The P0171 code, indicating a lean fuel condition in Bank 1, is a common issue for many vehicle owners, including those driving Infiniti models. Often, the initial suspects are vacuum leaks or a faulty Mass Air Flow (MAF) sensor. However, as one Infiniti owner discovered, the root cause can sometimes be more nuanced, requiring a deeper dive into the fuel and emission systems. This article explores a real-world case of diagnosing and resolving a P0171 code on an Infiniti, highlighting the importance of considering less common culprits like the low pressure fuel pump and purge control valve.
Initially, like many others encountering the P0171 code, the focus was on potential air leaks. Fuel trim levels, key indicators of engine fueling adjustments, were sporadically elevated, suggesting an unmetered air entry into the system. Even after reverting to the original equipment air intake system from a cold air intake, the improvement in fuel trims was minimal and inconsistent. This pointed away from a simple intake-related air leak being the sole problem. The frustratingly intermittent nature of the issue, with fuel trims fluctuating from acceptable to elevated levels, made pinpointing the exact cause challenging.
The breakthrough came during a detailed data analysis session. While monitoring live engine data, a discrepancy in the low pressure fuel sensor readings was observed. At 2000 RPM, the fuel pressure was erratic, sometimes exceeding the Infiniti specified range and occasionally dropping below it. This inconsistent fuel delivery immediately raised suspicion about the low pressure fuel pump’s performance. While a failing fuel pump might seem like an obvious cause for lean codes, the intermittent nature of the pressure fluctuations explained the sporadic P0171 triggering and the fluctuating fuel trim values.
Further investigation revealed another contributing factor: the purge control valve. At idle, this valve was improperly registering a 15% open state when it should have been closed (0% at idle per Infiniti specifications). A malfunctioning purge control valve, stuck open or opening at inappropriate times, can introduce unmetered air into the intake manifold, mimicking a vacuum leak and contributing to a lean condition. Interestingly, this issue also was intermittent, not consistently triggering a specific purge valve error code but subtly exacerbating the lean condition.
Ultimately, the solution involved replacing both the low pressure fuel pump and the purge control and vent valves. Since these replacements, the Infiniti has been running smoothly, with fuel trims returning to normal ranges. Current Short Term Fuel Trim (STFT) readings are around 100-101 for both banks, and Long Term Fuel Trim (LTFT) values are a healthy 104-105. This case underscores that while vacuum leaks and MAF sensor issues are common causes for P0171 codes, a comprehensive diagnostic approach should also include evaluating the fuel delivery system, particularly the low pressure fuel pump, and the evaporative emission control system components like the purge control valve. For Infiniti owners facing a persistent P0171 code, considering these less obvious components can be crucial for an effective and lasting repair.
