Multiple Injectors Show High Leak-Off in Field Testing but Pass Bench Inspection: A DR Congo Mining Service Team Eliminates Measurement Error by Standardising Hose Length, Container Position and Test Duration

Conflicting Injector Test Results at a Mining Site

A mining service team in the Democratic Republic of Congo faced an unusual diesel injector diagnosis. Several engines used in haulage and loading equipment showed difficult starting and unstable rail pressure after extended operation. Field leak-off tests appeared to identify multiple injectors with excessive return flow.

The injectors were removed and sent for bench inspection. Under controlled test conditions, however, the units produced acceptable return-flow and fuel-delivery results.

Rather than classifying the bench test as incorrect, the service team reviewed how the original field measurements had been performed at the DR Congo mine.

The investigation showed that the real problem was measurement consistency during injector leak-off testing.

Why the Field Results Could Not Be Compared Directly

Different technicians had assembled their test equipment using available hoses and containers. Hose lengths varied from cylinder to cylinder, some collection bottles were positioned higher than others, and test duration was estimated manually.

These differences can influence the apparent return quantity.

Unequal Hose Length

Longer hoses create additional flow resistance. If one injector uses a noticeably different hose length, its collected volume may not be directly comparable with another cylinder.

Container Height

A collection container positioned significantly above or below another can change the pressure relationship in the temporary return circuit.

Inconsistent Test Time

At the Congolese mining site, technicians sometimes stopped individual measurements when they considered the volume “high enough.” This made the final results impossible to compare quantitatively.

The Mine Introduced a Standard Field Procedure

The team rebuilt the test arrangement so every injector used the same hose type and similar hose length. Containers were mounted at the same height and clearly labelled by cylinder number.

Engine condition was also standardised.

Technicians recorded:

• Coolant temperature;
• Engine speed;
• Test duration;
• Fuel temperature where practical;
• Collected return quantity;
• Target and actual rail pressure.

The test was repeated under the same operating condition for all cylinders.

The previously large differences became much smaller.

Bench Testing Was Used as a Second Measurement Layer

The injectors were still evaluated on a model-specific bench programme. The report included starting delivery, relevant load points, internal return flow and nozzle sealing.

Instead of asking whether the field test or bench test was “correct,” the DR Congo team compared what each method actually measured.

The field test showed injector behaviour inside the running engine. The bench test controlled pressure, pulse width, temperature and timing more precisely.

When both methods were performed consistently, the results no longer appeared contradictory.

Mining Conditions Made Test Discipline Important

At remote DR Congo mine sites, temporary diagnostic equipment is often assembled quickly because machinery downtime affects production schedules. Yet speed can create measurement errors that lead to unnecessary injector removal.

Dust, heat and restricted workshop space also make repeatable testing more difficult than in a dedicated diesel laboratory.

For that reason, the service team treated the leak-off kit as measuring equipment rather than simply a group of hoses and bottles.

Technical Outcome

The central lesson was not that leak-off testing is unreliable. It was that leak-off comparison depends on controlled test geometry and timing.

By standardising hose length, container position and test duration, the Congolese mining team created field data that could be compared with confidence.

The process also reduced the risk of interpreting a test-setup difference as evidence that several diesel fuel injectors had failed simultaneously.