Pressures
System Pressure
On the Bosch KE-Jet systems, you will connect one leg of the “T” to the fitting on the fuel distributor that supplies the cold start valve; this is the one that the valve will turn off. The other leg of the “T” will connect to a test port located in the lower half of the fuel distributor. This port can be identified by a 8 x 1.25 mm plug with a 12 mm hex head. With the valve open and the fuel pump running, you will read system pressure. This is usually 5.4 to 5.6 bar on cars with a single fuel pump, 6.2 to 6.6 bar on cars with duel fuel pumps.
Lower Chamber Pressure
Changes in lower chamber pressure perform the same functions of mixture leaning and enrichment as control pressure does on the earlier Bosch K-Jet system. It does this by adjusting the opening clearance of the individual differential pressure valves in the fuel distributor rather than adjusting the resistance pressure on top of the piston. This is done by the electro-hydraulic actuator (EHA). The EHA is an electro-magnetic valve that controls the amount of fuel flowing to the lower chamber. The lower chamber pressure needs to be .4 bar less than system pressure (AKA upper chamber pressure). If you don’t have enough differential pressure separation, you will not have enough, if any, fuel delivery to the injectors. This pressure can be read by closing the valve on the continuous injection system (CIS) gauge so that the gauge reads only from the test port in the lower half of the fuel distributor.
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.
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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