What exactly does the fuel metering unit do? It is also called the metering proportional valve. Installed at the fuel inlet position of the high-pressure fuel pump, used to adjust fuel supply and fuel pressure, and controlled by ECU. Generally applicable to Yuchai National 3 and National 4 series electronically controlled diesel engines supporting BOSCH high pressure common rail fuel injection system.
The structural principle of the metering valve is also very simple: the fuel metering valve and the overflow valve are the two key components with a relatively high failure rate in the Bosch common rail high-pressure fuel pump. In most cases, the fault location and fault type are accurately located and disassembled separately. Inspection, cleaning or replacement of fuel metering valve and overflow valve components can avoid unnecessary replacement of the entire pump.
The fuel metering valve is a proportional solenoid valve, usually installed at the fuel inlet position of the high-pressure fuel pump, and controlled by the PWM signal output by the ECU; the ECU controls the opening of the fuel metering valve by changing the duty cycle of the PWM signal to control the entry plunger The amount of fuel to control the common rail pressure. The top dead center diagnostic instrument has all functions.
Different models of Yuchai Bosch common rail electronically controlled diesel engines, depending on the applied high-pressure fuel pump model and specific control strategy, the fuel metering valve structure, characteristics, installation position and electrical interface form are slightly different. Be sure to pay attention to the difference when replacing, especially according to the different characteristics of the default state (in the case of power failure), the fuel metering valve is divided into two types: normally open and normally closed. In practical applications, it can be judged according to whether there is a pressure relief valve on the common rail pipe: general commercial vehicle platform engine common rail pipe has a pressure relief valve, and its fuel metering valve is normally open; passenger vehicle platform engine common rail pipe There is no pressure relief valve, and its fuel metering valve is a normally closed top dead center diagnostic instrument with all functions.
For the normally open fuel metering valve, the default working state is normally open, and the flow is the largest in the non-energized state, and the current characteristics of the normally closed type are just the opposite. The overflow valve is a multi-stage mechanical valve, used to adjust the oil delivery pressure of the oil pump, which is essential to ensure the normal lubrication and oil return of the oil pump. When the oil pressure at the inlet of the fuel metering valve is too high (usually 0.1MPa), the overflow valve will open the oil return port, allowing the fuel to return to the inlet of the input pump or the fuel tank.
Different models, depending on the type of fuel metering valve used and specific control strategies, have slightly different fault phenomena and fault code definitions, but the basic troubleshooting ideas are the same.
There are several common failure modes of fuel metering valves.
- Turn on the fault light
- Generate corresponding fault codes: P0251 (fuel metering valve drive failure-open circuit), P0254 (fuel metering valve drive failure-short circuit to the power supply), P0253 (fuel metering valve drive failure-short circuit to ground), P160E (main relay 1 Drive circuit failure-short circuit to power) or P160F (main relay 1 drive circuit failure-short circuit to ground)
- The controller increases the oil supply of the high-pressure oil pump
- The fuel pressure is extremely high, and the pressure relief valve bursts open
- The diagnostic instrument shows that the rail pressure is 700-760Bar
- The engine speed is limited, and the throttle still works within the speed limit. Inappropriate operations during use, maintenance, disassembly, handling and assembly may cause bumps, deformation, internal structure damage, and even leakage of the fuel metering valve and its electrical connectors, resulting in abnormal electrical connections.
Overflow valves also have some common failure modes, such as:
- The relief valve sealing ring is damaged
- Air leakage from the overflow valve
- Abnormal flow and pressure parameters of the overflow valve
- The overflow valve is stuck
- The spring of the relief valve is broken or cushioned by foreign matter.
Some methods can be used to troubleshoot and reduce unnecessary losses. Check beforehand that the battery is sufficiently charged; the vehicle power supply lines and fuses are normal; all connectors are normal, and there are no loose connectors or rusty pins. You can also use some testing tools to troubleshoot, such as multimeters, diagnostics, jumper boxes, and wires.
It should be pointed out that the normal and effective operation of the electronic control system is inseparable from scientific and reasonable maintenance. The fuel metering unit is generally integrated and installed on the fuel pump and is not allowed to be disassembled at will. In addition, the fuel metering units of different electronic control systems are different. For example, Bosch, Denso and Delphi are different, and the specific electronic control system needs to be resolved.