Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Low pressure fuel system: Notes
The N55 6-cylinder spark-ignition engine relies on direct fuel injection. The direct fuel injection increases the performance. The maximum fuel pressure is 200 bar (idle: 50 bar, WOT full load: 200 bar). The use of direct fuel injection creates a homogeneous mixture preparation in the entire combustion chamber. Homogeneous mixture preparation means that the fuel-air ratio is regulated stoichiometrically in the same way as for intake pipe fuel injection (Lambda = 1). A stoichiometric air-fuel mixture contains a ratio of 14.7 kilograms of air to 1 kilogram of fuel. Homogeneous mixture formation renders it possible to use a conventional emissions control system. Fully-sequential multipoint injection with selective control for each individual cylinder offers the following advantages:
- Optimal fuel mixtures for each individual cylinder
- The injection duration is precisely adapted to the engine's instantaneous operating conditions (engine speed, load factor and temperature)
- Responds to changes in load by correcting the injection duration specifically for the individual cylinder (during the intake stroke the injection duration can be corrected with a supplementary post-injection discharge as well as by extending or reducing the injection period)
- Selective deactivation of individual cylinders is also available (for instance, in response to a defective ignition coil)
- Allows individual diagnosis of each fuel injector.
The fuel-pressure sensor transmits a voltage signal to the engine-management control unit (DME control unit) indicating the system pressure between the electric fuel pump and the high-pressure pump. It monitors system pressure (fuel pressure) upstream from the high-pressure pump. The DME digital engine electronics system runs continuous checks to compare the specified setpoint pressure with the actual pressure. The DME responds to deviations with action such as using the EKPS electric fuel-pump control to reduce the voltage to the electric fuel pump. This regulates the delivery pressure at which fuel is supplied to the engine to the correct level.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.