I first posted this on the site in an area for troubleshooting and didn't get any replies. I missed this section of the site when first encountering the whole thing...my bad I think they say nowadays.

The problem vehicle: 2003 F-150, 37500 mi., 4.6L V-8, 4 x 4

The problem: Just a couple days ago, my truck began idleing erratically. It had run just fine up until that morning. I've had no complaints, no codes, and no symptoms until this problem just arose on Tuesday. Two codes came up when I read those. They were the P0171 and P0174. I have now changed the Throttle Position Sensor thinking that may have gone bad but it didn't affect the problem in the least. I am next considering replacing the Idle Air Controller to see if that helps. I hate just trying the "plug and play" process and am hoping someone on here might have guidance about a troubleshooting protocol or other useful suggestions. I have also now ordered the Auto Tap software program and bought the enhanced Ford codes with it. I haven't yet gotten it delivered but it should be here in the next day or so. Any suggestions what I should look for using that diagnostic tool? I was thinking of monitoring the fuel pump, the fuel injectors, and any other circuits I can think of that might affect fuel delivery or idle settings and performance. I plan to keep this truck for a long time and I want to learn how to maintain it myself. I have plenty of time now that I am retired, I have a wealth of tools, and a strong desire to know what I'm doing. All I lack right now is a good source of knowledge that I can tap to shortcut the learning process somewhat...at least speed things up a bit. If anyone can help, I will certainly appreciate it. If not, I thank everyone who bothers to read what I posted and also thank you for all that you do for a wide ranging public, as well. It is clear that happens regularly!! Great job! Kevin

 

p0171 and p0174 are codes for the engine running lean on both banks. Most common cause that I have seen is the rubber hoses near the pcv valve. See if they are cracked, broken, or somehow not connected. Also won't hurt to check all other vacuum lines on engine. If this does not help check to see if you have a dirty MAF (Mass airflow Sensor).

 

Bank 1 and 2 lean are what those two codes mean. Check the 90 degree rubber elbow in the PVC hose at the back of the intake manifold. The hose tends to rot out and results in a vacuum leak. You may also try cleaning the MAF sensor too. Good luck!

 

It's like deja vu

 

Yes, you beat me to it. Caught me

 

I have the exact same truck with exact same problem. Found PCV Elbow and removed it. It had a tear in it. I Decided to replace PCV Valve while I had it off. Is there a trick to pulling out this valve. Its in a very tight spot so its hard to get my hands around. Any suggestions?

 

Same problem here, found it fast though.
Sudden onset but there has been poor performance for a while.

I removed the upper intake elbow assy to get at it.


And how the elbow looked once off.
The rubber was soft like acid ate at it. It was very porous.

 

Would this also cause the idle to bounce around when the truck is warmed up. Because that is when I am having trouble. I already replaced the IAC and it didnt help. Can you get one of them at the local parts store or is it a dealer item?. Thanks for your help.

 

Definitely. My idle was all over the chart and it stalled at the lights if I did not keep a foot on the throttle.
As for a replacement, I would think they'd be easy to find in some fashion.
I actually used the elbow off the other crankcase vent line from the driver's side valve cover that is attached at the intake tube in front of the throttle body. I had made my own 3.5" cold air intake and that elbow was now free to use so since it basically had the same function I used it. Worked fine.
So if you can find one like that you should be golden.