I have a 99 expedition with the 5.4L. It idles rough and the check engine light is on. When I hooked up my scanner it gave me the P0171 system 1 too lean code. While checking the run time data, I found that the long term fuel trim 1 never drops below 23%, while both short terms and the long term for 2 never go above 10%. Does anyone know what might cause these issues?

 

Yes. Bank 1 (passenger side bank) is running lean. This is usually caused by a vacuum leak that only effects one side of the engine, like an injector o-ring, maybe an intake manifold port gasket leak, (or maybe an injector that isn't flowing well, but then you should also be getting a P030x cylinder misfire DTC.) (An exhaust leak on the bank 1 side ahead of the HO2S can also cause this as in between exhaust pulse leaks the exhaust can suck in oxygen which the HO2S will interpert as being too lean.)

A vacuum leak after the MAF or one of the vacuum hoses usually effects both banks and will set the P0171 AND the P0174 DTCs.

The PCM looks at LTFT + STFT and if the total is ~30% or more it will set the P0171/P0174 code for Bank 1 and/or 2. If you had the engine at idle when you looked at the LTFT you most likely had a vacuum leak. A vacuum leak has a significant effect at idle speed while a fuel delivery issue is less of an issue at idle.

Also run the engine up to ~2500 RPM and look at the fuel trims again. At the higher rpm if the LTFT reads high you more than likely have a fuel delivery problem as a vacuum leak has minimal effect at higher RPMs. (Has the fuel filter ever been serviced?).

If your scan tool will let you look at the freeze frame data, that will show you what was going on when the PCM decide it was time to set the P0171 code.

So to make the story shorter, your looking for something that can only effect one side of the engine.