I have a '99 F150 5.4L 4x2 150k miles that idles too high. It's got me baffled. I was chasing a problem with pinging. I had covered the usual suspects such as cleaning the MAF and throttle body, put in a new EGR valve. I had a high idle problem before and cleaned the IAC and it fixed it. There was also an annoying whistle at near idle engine speeds, but I could never figure out where it was coming from. It sounded like a vacuum leak. I finally figured out it wasn't a leak at all, it was coming from the IAC hose that runs to the throttle body boot. With that and the fact that I had cleaned the IAC before, I put in a new IAC. So, now about 2 months later I have a high idle problem again. I have a data logging scanner to check stuff with.
Here's the symptoms:
Idle in Park - 980 RPM, should be 700 and it doesn't seem to surge
Idle in Drive - 670 RPM, should be 600 and it doesn't surge

Here's some of the readings I got from the scanner:
MAF - 8.05 Gm's, should be 5.10
DPFE - 1.64V, should be 1.52V
TPS - 0.95V
IAT - 62F (it was about 64F outside at the time)
Fuel Injector PW banks 1 & 2 - 5.1

Here's what I've done so far:
-Checked for DTC codes = no codes stored
-Cleaned the IAC = restarted motor no change
-unplugged the IAC = idle drops way down then stabilizes at normal (700)
=IAC fault code
-Put the original IAC I had kept back in and reset PCM = restarted motor no change
-Put the new IAC back in = restarted motor no change
-checked and cleaned PCV and hoses (PCV work fine) = restarted motor no change
-Checked TPS = 0.95v at idle 4.67v at WOT
-Pulled TPS plug = restarted motor no change
= TPS fault code
-Put plug back on TPS, reset PCM = restarted motor no change
-Checked the MAF, seems to have good output on the scanner
-Disconnected MAF = stumbled for a second, than ran as before,
= MAF fault code
-Reconnected MAF, reset PCM
-Checked for binding in throttle assy and cables. All looks perfect.
-Removed throttle body boot and cleaned throttle body and butterfly.
-Restarted motor pulled vacuum line from EGR = no change
-Put vacuum line back on EGR = no change

Take a step back and make sure everything is connected and normal.
restarted motor and the damn thing idles right. I watched the desired RPM and actual RPM on the scanner and the actual RPM followed the desired RPM within about 10 RPM as it incremented down the final desired idle of 688 RPM in Park and down to 592 in Drive. This lasted for about 5 mins then the idle creeped back up to 980 in Park.

-Unplugged the IAC (the new one)= no change in Idle. It didn't drop like before
-Replaced IAC with the old one = no change in idle
-Disconnected IAC = no change

I haven't found any vacuum leaks yet.

So, I'm pulling my hair out. Any ideas?

 

Excessive fuel? Could be too much fuel and rising RPMs that way? (just a thought...you have already done so much, I would be pulling hair too).

 

For anyone interested, I found the problem. I was having a hard time finding it because it was sometimes intermittent. As it turns out, it was the PCV valve after all.
The PCV valve was clean, unobstructed, and the internal poppet inside moved around inside as it should. The problem was that there was enough wear inside of it that the internal poppet would sometimes not seat and allow excessive airflow causing the high idle condition. I played with it for a while and once I figured out what was going on, I could replicate the failure quite easily.

$3.67 fix.

 

Thanks for letting us know what fixed it
PCV problems are easy to overlook because they are pretty rare.