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Common Rail Diesel Injector Back-Leakage Testing

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You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

(Solenoid and Piezo type injectors with accessible spill return)

This is a new Guided Test in BETA form. Please feel free to pass any feedback, suggestions, corrections or hints and tips to us via the comments function at the bottom of this page.Injector back-leakage can also be referred to as injector spill testing or leak-off testing.

The purpose of this test is to evaluate each common rail diesel injector’s contribution to back-leakage pressure, for low return pressure systems only.

Note: Diesel injector back-leakage, pressure and balance are dependent on fuel quality, the integrity of the high-pressure diesel pump priming circuit, the common rail high-pressure circuit, the engine load, and the common rail diesel control system/components.

All the below values obtained with the WPS500X are referenced to gauge pressure (0 mbar at atmospheric pressure).

All the numerical readings quoted in this help topic are typical and are not applicable to all common rail systems.

Make sure that the WPS500X is fully charged before you start the test.

How to perform the test

  • BNC to BNC cable
  • WPS500X pressure transducer
  • TA085 Vacuum hose
  • TA129 Vacuum adaptor
  • TA189 30 A current clamp
Various custom hose adaptors and containers may be required.

PicoScope settings:

Channel A

30 A Current clamp

 Input range -5 to 20 A

Channel B

WPS500X Range 3

± 69 mbar

Time base 20 ms/div 

pico auto

The WPS500X pressure transducer requires a single connection to the spill pipework, at the point where all injector back-leakage returns to the fuel tank. All injector leak-off will, therefore, pass through the WPS500X pressure transducer en route to a collection bottle.
  1. Connect the 20/60 A current clamp (TA189) to Channel A on PicoScope and link the clamp around the signal wire of the number 1 injector. This will enable identification of the pulsations in the back-leakage waveform, indicating offending injectors.
  2. Switch on the WPS500X and wait for the self-test to complete. (The LEDs will scroll from range 1 to 3 and revert to 1.)
  3. Remove the spill pipe from the injector that returns leak-off to the fuel tank. This is often located at injector number 1 or number 4 on a 4-cylinder engine. Make sure that the pipe is sealed to prevent contamination.
  4. Connect a donor spill type hose to the spill stub of the relevant injector located in step 3, and link the opposite end of this spill hose to the Vacuum hose (TA085) with the Vacuum adapter (TA129).
  5. Connect the Vacuum hose (TA085) to the quick couple connector on the WPS500X.
  6. Connect the bleed hose to the WPS500X and feed the hose into a sealed container.
  7. Open the bleed valve on the WPS500X and fix in the open position with a heavy duty cable tie. (The bleed valve should remain open throughout the test.)
  8. Press the range button on the WPS500X and select Range 3 (-5 to 5 Psi). Do not select zoom functions.
  9. Connect the WPS500X pressure transducer to Channel B of the scope with the BNC to BNC test lead.
  10. Run the PicoScope software by pressing either the space bar on your keyboard or the GO button in PicoScope.
  11. Crank and start the engine, and allow idle speed to stabilize.
  12. When the engine is at the correct operating temperature, you can carry out numerous momentary Wide Open Throttle (WOT) snap tests. This will purge the air from the injector back-leakage circuit and the WPS500X.
  13. Let the engine return to idle while you record the spill pressure value and the pulsation formations.
  14. While the engine is idling, select Zoom 3 from the front of the WPS500X to magnify the spill pulsations in preparation for closer analysis.

We advise that you recharge your WPS500X after each use to ensure it is ready for future measurements.

Example waveform 1.

WPS500X range 3 Zoom OFF

All the values included in the example waveforms are typical, and are not specific to all engine styles.

Channel A, Blue (1, 3, 4, 2, and 1) indicate the current drawn by injector 1 and the position of the remaining injection events taking place during two complete revolutions of the crankshaft. (The time taken to complete the 4-stroke cycle at an idle speed of 800 rpm is approximately 150 ms.)


Note: the firing order may change for the vehicle under test.

The engine tested above utilizes the firing order 1, 3, 4, 2.

 denotes atmospheric pressure of 0 mbar. 
  indicates the peak back-leakage pressure and pulsations resulting from “leak-off” contributions of each injector (approximately 25 mbar).

Example waveform 2.

WPS500X range 3 zoom level 3

indicates the highest back-leakage pulsation magnified 1000 times (contributed by injector no. 3). The waveform no longer represents the back-leakage pressure value (see Diagnosis section below).

 indicates the lowest back-leakage pulsation belonging to injector 2.



 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 your vehicle’s 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.

1. Engine idling ZOOM OFF (Example waveform 1)

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).

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.

Back-leakage pressure values have to be evaluated in relation to the customer's reported symptoms. Example 1: Symptoms of hard starting may be accompanied by an unusually high back-leakage pressure during cranking and uneven pulsations (see section 2 below).

Example 2: Symptoms of hesitation at high engine load will be accompanied by a momentary increase in back-leakage pressure and individually deformed pulsations (see section 2 below).

2. Engine idling Zoom level 3 (Example waveform 2)

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 Example waveform 1 and 2, 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. 

More information

Possibly one of the most valuable and conclusive test procedures we have when diagnosing common rail diesel systems is the Spill Test, alternatively known as Back-leakage test or leak-off test.

Spill or back-leakage is an essential feature of the common rail diesel systems, which confirms adequate lubrication of the injector assemblies while providing a degree of cooling as a beneficial side effect.

Traditionally we would measure the volume of back-leakage with graduated collection bottles while monitoring both the rate and the amount of fill during cranking or pre-set running conditions.

The back-leakage test will always remain relevant as manufacturers often specify the volume of back-leakage against time (Cubic Centimetres per Seconds CC/S)

The alternative of using the WPS500X does, however, offer some additional advantages.

Connecting the WPS500X in series with the injector spill return hose will enable the technician to measure the back-leakage pressure/pulsations under numerous engine speed and load conditions. The technician can now monitor the contributing back-leakage pressure from each injector while road testing the vehicle under the conditions described by the customer or DTC freeze frame data.
The WPS500X will need only one connection into the back-leakage return pipework as opposed to disrupting four injector spill hoses on a 4-cylinder engine.
Using the WPS500X, in conjunction with PicoScope, will provide a hard copy of test results under the conditions specified by the customer. When you use a 4-channel PicoScope, the remaining two channels can be used to include any additional relevant engine parameters, such as RPM, MAF, MAP or fuel pressure. This builds customer confidence and provides feedback to warranty/insurance companies.

Injector back-leakage pulsation formation  – Injector open

Injector back-leakage pulsation formation – Injector closed

pico auto injector closed

Injector back-leakage is seen as a pulsation when viewed using a PicoScope. The pulsation is formed as a result of each injection event as follows:

High-pressure diesel generated by the high-pressure diesel pump is fed directly to the common rail and into the injector assemblies. The pressure remains equal throughout the injector body until the solenoid or Piezo crystal is energised by the engine control unit.

Upon receiving the energizing signal, a control plunger in the injector will lift from its seat and release a small amount of diesel into the back-leakage pipework, generating the pulsation seen on channel B of the scope. The release of diesel into the back-leakage circuit forms a differential pressure inside the injector, allowing the high-pressure diesel to lift the nozzle from its seat and deliver a measured quantity of fuel into the relevant cylinder.

De-energizing the injector solenoid or Piezo crystal results in closure of the control plunger assembly, increasing the pressure acting upon the nozzle which will close under the influence of the high-pressure diesel and internal spring pressure. Back-leakage is essential to provide lubrication and cooling to the high-speed moving components of the injector. Any failures within the injector will be revealed in the formation of the back-leakage pulsations.

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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.


Suitable Accessories

WPS500X Pressure Transducer

ta085 vacuum hose Vacuum hose

ta129 Universal vacuum adaptor

ta234 current clamp 30 A (low amps) current clamp

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