The purpose of this test is to check the fuel system pressure characteristics on a common rail diesel engine by using the fuel rail pressure sensor.
This known good waveform has the following characteristics:
The fuel rail pressure sensor provides feedback to the ECM, which regulates the rail pressure by varying the volume of fuel delivered by the high-pressure pump and/or controlling the flow of excess fuel from the rail to the fuel return system. The ECM uses its internal calibration tables (maps) to calculate the desired fuel rail pressure and injection duration according to the accelerator pedal position (indicating driver demand), engine speed, load, and temperature etc.
The rail pressure sensor is calibrated to output a signal between 0.5 V and 4.5 V with changing rail pressure across the range 0 bar to 1600 bar. This provides two opportunities for the ECM to undertake a signal plausibility check: any voltage above or below these values, for example, either at 0 V or 5 V, will indicate that the sensor has failed.
A high-pressure system leak can be indicated if the sensor output rapidly drops to 0.5 V after the engine has been stopped, as might occur with a leaky injector, rail pressure regulator, inlet metering valve or pump.
Please note, the example waveform was captured with an unloaded engine; with the engine fully loaded over a prolonged period, the sensor output voltage will be above 2.5 V and, in the extreme case, up to 4.5 V.
Selection of component related Diagnostic Trouble Codes (DTCs):
P0087 – Fuel rail pressure sensor or rail pressure too low
P0088 – Fuel rail pressure sensor or rail pressure too high
P0190 – Fuel rail pressure sensor circuit open/short/failure
P0191 – Fuel rail pressure sensor circuit performance/range
P0192 – Fuel rail pressure sensor circuit low input
P0193 – Fuel rail pressure sensor circuit high input
P0194 – Fuel rail pressure sensor circuit intermittent
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