2003 Ford F150 4.6L Engine P0172 & P0174 Codes: Diagnosis and Repair Questions After Multiple Dealer Visits
My 2003 Ford F15-0 with 89,000 miles has been experiencing severe performance issues. The engine idles smoothly in park but stalls or runs poorly when shifted into gear—typically failing to exceed 30 mph, with symptoms including stalling, misfires, pre-ignition, and noticeable valve noise. I took the truck to two Ford dealerships for diagnosis. Both reported P0172 (rich mixture bank 2) and P0174 (rich mixture bank 1), and recommended replacing coil packs 5, 7, and 8 at the first visit. I followed their advice—replaced those coils, installed new spark plugs, cleaned the MAF sensor, and reprogrammed the ECM. No improvement. At the second dealership, they suggested replacing coil pack 1 and adding new plugs and a fresh MAF sensor, stating that the ECU does not need reprogramming. I performed these changes myself (including replacing coils 1 through 4 and 6), used existing spark plugs, and installed a new Ford OEM MAF sensor—still no resolution. I've noticed in this forum that many users have reported issues related to fuel pressure, catalytic converters, oxygen sensors, and intake gaskets. However, neither dealership mentioned these components during their diagnosis. Given the high cost of service at Ford dealerships, I'm wondering whether such common underlying causes should be routinely considered—especially when standard diagnostics fail to resolve the issue. Is it reasonable to expect that a comprehensive diagnosis would include checking fuel pressure, intake gaskets, or exhaust backpressure? If not, does this suggest potential overcharging or misdiagnosis by dealerships?
The core question here is whether you were misled by the dealership's diagnosis or if the forum discussions about underlying issues like fuel pressure, intake gaskets, or vacuum leaks are accurate. We can't assess the accuracy of their diagnostics without knowing exactly what they performed. It sounds like you received code readings and general recommendations—then proceeded to carry out repairs yourself. While this approach may work sometimes, it's not ideal because a diagnostic trouble code (DTC) alone is never a complete diagnosis. Codes are patterns based on sensor data, not definitive root causes. When symptoms persist after repair, the issue is often not mechanical but rather related to system-level factors such as vacuum leaks or low fuel pressure. For example, if intake gaskets were faulty, you'd typically expect more severe performance issues—like rough idling or stalling at idle—not just a lack of power above 30 mph. That said, timing and exhaust backpressure should also be verified to rule out other potential causes. Fuel pressure testing is especially critical in a 4.6L V8 engine with P0172/P0174 codes—low fuel pressure can cause rich mixtures that trigger these codes. A proper diagnostic process should include these checks, and if not, it raises concerns about the thoroughness of the dealership's evaluation.