Thanks, Tom!

So as y'all know, I have dropping/fluctuating idle about 10% of the time. The mechanic told me he yesterday he strongly suspected vacuum leak, so I thoroughly investigated my vacuum lines today. I even went to the corner market, bought a cigar, and blew cigar smoke into my vacuum lines. Totally fine, no vacuum leaks that I could tell.

So I started thinking on the physics of the matter. I have a new Motorcraft IAC valve on there that I bought from RockAuto in July. It's installed correctly, the mechanic yesterday checked it. But then I thought "Hey wait...just cause it's new and it's Motorcraft doesn't mean it's working right!"

I got my original IAC valve out of the closet (I replaced it in July because I was starting to have some idle fluctuations), but instead of just putting it right back on, I decided to lubricate it because my new hypothesis is that NEITHER of these IAC valves are opening properly: not the old one, not the new one. I sprayed some WD-40 into the old IAC valve and put it on my truck. I also examined my throttle body and it was clean as a whistle and closing properly. Then I started the truck with my freshly lubricated original IAC valve on it. The truck immediately staggered and almost stalled due to the fumes from the WD-40 but after a few seconds it was fine and I went for a drive: My idle was smooth as butter! Totally stable. Even when I toggled the AC off and on, it didn't dip down or fluctuate at all, which is definitely a new experience.

So I'm tentatively going to say that many IAC valves are under-lubricated, and from what I can tell, my problem is now fixed.

If the idle dropping comes back, I'll eat my words on this, but from what I can tell, both my IAC valves were under-lubricated inside and thus couldn't open optimally. I have initiated a return with RockAuto to exchange my newer IAC valve for a new one. I'll put the new one on when it comes and we'll see how it performs but if it starts acting weird I'm going to lubricate it. Wish I could've gotten a refund but it's been too long, they'll only do a warranty exchange.

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