Well a few weeks ago when my pickup (99 w/5.4) was in the shop getting the pinion seal replaced they said it was missing. Pulled up a code P0171 even though the CEL light wasn't on. They replaced the PCV valve/hose/plastic tube assembly. I couldn't tell any difference, but it seemed to idle not real rough, but not smooth either.

After doing alot of reading I decided to clean the MAF, IAC and throttle body. I took it all apart and cleaned everything. The MAF was easy. The throttle body wasn't all that hard. Just watch out for the cooling hose on the the passenger side of the TB. Thought that was a vaccum hose and was VERY surprised when antifreeze started coming out.

I didn't like the action of the IAC after cleaning so I bought a new one. $60 at Autozone Put it all back together reconnected the battery and no problems, but I can't tell any difference in the way it idles. However the throttle response seems alot better and smoother. Guessing that's because of the MAF. Also found a new product by CRC specifically made for cleaning mass air flow sensors. The kid at O'Reilly's told me to use CRC carb cleaner!