My 1998 Ford Expedition with a 5.4-liter engine is showing an OBD2 diagnostic code P0171, indicating a lean air-fuel mixture. The vehicle feels sluggish during acceleration and seems to lose power when speeding up. I'm looking for the most likely cause of this code and practical solutions that don't require expensive repairs. Any insights from owners with similar issues?
I now have both P0171 and P1131 codes, with a brief appearance of P0133 that has since disappeared. The engine knocking is worse, and I can only maintain speed up to 35-40 mph. I've already checked for vacuum leaks, replaced the oxygen sensor, added an engine treatment, but performance hasn't improved. I'm out of money and can't afford trial-and-error fixes. Any reliable diagnostics or affordable solutions for this P0171 issue on a 1998 Expedition?
Same diagnosis — vacuum leaks or fuel system issues are the top suspects for P0171 in this model year and engine size.
A vacuum leak or low fuel pressure is one of the most common causes of P0171 in a 1998 Ford Expedition. Check all hoses and connections, especially around the intake manifold and throttle body.