A 2017 Detroit DD15 engine is experiencing fluctuating NOx efficiency, sometimes reaching 95-100% then dropping to 0% or below 60%, despite extensive troubleshooting. The initial fault code indicated low NOx conversion efficiency and a faulty outlet NOx sensor. This guide explores potential causes and solutions for this persistent problem, focusing on diagnostic code 4364 Fmi 17.
Understanding the Problem: 4364 FMI 17 and NOx Efficiency
The diagnostic trouble code (DTC) 4364 FMI 17 often relates to issues within the aftertreatment system, specifically the Selective Catalytic Reduction (SCR) system responsible for reducing NOx emissions. Fluctuating NOx efficiency, as described in the original problem, points towards an intermittent fault that requires careful diagnosis. While the outlet NOx sensor was replaced and modules updated, the problem persists, suggesting a deeper underlying issue.
Troubleshooting Steps Already Taken
The following steps have already been taken to address the issue:
- Outlet NOx sensor replacement and module updates: Initially performed by the dealer, these addressed the initial fault code but didn’t resolve the underlying problem.
- Valve adjustment: Performed to address potential rich air/fuel mixture.
- Air filter and air filter box replacement: Eliminated potential air intake restrictions.
- DEF fluid replacement: Ensured fresh DEF fluid quality.
- Delta pressure sensor, MAF sensor, air intake temperature sensor, and HC doser replacement: Addressed potential sensor inaccuracies and doser malfunction.
- DPF cleaning and One Box cleaning: Ruled out clogged filters as a contributing factor.
- Inlet NOx sensor, inlet DOC pressure sensor, and hose replacement: Eliminated potential sensor and pressure issues in the DOC.
- Soot sensor and DEF doser replacement: Addressed potential soot accumulation and DEF dosing problems.
- DOC face plug cleaning: Ensured proper DOC functionality.
- Water pump and thermostat replacement: Addressed potential coolant temperature fluctuations.
Potential Causes of Persistent 4364 FMI 17
Despite the extensive work performed, the fluctuating NOx efficiency suggests several remaining possibilities:
- Wiring Harness Issues: While visually inspected, the wiring harness between the Aftertreatment Control Module (ACM) and the sensors might have intermittent shorts or opens, especially at higher temperatures. Thorough testing with a multimeter, including wiggle tests, is crucial.
- ACM Malfunction: A faulty ACM could be sending incorrect signals or failing to properly control the aftertreatment system.
- Exhaust Leaks: Leaks in the exhaust system upstream of the sensors can affect pressure readings and compromise NOx reduction efficiency. A thorough inspection of the exhaust system for leaks is necessary.
- Internal SCR Catalyst Issues: While less common, degradation or damage to the SCR catalyst itself can lead to poor NOx conversion. Specialized diagnostic tests may be required to confirm this.
Further Diagnostic Steps
To pinpoint the root cause, consider these additional diagnostic steps:
- Thorough Harness Inspection: Perform continuity and resistance checks on the entire wiring harness related to the aftertreatment system, paying particular attention to areas exposed to high temperatures.
- ACM Testing: If possible, use diagnostic software to monitor ACM performance and check for any unusual readings or fault codes related to the ACM itself. Consult the Detroit Diesel diagnostic manual for specific procedures.
- Exhaust System Pressure Test: Conduct a pressure test of the exhaust system to identify any leaks.
- SCR Catalyst Efficiency Test: Consult a qualified diesel technician to perform a specialized SCR catalyst efficiency test to assess its performance.
Conclusion
Resolving a persistent 4364 FMI 17 requires a systematic approach to diagnose and eliminate potential causes. The intermittent nature of the fault necessitates meticulous attention to detail and thorough testing. By following the recommended diagnostic steps and consulting with experienced diesel technicians, the root cause of the fluctuating NOx efficiency can be identified and rectified.
