This applies to Bosch KE-jet only, Changes in lower chamber pressure performs 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. The EHA is a 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.