The lower chamber pressure is regulated by the electrohydraulic actuator which is a solenoid valve controlled directly by the CIS-E control unit, which sets running conditions based on feedback from the temperature sensors (among other sensors, switches, etc). The CIS-E fuel injection system is the result of taking a bullet-proof mechanical injection system and adding whiz-bang gizmos to make the system run leaner to yield better fuel economy and extended service life without adjustment. Do a little research on the system, there should be some good online info available (I've got the MB CD's at work with the testing procedures and how the injection system works, its a lot of information but impressive the engineering put into this injection system - I'm pretty sure I've seen the CD's linked here before).