I have a 2005 with a rebuilt 5.4L installed last fall and I have 8k miles on it so far. Last spring, I noticed that the motor was running a little rough at start up. When I start my truck, rpms are at about 11k and it is running smooth. When the rpms drop to around 800-900 rpm, the motor is not running as smooth and almost feels like it is shaking a bit. The roughness went away as the motor warmed up, usually after a couple of minutes. The last couple of weeks now, I am starting to feel that roughness when I am sitting at a light or stop sign, but not every time, even when the engine is warm. The tachometer is always steady with hardly any fluctuation during the roughness. I feel like the motor sounds good and runs well otherwise, I have plenty of power and great acceleration, and it feels pretty peppy. I have no pinging or knocking going on and it has never stalled for felt like it would. The rebuilt motor came with new spark plugs and I have cleaned the throttle body, changed air filter, cleaned the MAF. I have been doing a lot of reading here, and it sounds like the IAC, so I am thinking about replacing the throttle body, what do you guys think?

 

Don’t replace the throttle body yet. Start with the IAC valve (if equipped), look for vacuum leaks, and check mounts. These are common causes of rough idle in a 5.4L and are much cheaper and easier to deal with first.

 

I would agree, I don't see why the throttle body would be doing anything weird at idle. Maybe check the foam on the air filter box, and maybe monitor your valve timing make sure there's no errors at idle. That's why I finally did my timing job one of the phasers wouldnt come back to zero it would stick at 20 degrees and it made the idle rough.
And check all your vacuum hoses around the valve covers make sure they are snapped onto their fittings.

 

As I understand it, the IAC is integrated in the throttle body and there is no way to clean or service it. I have done some checking for vacuum leaks and I did not hear anything. What is the best way to check for vacuum leaks?

 

Have you cleaned the throttle body & MAF sensor recently? If not then I'd start there. Then do a pedal re-learn - courtesy of Diamondbird:

 

If you've done all of this, I would replace the throttle body too:

• scanned for OBD codes but didn't get any related to the problem,
• done the idle relearn and that didn't work,
• made sure that the throttle plate and where it contacts the throttle body are carbon free,
• inspected the wires from the PCM to throttle body, and
• inspected the pins inside the throttle body wire connector.

 

Thanks, I will try those

 

Any luck with you idle issue? I'm experiencing the same thing on my 2008 f150.

 

After checking out all the suggestions above, I ended up replacing the throttle body and it didn't really change anything. I did the throttle body relearn a couple of times, still not much improvement, and then I noticed that the start up rpms were shooting up to1800-1900 rpms. Took my truck in to a Ford dealer and they did a full diagnostic and did not turn anything up. Had them re-flash the PCM, which they were afraid to do, they fried it and replaced it with another one.
The high startup RPM went away, and the idle smoothed out back to normal. It has been good for 2 months now.

 

Thanks I'm going to do the same