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.
Functional description
Depending upon conditions, one of the following 3 available closed-loop control strategies is used to adjust the rail pressure to the correct level:
- Volumetric flow regulation by the flow regulating valve:
The flow regulating valve only allows the amount of fuel to flow into the high pressure pump from the low-pressure side that is required in order to generate the required fuel rail pressure. The high-pressure pump cylinder is not completely filled with fuel. The higher the control signal current, the lower is the rail pressure that is generated. The rail-pressure regulating valve is not supplied with current for the maximum pressure. The rail-pressure regulating valve is supplied with slightly higher current than would be necessary for setting the target pressure.
- Pressure regulation by the rail pressure control valve:
The high-pressure pump provides a constant supply of highly-pressurized fuel to the rail. The rail pressure regulating valve diverts excess fuel arriving in the rail into the return line. The higher the control signal current, the higher the rail pressure that is maintained. The flow regulating valve is set to maximum flow.
- Combined closed-loop control with simultaneous regulation from the rail pressure regulating valve and the fuel-quantity control valve:
At extremely low fuel-injection quantities of less than roughly 4 mg (on trailing throttle/overrun) the rail pressure regulating valve must discharge a certain amount of fuel from the high-pressure system. The reason is that the high-pressure pump cannot run with zero delivery. This means that the high-pressure pump supplies fuel to the high-pressure system even when the flow regulating valve is closed. This would lead to excessive rail pressure and thus to a control deviation.
The two control methods are used under the following engine operating conditions:
- During engine starts: Pressure regulation is always active.
- When the engine is running: At a coolant temperature below 1 °C, the pressure regulation is always active. Once the coolant temperature rises above 15 °C the combined closed-loop control system assumes operation.
| Index | Explanation |
|---|---|
| 1 | Fuel quantity control valve |
| 2 | two-pin plug connection |
| 3 | High-pressure pump (CP4.2) |
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
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.