99 F150 5.4L
125,000 miles
My idle rpm was bouncing up and down, and eventually it would die. No engine codes.
Replaced spark plugs for the first time. No change.
Cleaned MAF and throttle body. No change.
I held the throttle at a constant 1000-1200 rpms and eventually got an engine code. I think it was p1121; Throttle Position Sensor Inconsistent with Mass Air Flow Sensor.
Replaced the IAC... problem solved.

 

rid UR self of the K&N !

 

check out this tech on YouTube my truck was doing the same thing I did what he says here and the truck is running great, and shifting perfect. Let me know if this works for you.

 

This thread is over 3 years old, and the 99 did not have an electronic throttle body.

 

I was just reading hru this thread and just realized that this thread is olddddd! Just trying to do some research for my p1451 or p1452 codes.