Hello, this is my first time on here. I have recently purchased a 2001 f150 supercrew 5.4L . (About month ago) Well to make a long story short... I drove for a week It was fine, but the I went to pull out one day and It started missing very badly. Then the miss went away. No trouble codes or nothing. I had new plugs put in, but the truck still shakes (like a slight miss) when at an idle (550 to 1500 RPMs) Everyone else thinks I'm crazy. I had water in #7 boot unber the coil. That put the light on... fixed that hoping it was the cause of my underlying complaint (poor idle). Not at all... This idle thing started after the first time I had a miss, and has been getting worse. I have checked for vacuum leakes cleaned the MAF and checked the coils. Also a new fuel filter. I even changed where I purchase gas. This is an extremely clean truck... (No rust, stickers still on the frame and axles) and I live in the salt belt in PA.
Please, anymore advice is appreciated... Thanks!

 

If it's ONLY happening at idle, I'd guess the idle air controller (IAC). Usually when they stick it can or almost dies when sitting at idle, shaking and shuddering as it tries to stay running.

 

I found out (by accident) that a O2 sensor not reading correctly can affect Idle quality. I had a Secondary O2 sensor on my 04 go bad, I didn't change it for 20k miles, Truck ran fine, but the idle was rough. Thought about trading the truck, dealer wanted the SES light off, I said I'd change it, knew what the problem was. Well, long story shorter, did change the o2, did not trade my beloved truck. and now, my Idle is smooth as silk.

Just my $.02

 

I'm having a problem with idle also. Idles fine in park @ about 1k rpm but when in gear (brake on) drops to about 500rpm and sometimes stalls when makeing a slow turn. 245k miles 4.6l and replaced the IAC at about 145k miles with a autozone one. IAC again?

 

Advice for both: Try removing the IAC (back of the intake) and giving it a good cleaning with Carb Cleaner.
These do tend to get a build up over time. Cleaning is much cheaper than replacing!