VEHICLE: 2009 Ford F-150 XLT 4.6L, V8, MFI, SOHC, VIN 8, Eng Version Triton
Problem: The Malfunction Indicator Light (MIL) is illuminated and the Powertrain Control Module (PCM) set Keep Alive Memory (KAM) codes P0171 and P0174.
The scan tool is showing one Long Term Fuel Trim (LFT) value at (+)6% and the other at (+)129%.
The LFT value that is displayed at 129% is an illogical reading. Fuel trim is a closed loop adjustment to the air fuel ratio sensor input readings. These values go positive, in the case of a lean running condition, or negative, in the case of a rich running condition. The value itself is a percentage of adjustment one direction or the other away from the base calculation of fuel injector pulse width. The base calculation is made based on the input values of the open loop sensors such as engine RPM, Mass Air Flow (MAF) sensor input, Intake Air Temperature (IAT) sensor and Cylinder Head Temperature (CHT) sensor. The LFT maximum movement from 0% is 25% positive or negative, and the SFT maximum from 0% is 33%, positive or negative, on most scan tool displays.
2. For the PCM to set codes P0171 and/or P0174, the fuel trim (SFT or LFT) needs to reach its maximum positive limit. Any double digit value of SFT or LFT can be considered excessive correction for a lean running condition.
3. When both banks are showing lean operation at idle speed but the values decrease to single digit readings in an off-of-idle running condition, the suggestive cause is a vacuum leak concern.
4. If the fuel trim values are seen to be high in off-of-idle conditions, it would suggest a MAF sensor input or fuel delivery issue.
Check fuel pressure. This vehicle has a mechanical returnless fuel system. Fuel pressure should indicate a constant reading of 50-70 PSI.
5. If the MAF sensor input is suspect for cause, inspect the MAF sensing wire for contamination and clean it using a MAF sensor safe cleaning agent. Do not blow dry the sensing element or wipe it with a cloth or cotton swab. Touching the sending element could cause it damage. Inspect the air filter element and air cleaner assembly for a potential airflow diversion which could cause a sensing issue for the MAF and address as needed.
6. If the MAF sensing element is cleaned or the MAF is replaced, clear the PCM's adaptive memory and recheck the SFT and LFT readings on the scan tool during closed loop operation to determine if there is still indication of lean running.
Potential Causes: Vacuum Leak
Air Induction Leak
Mass Air Flow (MAF) Sensor
Confirmed Fix: Replaced Mass Air Flow (MAF) Sensor