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Diesel Common Rail Systems Troubleshooting
common rail systems
The main advantage of high pressure common rail system is that it can change the injection pressure and injection time in a wide range.But traditional methods can not be used for fault diagnosis.Before fault diagnosis, the maintenance manual and operation guide of high pressure common rail diesel engine must be read in detail and strictly followed the maintenance steps and then process execution. Otherwise, it may cause personal safety accidents due to high pressure oil spout in common rail diesel engine during operation.
1.Electrical system fault
First, Read the fault information through the fault decoder.For most common-rail diesel engines, if the crankshaft speed sensor is damaged, the engine will not start.But the cummins electronically controlled high pressure common rail diesel engine has two crankshaft speed sensors, one mounted on the flywheel housing to measure crankshaft speed and piston top stop.The other is mounted on a high-pressure oil pump to measure the phase of the oil pump camshaft.If any of the speed sensors are damaged, the diesel engine can still start, but if both two speed sensors are damaged at the same time, the diesel engine cannot start.If the speed sensor located on the flywheel shell is damaged, its connector can be removed and the speed sensor on the high-pressure oil pump can be used to adjust.n case of fault code, the fault code can be used to find the fault of related electrical components.Usually the electrical components are rarely damaged except  connection loose.As the common rail parts suppliers & manufacturers,China Balin Power Co.,Ltd recommend that all connectors can be replugged if no fault point is found.
2.Low pressure oil circuit failure
The engine has no fault code during the inspection. The engine cannot be started after being driven by the starter for several times. The pressure of low-pressure oil supply pipeline can be measured under the operating condition of the starter.If it is lower than 0.15mpa, there is a fault in the low-pressure oil path.Try to discharged the air in it and then engine could be started smoothly.But if it is difficult to start again after being put out or put out for about 5min after a fire. There is possiblity that the low-pressure oil path seal is not tight.Generally, there is leakage between the fuel tank and the low-pressure oil pump, which leads to the low-pressure oil road entering air and directly leads to the difficulty in establishing the rail pressure.At this point, it is necessary to check whether the fuel tank oil level is too low, whether the oil port of the oil tank cap is blocked, whether the oil and water separator has a large amount of water,fuel filter and its bypass valve are blocked, whether the tubing is bent, and whether the pressure of the low-pressure oil transfer pump is too low.
3.Pressure limiting valve problem
In the condition of high idle speed of diesel engine, if the fuel pressure of high-pressure oil rail exceeds 140MPa, the pressure limiting valve should open the pressure relief protection system to avoid damage.When it is lower than 30MPa, the pressure limiting valve should be closed to increase the system pressure.If the pressure is not within the specified range when measuring the common rail oil pressure, it's possible that the pressure limit valve don't open and close according to the specified pressure,then the pressure limit valve should be replaced.In addition, the return oil amount of pressure limiting valve should be measured. If it is higher than the maximum allowable value (10mL), it can be judged that there is leakage in the pressure limiting valve. Replacing the pressure limiting valve can eliminate the fault.
4.Injector failure
The return oil amount of the injector is measured at the rated speed. If it exceeds the normal working limit (960 ~ 1200mL), it can be judged that the injector internal parts such as the electromagnetic valve or the injector nozzle needle valve are stuck.The above components are stuck with a lag, leading to an abnormal increase in the return oil volume. The high-pressure oil is recirculated to the oil tank through the overflow hole of the fuel injector, which causes the diesel engine to flameout. At this time, the fuel injector can be replaced.When removing the injector of the high pressure common rail system, it should mark the cylinder number where it is located. When installing, it must be "positioned with the same number", so that the injector and cylinder correspond one by one.It is not allowed to loosen the joint of the fuel injector for "breaking cylinder" test.Usually, We release the high-pressure oil pipe joint of a cylinder injector,If the speed of the engine does not change,it means the injector of this cylinder does not work well.However, this "breaking cylinder" test cannot be used for electronically controlled high pressure common rail diesel engines. Once the high pressure oil pipe joint on the common rail diesel fuel injector is released, the common rail diesel engine will be shut off immediately.This is because loosening the high-pressure tubing joint on any injector will not establish the common rail high-pressure oil pressure.If necessary, the oil return pipe joint of the injector can be released and we judged by the oil return condition.The method is to remove the oil return pipe of each fuel injector and put it into a measuring cup separately. When starting the engine, the oil flowing out of the fuel return pipe of the fuel injector will be directed into the measuring cup.By comparing the difference amount of oil in each measuring cup, the problem injector can be determined quickly.If the return oil of a cylinder is significantly more or less, it indicates that the fuel injector parts is stuck or leakage is too large.
Note: the seal ring of the injector is a disposable part, which cannot be used after disassembly
5.High pressure oil path failure
After starting the engine, it will turn off automatically after running for about 5 minutes.Spot check, no flash code.Preliminary analysis, belonging to the oil road problem.After checking and removing the air from the low pressure oil pipeline, it is still unable to start.It may be problem that there is air in the high-pressure oil path.Loosen the two high-pressure tubing interfaces at the end of the oil pump and drive them with the starter. If no fuel is discharged, it can be confirmed that there is air in the high-pressure pump.Fold the return pipe and use the starter again to drive the exhaust. After several times of driving, the fuel can flow out of the interface.Connect the tubing joint and start normally.