I have a 2004 F150 w/5.4L. Two days ago it belched and yesterday the check engine light came on. It's been running rough since, occasionally shutting off. Took it to Advance Auto for code check, he said it indicated it was running lean on both banks. Suggested it may be a vacuum line has come off. If this is possible, where and how do I check it?

 

This is currently happening to mine (04 FX4). Rough idle a few days ago, hesitation while accelerating, codes p0171 and p0174 - lean on both banks. Started after a heavy rain but that may be a coincidence. Things I've read it could be: vacuum leak, MAF sensor, dirty air and/or fuel filter, weak fuel pump, intake leak (gasket or manifold).
Mine ended up being a dirty MAF sensor, I cleaned it last night and it is back to running perfect today. I did have one hiccup after cleaning it would not start at all (first time ever) but I believe it may have flooded due to the LT fuel trims. I reset the PCM and went through flood procedures (start with pedal depressed fully) and it started on the next try. Been fine since.
What I do to check for vacuum leaks is spray electric contact cleaner around vacuum hoses and fittings while idleing and watching for a rise in RPM. Don't use ether or brake cleaner as it can degrade the hoses. Others here may have a better method but this works for me. Look up the procedure for cleaning your MAF sensor, it's not hard to do either. If those don't work try a new air and fuel filter. Get yourself a code reader also, you can check your own codes and monitor fuel trims and fuel pump PSI to help narrow the issue down if the common stuff doesn't fix it. They are pretty cheap now-a-days. Good luck.

 

Thanks, I'll give that a try. Will update whether this works or not.

 

There is a hard red line that runs behind the intake near the firewall. I use an inspection camera to see back there. It has been known to pop off and cause lean cond and rough idle. i have the 2 lean codes and have yet to figure it out. However it could be something else as suggested

 

I will definitely check that. Took the truck to AutoZone to get the codes read, they say the computer suggests that it may be the PCV. Unfortunately, on this truck, the PCV is part of one of the valve covers, not just "plug and play" (so to speak). Anyone know of a way to service this without having to replace the entire valve cover, should the problem indeed be the PCV?

The only red line I found runs from an electrical thing (guessing it's some kind of sensor) on the driver side fuel ramp, around the back of the motor, to the passenger side and terminates in what looks like some kind of plug. Any ideas?

 

It also goes down behind the motor as you said and i could only see it with my inspection camera. I believe that "plug" is a vapor unit of sorts. I actually replaced that too because rain gets on it and the new one has a lid. I'm not sure about the pcv and yes its in the valve cover so its hard to believe its that considering its not rubber. i believe you can undo that plastic red line at the fuel rail while running to see if theres vacuum by putting your finger up to the rubber end of that line

 

Also as tbc said it seems my lean codes have gone away for now after cleaning my MAF sensor. It wouldn't hurt to do the easier things like that and fuel filter etc. check air filter and foam around the plastic tray, its known for disintegrating and that would cause too much air to enter the housing. Simple fix with foam rubber from depot. All I can think of for now

 

Forgot to mention another thing that's known to go bad. FPDM. I believe its fuel pressure driver module. Its under the spare tire mounted and plugged in. It gets rusty and corroded and the electrical components inside get messed up. Not a hard fix and i believe is under $100

 

Check seal around the air filter. Over yrs it will deteriorate and will no longer seal properly.