A 2014 Ford Fusion 2.0L engine presented with a persistent P0171 code (lean condition) setting immediately upon starting, even without driving. This article outlines the extensive diagnostic process undertaken to identify the root cause of this issue.
Troubleshooting a Stubborn Lean Code
The vehicle exhibited a lean condition with a long-term fuel trim (LTFT) at 28% and short-term fuel trim (STFT) at 7%. The engine ran smoothly with no noticeable drivability problems once the fuel trims compensated for the lean condition. However, clearing the Keep Alive Memory (KAM) resulted in the engine stalling until the LTFT reached 28% again, indicating a genuine lean condition.
Numerous potential causes were systematically investigated and ruled out:
- Fuel System: Fuel pressure (both high and low side) was within specifications and matched desired values. The fuel pump command and monitor were also checked. A fuel sample analysis revealed no abnormalities, and the alcohol content was below 10%. Fuel pressure sensors were validated by unplugging the Fuel Vapor Return (FVR) and confirming consistent readings.
- Vacuum Leaks: A thorough smoke test of the intake system revealed no leaks. The Positive Crankcase Ventilation (PCV) system was also inspected and tested for leaks.
- Exhaust Leaks: The exhaust system was pressure tested, and no leaks were found.
- Sensor Data: Various sensor readings were examined and compared to expected values:
- Manifold Absolute Pressure (MAP) sensor readings matched actual pressure.
- Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors reported correct values.
- Vehicle speed sensor
- Mass airflow (MAF) sensor
- Oxygen sensors
- Other Components:
- No misfire data was found in Mode 6.
- Variable Valve Timing (VVT) actual and desired values aligned correctly.
- There were no oil consumption issues.
- Injector tips were inspected with a borescope and found to be clean.
Unresponsive to Propane Enrichment
Introducing propane into the intake upstream of the air/fuel sensor did not alter the fuel trims, further complicating the diagnosis. The upstream air/fuel sensor remained at 1V, potentially indicating a faulty sensor. However, the downstream oxygen sensor readings did not indicate a rich condition, and the lack of drivability issues cast doubt on this being the primary cause. A stuck sensor at 1v would typically result in excessive fuel enrichment, leading to a rich condition and potentially other driveability concerns.
Possible Causes and Further Steps
The sudden onset of the problem after a dealership visit for a recall related to the shifter bushing, while seemingly unrelated, raises questions about potential disturbances during the service.
Possible remaining avenues for investigation include:
- Wiring Harness: Inspecting the wiring harness for damage or chafing, particularly around areas that may have been disturbed during the recall service.
- PCM/ECM Issues: Exploring potential issues with the Powertrain Control Module (PCM) or Engine Control Module (ECM), including software glitches or faulty connections. This could involve checking for related Technical Service Bulletins (TSBs) from Ford.
This case highlights the complexity of diagnosing intermittent lean conditions. Further diagnostics are required to pinpoint the root cause and implement the appropriate repair.
