2001 Screw 5.4 all stock ( except for Flowmaster ) and formerly a K&N...Removed to diagnose Idle prob. Seems to happen when it is raining, or has just rained, but idle drops out, or just will not idle at all, even turning the steering wheel will kill the engine after cold start up. Tempurature seems to be irrelevant. I replaced MAF, installed paper Motorcraft Filter, cleaned throttle plate and air intake. I CAN NOT FIGURE it out. No Codes, No CK engine light.

 

Sounds like an idle Air control motor that is stuck or not workink corectly. The iac is held on by 2 small bolts 5/16 socket will work. remove teh valve and try cleaning it with choke cleaner or brake parts cleaner. If this does not help replace it, not that expensive of a part. Hope this helps, also chec eletrical conection to the Iac.

 

Before you go bustin your knuckles on the IAC.

Next time the idle acts up tap the IAC with a screwdriver handle. If the idle pops back into shape you have a dirty/bad IAC. Very very common on your year and the 5.4.

I had the same problem and decided to clean it instead of replacing it. So far so good and it has been close to 10k miles since the cleaning.

Do a search on "5.4 IAC"

 

I cleaned the IAC, and all is well for the past week. THANKS SO MUCH!