1993 4.0 low idle, wavers up and down

[EHeaney]

Alright so as the title says, my truck idles pretty low once warmed up, and it wavers up and down about 100 rpm. It's especially bad with the A/C on, sometimes it drops under 500 rpm and the truck shudders a bit. Here's a vid, vertical vid warning, my bad:



Direct Link: https://www.youtube.com/watch?v=LWYOgOAcxPA

Manual transmission so no torque converter load. A few seconds in, I kick on the A/C and you can see how much it dips. Can't tell on the vid but when it's at it's lowest speed, the engine shudders. I revved it up and held it and it has no problem holding a certain speed with the throttle. Also it drives perfectly fine. Just seems to struggle at idle. Which is a problem because it's overheating on 100+ degree days when idling with the A/C on, and I think it's because there's not enough fan blowing/water flowing at such a low speed with the A/C load.

I replaced the IAC and PCV valve, and I've been searching for a vaccuum leak and can't find one, though I wouldn't be surprised one bit to find out I just missed it. I'm wondering what you guys think should be the next culprit.

I can't tell if this is a mechanical failure such as a vaccum leak or sticky valve somewhere or if it's more of an electrical/sensor failure, as in something is telling the IAC to open, close, open, close, open, etc., though this could also be caused by some mechanical failure I guess.

 

[RayInStl]

That's classic vacuum leak. Your A/C puts load on the engine, so it's not surprising that it gets worse. Your A/C controls are vacuum operated, also. I suggest getting a can of starting fluid and spraying likely leak spots (hoses, intake at the gaskets, brake booster, etc. If the engine revs, you found your leak.

Good luck!