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P0171 and P0174 Codes on 1999 Ford F150: Causes, Fixes, and Diagnostics for Lean Fuel Conditions

Model: 1999 Ford F150 Fault Code: P0174 Posted: 2010-08-31 22:23

I'm a DIY enthusiast with solid experience working on vehicles from 1985 to the present. I'm currently troubleshooting a 1999 Ford F150 (2WD, 5.4L V8) that has triggered OBD II trouble codes P0171 and P0174 — both indicating 'System Adaptive Fuel Trim Lean' for Bank 1 and Bank 2. I've already replaced the fuel filter and air filter, and cleaned the MAF sensor. While these steps often resolve such issues, they haven't helped here. Using a Genisys 2.0 diagnostic tool, I monitored all sensors feeding into the Powertrain Control Module (PCM). All sensors appear functional during operation, but several readings are abnormal: - The heated oxygen sensors initially read 0.0 volts (normal range: 0.2V to 0.9V), though they stabilize to normal values while driving. - The Manifold Absolute Pressure (MAP) sensor reads 25.83" Hg, which is significantly below the expected value of around 29.60–30.10" Hg based on current barometric pressure. My questions: - Has anyone else experienced these symptoms and what was the root cause? - Could a vacuum leak or clogged vacuum line be responsible? If so, how did you diagnose and fix it? - What are the most effective troubleshooting steps for P0171 and P0174 on this model? Thanks in advance for any insights — I'm committed to resolving this issue safely and effectively. — electra188

Related fault codes

P0171P0174

Comments (8)

Anonymous 2010-09-01 05:07

Vacuum leaks are the most common cause of P0171 and P0174 on a 1999 Ford F150. Look closely for cracked hoses, especially near the PCV valve or intake manifold gaskets. Intake bolts with deteriorated seals can also allow air to enter the system, causing lean conditions. If you have access to a Genisys tool, record the fuel trim values at idle and 2500 RPM — this helps determine whether the issue is due to vacuum leaks or actual fuel delivery problems.

Anonymous 2010-09-01 06:31

Thank you for the helpful advice! I’ll ask my daughter’s boyfriend to come over today and take the fuel trim readings. Appreciate your time and detailed response — I’ll post the results as soon as possible. — electra188

Anonymous 2010-09-01 19:42

The Fuel Pulse Width readings were inconsistent: at idle, values ranged from 3.3 to 5.0 milliseconds, and the engine idles roughly or stalls occasionally. At 2500 RPM, the readings stabilized at 4.3 ms (Fuel Pulse Width 1) and 4.5 ms (Fuel Pulse Width 2), with smooth engine operation. Other confusing data: - EGR Vacuum Regulator: 0% - EVAP Can Vent DC: 0% - Barometric Pressure: 25.78" Hg I suspect a vacuum issue, but as a DIYer, diagnosing these can be challenging. I plan to inspect the EGR system in the coming week or two. Given that the MAP reading is low, it may point to an intake leak — especially if air is being drawn into the engine. Thanks for your help! — electra188

Anonymous 2011-02-02 18:21

I recently posted about a similar issue in another thread: one of the first things to check is a plugged hole inside the throttle body. This can cause vacuum imbalances and trigger P0171/P0174. Reference: http://www.ford-trucks.com/forums/21254...-flow.html

Anonymous 2011-02-02 18:22

I had the same P0171 and P0174 codes on my 1999 F-150. After scanning with a Snap-On Modis, I checked O2 sensors, MAF, intake air flow, and fuel pressure — all within normal ranges. I opened the PCV valve and inspected plugs; everything looked fine. A Mode 6 test revealed a failed $41 code on component $12, which corresponds to a DPFE (Downstream Pressure Feedback) hose leak. This was previously linked to P0440 (insufficient EGR flow). After further research, I believe the root cause was inside the throttle body — something we initially overlooked. Reference: http://www.ford-trucks.com/forums/21254...-flow.html I also experienced hesitation between 1500–2000 RPM after clearing codes. The TPS voltage across RPM range may be sticking at that point, which could cause a lean condition and trigger the codes. That’s my next step if the codes return. This was a valuable lesson — I wish I’d known about DPFE hoses earlier; it cost me $300 in unnecessary repairs. Thanks for sharing! — [Anonymous User]

Anonymous 2011-08-10 21:30

If both P0171 and P0174 appear simultaneously, 'bad gas' is a strong possibility. Why? Because these codes are bank-specific — P0171 affects Bank 1 (cylinder #1 side), while P0174 affects Bank 2. Having both at the same time suggests air or fuel contamination in the tank. If only one of the codes appears, then typical fixes like cleaning the MAF or replacing the fuel filter may suffice. However, if both are present, it's likely due to contaminated fuel. Try adding a dry gas additive such as Heet (yellow bottle) or Iso-Heet (red bottle) and monitor whether the codes clear up after a few tank fills.

Anonymous 2011-08-19 14:31

Bad gas — I didn’t know this could be a consistent cause. This is an important point I hadn't considered before. Thanks for highlighting it!

Anonymous 2011-08-20 20:03

I recommend performing a smoke test to detect vacuum leaks, especially around the intake manifold. Do this when the engine is cold for best results. Also inspect all vacuum hoses — particularly the PCV hose. Additionally, please share both short- and long-term fuel trim values at idle and 2500 RPM (not just pulse width). The 1999 F-150 has isolator bolts that must be replaced during intake gasket work; these can also cause vacuum leaks if not properly torqued or replaced.