Further guidance
When selecting ZOOM LEVEL 3 of your WPS500X, the back-leakage pressure value obtained in Example waveform 1 (25 mbar) is centered around the 0 mbar marker (black dotted line) and no longer represents the pressure value. Zoom level 3 now amplifies the dynamic fluctuations within the back-leakage pressure signal, by magnifying the pulsations 1000 times. This lets you analyze the waveform for irregularities attributed to injector malfunction. While the vehicle tested above did not exhibit any running issues, there is a clear difference in contribution (at idle speed) to back-leakage pulsations between injectors 2 and 3.
Refer to vehicle technical data for specific test conditions and results.
Typical values with an engine at correct operating temperature, idling and no load applied.
Note: Back-leakage pressure and pulsations will change in relation to engine speed and load. Analyse back-leakage pulsations and pressure under conditions equal to the ones reported by the customer.
Engine idling ZOOM OFF (Figure 2)
It is vitally important that all air is bled from the spill hoses and the WPS500X pressure transducer before you analyze the waveform for irregularities. With the engine at the correct operating temperature, you can carry out numerous momentary Wide Open Throttle (WOT) snap tests to purge the air from the injector back-leakage circuit as well as the WPS500X pressure transducer.
When the engine idle speed is stabilized and all loads applied to the engine have settled, you can measure the maximum peak back-leakage pressure (approximately 25 mbar in the example above).
When the engine idle speed is stabilized and all loads applied to the engine have settled, you can measure the maximum peak back-leakage pressure (approximately 25 mbar in the example above).
Note: An increase in common rail pressure will be accompanied by an increase in back-leakage pressure.
Peak back-leakage pressure may vary depending on the system under test. Typical values average between 20 to 300 mbar.
Peak back-leakage pressure may vary depending on the system under test. Typical values average between 20 to 300 mbar.
Symptoms of hesitation at high engine load will be accompanied by a momentary increase in back-leakage pressure and individually deformed pulsations.
Engine idling Zoom level 3 (Figure 3)
While peak back-leakage pressure is important, you can use the zoom function of the WPS500X, with the firing order, to reveal the origin of uneven pulsations in the back-leakage waveform.
You can identify the source of the pulsations by selecting ZOOM LEVEL 3 (press the ZOOM button 3 times on the front panel of the WPS500X). This has the effect of magnifying the pulsations in the back-leakage waveform only. These are formed by leak-off contributions from each injector. Do not refer to the pressure scale on the scope when you use the zoom function, as only the pulsations are displayed on the screen, not the back-leakage pressure value.
With the pulsations magnified like this, you can analyze the formation of each peak for uniformity and balance between peaks.
Peaks that are unevenly formed, or exhibit an irregular shape, indicate a potential injector failure due to internal sticking or worn components. Pulsations with amplitude high above other injectors indicate an injector that is contributing an increased amount of back-leakage in comparison to others. Equally, pulsations with an amplitude below other injectors indicate an injector with a reduced contribution to back-leakage and so a potential blockage or sticking components in the injector.
When you attempt to identify an offending injector due to irregularities in the magnified back-leakage pulsations, use the injection events on channel A of your oscilloscope.
Injector back-leakage pulsations are created during, or shortly after, the main injection event. Looking at Figure 2 and 3, the pulsations for each injector occurs directly above each injection event.
By using the Rotation markers to denote 0 and 720 degree rotation between switching events in injector 1, with four rotation partition rulers to divide the 720 degree rotation into 180 degree intervals, we can reveal the approximate position of the remaining injection events that take place during two full revolutions of the crankshaft (720 degrees).
You can now identify every injector back-leakage pulsation by using the firing order 1, 3, 4, 2 to identify offending injectors.
Alignment of the back-leakage pulsations to each injection event will vary from system to system. To confirm a suspect pulsation (and so an injector), you can introduce a misfire with a Cylinder Cut feature in a scan tool, or substitute an injector with a load box. This will remove the back-leakage contribution for the injector that has been cut and confirm if it is the offending injector.
