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.
Connection for diagnostic work will vary dependent on the application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on the difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
This known good waveform has the following characteristics:
With the ignition on and the engine off, the sensor output is at 0.5 V, corresponding to a rail pressure of 0 bar.
As the engine cranks the sensor output rises rapidly to about 1.3 V, indicating the idle operating rail pressure of around 280 to 320 bar.
With the accelerator pedal fully depressed, the engine accelerates creating a rise in rail pressure.
After reaching a peak around 3.5 V, the rail pressure is reduced whilst the high engine speed is maintained and the sensor voltage drops to 2.5 V: with the vehicle stationary, the engine is unloaded and a high rail pressure is not required to maintain its speed.
When the accelerator pedal is released, the engine returns to idle and the sensor output returns to 1.3 V, as originally observed.
After the engine is stopped and the ignition is switched off, the signal drops slowly back to 0.5 V over about 10 s, after which the ECM powers down and the sensor output falls to 0 V.
The fuel rail pressure sensor provides feedback to the ECM, which regulates the rail pressure by either varying the volume of fuel delivered by the high-pressure pump 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
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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