A Ford F-150 presented with a rough idle and a check engine light (CEL). Initial diagnostics revealed a P0175 code (System Too Rich Bank 2) and significantly different fuel trims between Bank 1 and Bank 2. Bank 2 showed a -25% short term fuel trim (STFT) and Bank 1 a +25% STFT, indicating a rich condition in Bank 2 and a lean condition in Bank 1.
Several components were checked and replaced, including a weak spark coil and spark plugs on cylinder 6, a broken vacuum line, and a small intake leak. The Bank 2 fuel trim corrected itself, but Bank 1 persisted with a positive (lean) fuel trim. Even moving the injectors from Bank 1 to Bank 2 didn’t resolve the issue.
A compression test revealed an anomaly in cylinder 1, initially showing 250 psi before dropping back down to a normal 180 psi. Suspecting a valve issue, the exhaust valve lifter was replaced, but the lean condition remained. The engine ran smoothly until reaching operating temperature (170°F), at which point the rough idle and positive fuel trim returned.
Further tests included checking backpressure (within normal range), probing the Bank 1 sensor 1 oxygen sensor (B1S1) signal wire (matching the data stream), and substituting a known good oxygen sensor. No exhaust leaks were found. Common potential causes like PCV, purge valve, brake booster, EGR, and fuel pressure regulator were also checked and ruled out.
While the engine runs smoothly and has power, the P0171 code (System Too Lean Bank 1) and rough idle, especially noticeable in gear, persist. One observation is the difference in upstream oxygen sensor voltage ranges between banks. Bank 1 ranges from .2-.8V while Bank 2 ranges from .2-.6V. This could indicate the computer is compensating for a richer condition in Bank 1, possibly due to engine protection logic or overcompensation.
The question remains: what’s causing the persistent lean condition in Bank 1? Could an Autel scanner provide more granular data on rear fuel trims and other parameters to pinpoint the root cause? Further diagnosis, potentially including a hot compression test and timing chain inspection, is necessary. An Autel scanner’s capabilities, particularly in analyzing live data and specific Ford parameters, could be invaluable in this situation. Using an Autel device, technicians can monitor long term fuel trims (LTFT), short term fuel trims (STFT), O2 sensor readings, MAF sensor data, and other critical engine parameters in real time. This allows for a comprehensive analysis of the fuel system’s performance and can help identify subtle issues that might be missed with traditional diagnostic methods. Autel’s comprehensive diagnostic software may also offer specific guided tests or troubleshooting procedures tailored to Ford vehicles, potentially leading to a quicker and more accurate diagnosis of the lean condition.
