money, and headaches in the long run. It enables you to identify issues early on and make informed decisions about maintenance and repairs, ensuring the longevity and optimal performance of your Bosch K-Jetronic fuel injection system.

Fuel flow from the pump

The fuel pump is a crucial component of the Bosch K-Jetronic system. It must be capable of producing more than 6 bar of pressure as the system pressure of most K-Jetronic systems is approximately 5 to 5.5 bar, and if the pump can’t keep up, the system will starve. The pump should always be producing more fuel than the engine needs.

Testing the fuel pump requires measuring the delivery volume at system pressure. This is done by attaching a hose to the return line fitting on the fuel distributor on the Bosch K-jet system, or to the return line fitting on the external fuel pressure regulator on the Bosch KE-jet system. When this hose is connected and directed into a container, run the fuel pump for 30 seconds and measure the amount of fuel in the container.

You should have a minimum of 750cc to 1200cc of fuel in the container (this figure differs by model of car). If you don’t get the specified minimum amount for your car, then check for problems or blockage in the fuel delivery system.

If the fuel flow is OK, reconnect the return line and proceed with testing the System and Control pressures.

System and Control Pressures

To test the system and control pressures, connect the pressure tester between the fuel distributor and the WUR (Warm-Up Regulator). The shut-off valve must be on the side of the gauge that connects to the WUR.

You don’t need to run the engine to test the pressures. Disconnect the electrical connector from the WUR for now (to stop it from heating up and changing pressures) and run the fuel pump.

With the shut-off valve open, you should be reading the Cold Control Pressure from the WUR. The Control Pressure is what controls the enrichment.

The Cold Control Pressures are on a scale depending 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.

If the control pressure is within the correct range, you’re good to go. If not, there is a problem, which may be due to incorrect control pressure or a failing fuel pump.

Next, close the shut-off valve, and you’re now reading the System Pressure. System pressure is the main overall pressure the injection system is running at. If it’s not set correctly, it will throw the Control Pressure out too.

This should read about 5.1 bar (0.4 bar +/-). If not, as long as the fuel flow test was OK, you may need to shim the regulator or you could have a failing fuel pump that cannot produce the required pressure.

Warm Control Pressure

To test the Warm Control Pressure, you can either warm up the engine or leave the ignition and pump running with the electrical connector connected to the WUR. The heater should warm the WUR up and allow us to test the warm pressure.

The Warm Control Pressure should start from the Cold Control Pressure (if it’s low enough; if it’s already 3 bar or higher, there is no point in testing for Warm Pressure) and steadily rise until it hits a plateau. The Warm Control Pressure should be about 3 bar depending on the spec.

System and Control Pressures

The system pressure is usually 5 to 5.5 bar (75 to 80 psi), but turbocharged cars will be 5.8 to 6.8 bar. This can be measured at the top of the fuel distributor by connecting a continuous injection system pressure gauge from the top fitting in the fuel distributor to the warm-up regulator. The gauge will be at the top, and the valve will close the horizontal leg between the “T” and the warm-up regulator. With the valve closed and the fuel pump running, you will read system pressure.

Control pressure, on the other hand, is what “controls” the mixture, rich when cold, lean when warm. This is done by applying pressure to the top of the piston in the centre of the fuel distributor. A low-pressure allows the piston to move higher in the fuel distributor to provide more fuel for a given airflow. Increasing the pressure forces the piston lower for a leaner mixture for the same airflow.

To test the system and control pressures, connect the pressure tester between the fuel distributor and the WUR. The shut-off valve must be on the side of the gauge that connects to the WUR. You don’t need to run the engine to test the pressures. Disconnect the electrical connector from the WUR for now (to stop it from heating up and changing pressures) and run the fuel pump.

With the shut-off valve open, you should be reading the Cold Control Pressure from the WUR. As mentioned, the Control Pressure is what controls the enrichment. The Cold Control Pressures are on a scale depending on the ambient temperature. For example, at 20°C, control pressure should be 0.6-0.9 bar, and at 30°C, it would be 1-1.3 bar.

If the control pressure is within the correct range, you’re good to go. If not, there is a problem. Next, close the shut-off valve, and you’re now reading the System Pressure. If this is not set correctly, it will throw the Control Pressure out too. This should read about 5.1 bar (0.4 bar +/-). If not, as long as the fuel flow test was okay, you may need to shim the regulator or you could have a failing fuel pump that cannot produce the required pressure.

To test the Warm Control Pressure, either warm the engine up if it runs, or leave the ignition and pump running with the electrical connector connected to the WUR. The heater should warm the WUR up and allow us to test the warm pressure. It should start from the Cold Control Pressure (if it’s low enough) and steadily rise until it hits a plateau. The Warm Control Pressure should be about 3 bar depending on the spec.

Accumulator and leaking injectors

The accumulator and injectors are integral parts of the Bosch K-Jetronic fuel injection system. The accumulator has two functions: it dampens out pulsation in the fuel from the fuel pump, and it holds pressure in the fuel lines when the engine is shut off, which helps with hot starting and reduces the possibility of vapour lock. Inside the accumulator is a large diaphragm that pushes against a spring.

The injectors are the final part of the fuel injection system. They consist of a metal cylinder housing a small valve, a spring, and a filter. During operation, a correctly functioning injector makes a squealing noise. The valve is set to open at a certain pressure—in the case of the Cologne V6, this is 3.3 BAR. Over that pressure, the injectors are open and constantly injecting fuel. Below that pressure, the injectors should be sealed and must not leak.

To test the accumulator and for leaking injectors, you will need a KJet-specific pressure tester. 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 2.5 BAR. This is the accumulator pressure. After 10 minutes, it should still be 2.5 BAR. After 20 minutes, it should drop to 2.0 BAR. If it's lower than this, either the accumulator is leaking internally, the injectors are leaking, or the cold start injector is leaking.

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 it is likely to be the accumulator.

Auxiliary components

The auxiliary components of the Bosch K-Jetronic fuel injection system include the Cold Start Injector, the Thermotime Switch, the Auxiliary Air Device, and the WUR Warmup Circuit.

Cold Start Injector

The Cold Start Injector is a primitive electric injector inserted into the plenum chamber, before the main injectors. When triggered, it fires a mist of atomized fuel into the intake system. To test the Cold Start Injector, remove the injector from the manifold and direct its nozzle into a container. Now, connect the power connector from the Aux Air Device to the Cold Start Injector and turn on the ignition and run the fuel pump. This should cause power to open the injector and spray fuel. Next, disconnect the power from the injector, wipe the nozzle dry, and hold the injector (with the ignition and pump on) in the container for 60 seconds. If the injector is still dry, it has passed the test.

Thermotime Switch

The Thermotime Switch triggers the Cold Start Injector based on temperature and time. To test this, check for resistance and continuity between the circuits. When cold, the resistance of Terminal A should be lower than the resistance of the same terminal when the coolant temperature is hot. When warm, Terminal A should have a higher resistance. When cold, terminal D should have continuity with the body of the sensor, with no resistance. When warm, the same terminal should show as an open circuit when tested against the body.

Auxiliary Air Device (AAD)

The AAD controls the flow of additional air into the engine when cold, increasing the cold idle speed. To test the AAD, pinch off the hose on either side of the unit when the engine is at operating temperature. If the idle speed drops when the hose is pinched, air is still flowing through the AAD. Another test is to check if there is power at the connector when the ignition and fuel pump are on.

WUR Warmup Circuit

The electrical side of the Warmup Regulator (WUR) is the last item on the list. To test this, first, check if there is power at the plug. Turn the ignition to ON with the pump on and use a test light to see if one of the terminals has power. Now, shut off the ignition and test for resistance in the heater coil in the WUR.

Frequently asked questions