I have a 2001 F-150 5.4L. I have a rough idle, but around 1200 RPM's it feels a pretty smooth. I pulled the codes and I am getting a P0171, P0174, P0457. I know that P0457 is for the fuel cap, but I believe that it is not the fuel cap but one of the solenoid valves for the EVAP system causing this problem.
Can the P0457 issue cause a rough idle?
Are P0171 and P0174 related to the P0457 issue?
I should also mention that I just replaced 2 coil packs. Could they have caused the P0171 and P0174 codes?

 

You probably have a vacuum leak. Check the PCV elbow behind the intake.

 

171 &174 are lean codes, most common cause on the 5.4 is the PCV elbow at the back of the intake.

However if you have a solenoid stuck open so the motor is sucking extra air then that MIGHT cause the lean condition.

 

I will check the PCV hose tomorrow. Thanks I will let y'all know what I find.

 

Okay, on Sunday I pulled the PCV hose apart. I cleaned and inspected it, including the elbow. Everything looks good there. I cleared the codes using my scanner since I just replaced those coil packs I wanted start fresh. Today the check engine light came back on. The only codes that are showing now is p0455 and p0457. So I guess my next step is to go through this EVAP system.

 

Start with a new Motorcraft (ONLY!) gas cap.