Back-leakage testing (spill testing)
by Pico Technology

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 a 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.

  1. 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.
  2. 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.
  3. 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

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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