Cummins diesel engine idle instability (rough idle, surging, stalling, etc.) stems from fuel system issues (contamination, air intrusion, faulty injectors/pumps), air intake/exhaust problems, sensor/ECM faults, mechanical issues, or governor problems. Troubleshooting involves verifying symptoms, checking DTCs, inspecting fuel/air systems, testing sensors, and mechanical checks, using tools like INSITE™ for diagnostics, with safety and model-specific manuals prioritized.