I have had a check engine light on for as long as I can remember for these codes. Only in the last few weeks have I noticed a big decrease in power. I just bought a scanner to try to pinpoint the problem. The long term trims are maxed out or close to it most of the time ( freeze frame data of the codes shows LTFT only at 10%).
I replaced to PVC lines that were in bad shape, the elbow the drivers side and another large hose going to the TB. This fixed a surging problem I was having. The hose coming of the back of the TB looks in good shape, possibly replaced by P.O. Replaced TPS and cleaned the MAF (actually found a bug on one of the elements ) and switch back to a paper filter from the K&N that was in there. I have 2 O2 sensors sitting new in the box, but after reading some other posts they suggest that the O2 sensors are not the problem. Im running out of ideas and wondering what else my scanner data can tell me??
99 F250 5.4

 

Those codes almost always mean you have a vacuum leak. It could be anywhere between the MAF and cylinders, possibly even an intake gasket. I know that many people overlook the hose and grommet to the brake vacuum booster.
After each thing you "fix", clear the codes to see if that was it.

 

After repairs clear the codes and see what comes back.
If the 171/174 returns look at the LTFT both banks again.
If shifted, you still have a leak.
It's not the OX sensors both banks because they are only reporting the problem by detecting to much Ox left in the exhaust after combustion.
What that causes is the program to 'richen' up the fuel.
This is the shifted tables you see that actually triggers the codes..
This is what they do to control fuel injection and the first detection report for issues out of limits, by there detection ability.
The shifted tables are the resullt that set code and turn on CEL..
Good luck..

 

I had chronic P0171 and P0174 issues for a long time. Finally made a fuel pressure gage rig that I could watch while driving. Turned out to be the fuel pressure dropped only at high loads. Replaced the fuel pump. Problem solved.

 

my 171- 174 problems were caused by the two small 1/4 inch black hoses on the EGR system above the left valve cover........ each about 8 inches long...old/ hard/ cracked.

 

Ive replaced mine a year or so ago after I had a warm starting issue. I dropped the tank...I will never EVER do that again. Pulling the bed next time.

 

Hoping I will get some time in the next few days to do the carb cleaner test, or the cigar test. Ive had the codes for a long time but it didnt effect driveability until recently when i've lost power and my fuel economy has gone down.