Alright, I've been having this issue since I put in the replacement parts to make the intake work the way it's supposed to. What happens is, when I first start the engine, the RPM's go WAY up (about 3k) and hold there for a few sec before the drop so low the engine almost stalls out. Same happens if I release the clutch coming to a stop, but the engine speed only goes up about 500 RPM or so. If I close the IAC (using the recommended method) and adjust the min air rate to 700 RPM (yeah, I know it's too high for an MT, but that's where I normally keep it so I don't have to screw with it in the wintertime), then the idle is WAY too high at hot idle (around 1200 RPM or so). The only way to prevent this is to put a piece of vac hose into the IAC port inside the TB, which is a real PITA, and doesn't really help all that much. However, if I close the IAC and unplug it, everything is peachy. I am running 17" hg vacuum at idle, even with EVERYTHING unlugged from the vac ports (including the brake booster) and the ports plugged up. I have searched for vac leaks, even around the injectors, and found NONE. Am I ever going to get this thing to idle properly, or is it a lost cause? Am I okay with running an 700R4 PROM with my T5, cause that's all I have (HNR, APFR, and some other one I'm running now I can't recall)? I have a really long crank time before the engine lights off when cold, and nothing has seemed to help. I am stuck using the 3.1 plenum, as opposed to my 2.8 plenum, since that one has a bad TB mounting face, and the 2.8 unit worked a LOT better...

 

You mean to say that there is NO ONE on here who can even HINT as to why my IAC has NOT worked properly since the 3.1 was dropped in 3 years ago, even though I have gone through 2 ECMs, 2 brand new IAC motors and a newer style pigtail, intake repairs, and other things?

 

Faulty EGR valve? That's what was causing my mystery idle problems a few months ago.

 

Bad EGR would cause stalling. My EGR works like it should (it better, I spent $50 on a NEW solenoid!).

Will I pass inspection, in NY, just leaving the damn thing (the IAC) unplugged? The engine runs sooo much better without the thing plugged in and being open.

 

I think a bad egr could also cause a high idle, since it closes to create vaccuum correct? Sometimes when I have a high idle - around 2200, if I pop the hood and use a socket wrench to press up on the diaphram, my idle will drop to normal. But it won't stay once I let go.

 

That's cause your engine isn't running right and the EGR is SUPPOSED to drop the engine speed. The EGR DOES NOT open except when the throttle plate is open (ported vacuum source only works when throttle plate is open).

I've now replaced the lower injector o-rings, replaced the rear fuel rail bolt (the old one was stripped and wouldn't stay in the hole), disconnected EVERYTHING from the engine and tested the idle vacuum to be about 17" Hg, sprayed carb cleaner around to look for vac leaks and found none, and am about to go to my fav j/y and pull a new TB bellows from a newly wrecked 91 I am slowly pieceing out for my car, so I can upgrade to a freer-flowing PVC pipe unit (that hopefully won't leak like my busted a** air box right now does). The engine STILL won't use the IAC if I plug it in. It's fine in the driveway, but as soon as the VSS reads speed, forget having a decent idle speed (read as: less than 1500-2500 RPM) for a few seconds before dropping to normal.
Anyone know where I can get that elbow that is in the PCV port on the passenger's side valve cover? My original somehow broke and I replaced it with one made for a V8 that I bought at a parts house, and this one won't stay in the valve cover!

 

Update:
Now, the thing likes to backfire on me when I hit the gas from a cold start, but NOT when hot. This may be due to the IAC being disconnected (I don't know why that would be, I ran without any sort of idle air control or inlet for TWO YEARS without this problem). I know I need to change the fuel filter, that's a given. I haven't had a chance to test the CTS when cold, but when I tested the IAT the other day, it was off by a few ohms (don't remember how many, exactly, but it was enough to be noticable). I have a spare CTS I can use to maybe see if that helps or not, but I would have to go and buy another IAT to replace that. Resetting the ECM doesn't help. I don't have any codes set. And also, when cold, the car will stall when first started if I don't run the engine speed up to 1200-1400 RPM and hold it for a few seconds. This almost seems like a bad MAF to me, except for the fact that I'm not coding and the MAF looked good the last time I looked at it (film wasn't warped), and it's also just over a year old, IIRC.
Any suggestions? I'll have to look up the warranty on the MAF, I think it's still good, so I won't have to spend the $80 to replace that AGAIN...

:edit: Found receipt for MAF, it's got a lifetime warranty, so I can go out and bag myself another one if it's recommended...