CNG Injector Misfire Diagnosis
A dedicated CNG bus has a misfire at idle and under light load. The check engine light is on with DTC P0302 (cylinder 2 misfire). The bus has 100,000 miles. The technician swaps the ignition coil and spark plug from cylinder 2 to cylinder 4, but the misfire stays on cylinder 2. A compression test shows all cylinders within 10% of each other. The technician performs a fuel injector balance test: cylinder 2 injector has a slightly lower flow rate than the others. The CNG injectors are solenoid type, fed by a common rail at 90 psi. The injector driver circuit is integrated into the ECM. The technician suspects a faulty injector, but also considers wiring or ECM issues. The bus uses a wideband oxygen sensor and has a long-term fuel trim of +10%.
1. What is the most likely cause of the misfire, and how can the technician confirm it?
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The misfire is isolated to cylinder 2, and swapping ignition components did not change it. Compression is good, ruling out mechanical issues. The injector balance test shows lower flow from cylinder 2 injector, indicating a partially clogged or faulty injector. The positive fuel trim (+10%) suggests the engine is running lean overall, possibly due to the weak injector. To confirm, the technician can perform a cylinder drop test: disable each injector one at a time and observe RPM drop; cylinder 2 may show less drop. Also, check the injector resistance and compare to spec. Inspect the injector connector and wiring for corrosion or damage. If the injector is faulty, replace it. However, since the injector driver is in the ECM, a faulty driver could also cause low flow. To rule out the ECM, the technician can swap the injector with another cylinder and see if the misfire moves. If it moves, the injector is at fault; if not, the wiring or ECM may be faulty.
2. If the injector is replaced and the misfire persists, what should be checked next?
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If the misfire persists after replacing the injector, check the injector driver circuit. Use a noid light or oscilloscope to verify the injector pulse signal at the connector. Compare the signal on cylinder 2 to a known-good cylinder. If the pulse is abnormal (e.g., low amplitude, wrong duration), check the wiring from the ECM to the injector for shorts, opens, or high resistance. Also, check the ECM ground and power feeds. If wiring is good, the ECM may have a faulty driver. Some ECMs can be tested or reprogrammed. Additionally, verify that the new injector is not defective and that the fuel rail pressure is correct. Check for intake leaks near cylinder 2, as a vacuum leak can cause a lean misfire. Also, perform a leak-down test to rule out valve issues.