This issue is actually quite common. Except for electrical faults that can be detected using diagnostic instruments, wear of mechanical fuel system components can only be addressed by replacing all injectors simultaneously. Worn components lack sensors, so the engine control unit (ECU) cannot detect them. Human analysis and judgment remain essential. For example, just as the ECU cannot detect a missing track shoe, it cannot detect individual injector wear.

In an ideal scenario, all six injectors should not fail simultaneously. However, due to the intervention of the electro-hydraulic control system during operation, when engine fuel system components experience wear, the ECU adjusts hydraulic pump flow as engine speed drops to compensate for reduced engine power. Operators are unlikely to perceive this adjustment by feel. Confirming this requires precise testing, which end users typically cannot perform. This explains a common observation: “the engine is difficult to start, but once started, it operates normally.” In reality, it is not entirely normal—fuel system wear does not only affect starting fuel, but this goes unnoticed.

When fuel system components reach a critical wear level—on a six-cylinder engine, at least four cylinders—starting difficulty or complete starting failure occurs. Typically, the first symptom is hard hot starts, followed eventually by cold start issues. Extensive testing has confirmed this phenomenon, although it is rarely communicated to end users.

Among the removed injectors, some may still have usable life if properly tested and selected. These can be combined with other serviceable injectors to form a set for continued use. While this practice is feasible, it should not be expected to restore full performance—only basic functionality. CAT does not recommend this approach, as combining injectors with different wear levels may affect fuel distribution across cylinders, potentially increasing wear on the crankshaft and connecting rod assembly. Such combinations are essentially a temporary workaround.

Under field repair conditions, qualitative assessment of injector failure can be performed using starting fluid tests, while quantitative evaluation is generally impossible. This limitation is inherent to electronically controlled fuel systems.

These field practices were developed through on-site service experience. Failures caused by mechanical wear cannot be detected with standard diagnostic tools (CAT ET with specialized software), and qualitative evaluation through return fuel measurement remains the only practical method. Consequently, replacing all six injectors together is necessary, as it is not feasible to identify the specific failed units.