Q: Code p0171 along with p0301 & p0304. New upstream o2 sensor, exhaust manifolds, fuel filter, pcv valve, no vacuum leaks

asked by on December 26, 2017

I recently replaced the intake gaskets, exhaust manifolds, pcv valve( hose and elbow looked ok), cleaned maf sensor. After the intake gaskets were replaced I developed a stutter between 40 and 55 mph. I now have code p0171, I also changed the upstream sensor on bank 1. This has turned into misfires on cylinders 1 & 4. I have the p0301 and p0304. What can I do? The misfires seem to start after it has warmed up. This is a 2000 f150 with the 5.4.

My car has 276000 miles.
My car has an automatic transmission.

A number of things may cause code P0171 and P0174 to be triggered. As the codes indicate, this means there is a lean fuel condition which may be caused by a lack of fuel supply or in some cases, a vacuum leak. These codes are also often times associated with misfire codes as well. When a vacuum leak is present, this introduces an excessive amount of air into the fuel/air mixture which results in leaning out the fuel content. Other problems such as a faulty fuel pressure regulator, a faulty fuel pump, a clogged fuel filter, faulty fuel injectors, faulty oxygen sensors, or a faulty mass air flow sensor may also cause this to happen. I would suggest having a professional from YourMechanic come to your location to diagnose and inspect your vehicle.

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