Pressures
System Pressure
On the Bosch K-jet systems, this pressure 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 (this is the horizontal one) to the warm-up regulator. When installed, the gauge will look like an upside down “T”. 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. This is usually 5 to 5.5 bar (75 to 80 psi); however, most turbocharged cars will be 5.8 to 6.8 bar.
Residual Pressure
Residual pressure is very important for proper hot restart. What you are looking for here are leaks in the system. When you shut down the engine or fuel pump, the system pressure will fall to 2.4 to 3.0 bar and stop. Pressure will slowly bleed off over time from there. If the pressure falls too quickly, look for problems in the accumulator, fuel pump check valve and fuel pressure regulator. On the Bosch KE-jet system, it is very important to have “Free Play” in the airflow sensor plate or the piston in the fuel distributor will not rest on its seal and you will have a leak down through the piston into the intake manifold.
Control Pressure
Control pressure 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 center of the fuel distributor. A low pressure allows the piston to move higher in the fuel distributor to provide more fuel for a given air flow. Increasing the pressure forces the piston lower for a leaner mixture for the same air flow. Control pressure needs to be checked cold and warm. The two-wire plug on the warm-up regulator heats the warm-up regulator to lean the mixture early before the engine is completely warm. This is done mostly for emissions regulations. Disconnect this plug for cold testing. While the engine is cold, open the valve on the continuous injection system CIS gauge and you will read cold control pressure. Cold control pressure will vary according to temperature. For example, at 10 degrees Celsius you may read 1 to 1.5 bar, and at 20 degrees Celsius you may read 1.8 to 2.3 bar. Reconnect the plug to the warm-up regulator and the control pressure should increase over the next few minutes until you reach warm control pressure, usually 3.4 to 3.8 bar. If the car is equipped with acceleration enrichment by means of a vacuum line connected to the top of the warm-up regulator, disconnect it and control pressure should drop 1 to 1.3 bar. These pressure values are approximate and actual pressure values need to be obtained from the manufacturer or Bosch.
Setting the Initial Mixture
Initial Mixture
Before you begin, get a clean rag and keep it nearby. Jump the fuel pump relay so that when you turn the key the fuel pump runs. Next, remove one injector line from the top of the fuel distributor so you can see when fuel begins to flow out the top. Now turn the key in the ignition. Turn the mixture screw clockwise until you see fuel flowing into the open port. When you see fuel flowing, slowly turn the mixture screw counterclockwise until the fuel stops flowing. Use the rag to dab at the excess fuel. When the fuel stops flowing, turn the mixture screw 1⁄2 turn counterclockwise.
This is Bosch’s recommended procedure in the K-Jetronic service guide.
Testing
Accumulator
The accumulator is a pressure reservoir that has a diaphragm with a spring behind it to hold a small amount of pressurized fuel in the system to smooth out spikes in delivery and to provide residual pressure. It is generally located in the area of the fuel pump and fuel filter. It will generally have a large line connected to the fuel line on the discharge side of the fuel pump and a small vent line attached to a steel nipple on the back side (in some cases this will be vented to the atmosphere), and it is usually vented back to the fuel tank or fuel intake on the fuel pump. Disconnect this vent line and plug it so it does not leak and run the fuel pump. While the fuel pump is running, if fuel leaks out of the steel nipple, the accumulator is bad and must be replaced.
Fuel Pump
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.
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