03 Ford Expedition getting both P0171, P0174 and P0304

Hello, that PCV tube is a main cause of the lean codes (P0171 / P0174), but isn't the only possible cause. Since both banks are "lean", I would look for a vacuum leak somewhere. Sounds like you have resolved the P0304 code, so that is positive. You could also have a mass air flow sensor issue that affects both banks, but I would not guess at this, you'll need a diagnostic tool that is capable of giving you a data log and can show you what the MAF is reading -- specifically the BARO reading -- barometric pressure. Compare this reading to the relative reading expected at your particular altitude, and if it is way way off, then the MAF could be causing the lean condition. No guessing - test as needed and repair as a result of the findings.

1st thing to check for when you get P0171/P0174 together is for a vacuum leak. those codes have nothing to do with the plugs/COP's(coils)/egr components.check the PCV valve vacuum hose at the pcv valve,may have to trim back foam insulation to see the little hose(might be collapsed/split/turned to mush) then follow the pipe/tube to where it connects to the back of the intake(check the rubber boot/elbow)where it connects for splits/holes/cracks,most of it is a metal tube except for the two ends. listen for hissing sound(air sucking sound) run fingers around all vacuum hoses,you will know when you've found the leak(engine will smooth out).there are other possibilities but this is the main one to check.like I said,could be other,but check above 1st,also it could be a dirty MAF Sensor or bad MAF Sensor.post findings,I've been down this road with my Ford also....

Too much air - OR - not enough gas .. As stated - TESTING is definitely required.