Hello, my 04 new body style F150 was giving me a P0171 Bank 1 Lean code a few months ago so I've been slowly diagnosing the problem but I just cannot get to the bottom of it. After about 2 months I pulled the codes again and it had a P0171, and P0174 for bank 2 lean, but I read that happens after having bank 1 lean for so long. I've had it for 4 years now with no other problems. It already had a K&N cold air intake and 3" banks single exhaust system when I bought it. After researching online I got a live data scanner and I think it is a vacuum leak somewhere because at idle the long term fuel trim stays at 26% at bank 1 and 24% at bank 2. When I rev it up to 2500rpm both go down about 8-10% each. I started with the easy things like replacing the PCV valve and hose with new motorcraft ones, not the problem. I cleaned the MAF sensor, no change. I checked for bad O2 sensors by spraying a little carb cleaner on air filter and watching the voltage, the response seems normal. I sprayed all around the engine looking for vacuum leaks and did not see a difference in 02 sensor voltage or engine reving up at any point. Fuel rail pressure is where it should be. Because it is due for inspection this month, I took a friends advice and reset the codes to see how long it takes for the CEL to come back on but it's been 190 miles already and no CEL, but it still saves a freeze frame because of Bank 1 too Lean. I've been at it for several months now with no luck so if anyone has any other ideas or tests for me to do, I would greatly appreciate it. Thanks in advance.

I forgot to mention, it has never idled rough or misfired while accelerating or anything like that. The only hiccups it has had is several times when I'm coming to a stop it jumps for a second almost as if I tapped the gas pedal while braking, but sometimes I can barely feel it. That has only happened on really hot days.

 

If you have a 3 valve 5.4, it's the K&N intake. Your MAF is not reporting correctly and it will run too lean. Either put the stock intake back on or get a custom tune for the K&N.

 

It's a 2 valve 4.6 liter V8. I've suspected the air intake being the problem but I can't find a stock air intake anywhere. Do you know of any way to test if that is the problem without having to swap it out?

 

If your truck is a Heritage model it has the 4.6 2 valve,.
If not it's a 3 valve.
The 171/174 codes are excess air in metered into the intake tract no matter what model you have..
You can verify that by looking at the long term fuel trim tables that will show they are shifted at least 25% setting the codes for both banks.
Good luck.

 

No, the 4.6 didn't go 3 valve in the F150 till 2009.

 

It is a 2 valve, and like I said in the original post the long term fuel trims are at 24% and 26% at idle but drop about 9% when rev'd up to 2500rpm. I finally found a stock intake and ordered it, should be here next tuesday... I'm hoping that solves it, if not at least remove that doubt.