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As you were originally chasing basic idle problems, let me start by saying that simply doing a thorough PCV system and throttle body cleaning (remove for proper cleaning) followed by an IAC valve rinse and a check for loose or split air intake/vacuum hoses works wonders in many cases. If the engine is suddenly running noticeably worse after your cleanings then something has definitely gone out of whack (as opposed to in whack?). The following are general notes that I've edited to address your problems.
When you get idle swinging way up above 1500 rpm into the 2500-3500+ range, the idle control system simply may not be active. The IAC valve would remain at the rest (semi-closed) position. This can happen with a disconnected, misadjusted or stuck TPS throttle position switch (the TPS is on the back side of the throttle body). If the TPS is dirty or the throttle plate is misaligned/dirty and not quite fully closed then a few solid taps will sometimes close the contacts and signal the ECU to control idle. TPS operation can be verified in the diagnostic test mentioned below. If there's any chance that random adjustments have been made to the throttle area then the throttle body, the TPS and throttle linkage should be properly readjusted. Make sure the electrical connector on the TPS has clean contacts and the connector is fully seated (do the IAC valve and AMM connectors as well). It's a good idea to occasionally use spray contact cleaner followed by packing the female contacts with dielectric grease to seal out moisture.
It's very simple to use the OBD diagnostic system to diagnose the TPS. If you haven't played with the OBD socket before, now's a good time to learn. In the 700/900 FAQ feature section, under "Engine:OBD Diagnostic Codes", there are some basic "Fuel Injection and Ignition Diagnostic Procedures" described. The Mode 2 sensor diagnostic can be used to detect a faulty TPS signal. You should be able to hear the TPS click just slightly after the throttle is raised from the closed position and additionally just slightly after it is closed from the full open (WOT) position. You should get the appropriate blinking LED code right after the click. You can repeat opening and closing the throttle as often as you want.
If it's not the TPS, then here's one way to spot a sticky IAC valve. When the engine is idling erratically, rapidly tap the IAC valve body with the end of a stick. If the idle smooths out and/or returns toward normal then a sticky/worn IAC valve is the culprit.
Idle problems can be related to a vacuum or air intake hose leak. Unmetered air (any air entering the intake system after the AMM) will usually only cause rough idling and poor running, not a high or wildly fluctuating idle. To distinguish a sticky/fluctuating IAC valve from a vacuum leak, remove the IAC valve electrical connector (or pinch off the air supply through the IAC valve) -the revs should drop to around 450-550 rpm (the engine may also die). If the revs stay up, and presuming it's not the TPS, then go looking for vacuum leaks.
Now, after all that prelude, you were able to block off some input air and get a steady cold idle. From an arm chair position, that certainly makes the IAC valve the prime suspect rather than the TPS, but I don't usually associate a bad IAC valve with idle in the 2000+ rpm range, more just like 1300-1800. Be that as it may, if it is a sticky/worn IAC valve then the most likely explanation would be as follows. The IAC valve can operate smoothly in a more open position as the ECU was trying to maintain idle when you reduced the main intake air source. If you have stable warm idle then the IAC valve can operate smoothly in a more closed position. In the middle there is likely a region with a contact wear groove in the commutator with its trailing edge right where the ECU is normally trying to operate the IAC valve at cold idle. Your wild rpm symptoms would mean the IAC valve is fairly badly worn at that point. The other possibility is that the IAC valve return spring is broken and the valve is simply out of control. You should have been able to notice that when you were cleaning the valve.
Unfortunately, your style of IAC valve cannot be cracked open for DIY repair. Do inspect the valve to make sure there isn't anything accidentally stuck inside and that the piston edges haven't been damaged by someone with a screwdriver accidentally trying to force a closed valve in the wrong direction (the rest position leaves an air gap beyond the closed position that fools some people). Open the valve with something non-scuffing (like popsicle sticks) and release it to see that it freely returns.
You can attempt to resurrect a worn IAC valve, but yours may be too far gone. I have had some luck with this, but not with a valve that misbehaved as badly as you describe. This uses the OBD diagnostic system to exercise the IAC valve by forcing it through full travel. This can sometimes work sticky bushings free and smooth out the ends of a commutator contact wear groove. I do this after a thorough cleaning. As I'm usually getting desparate, I'll often spray in a little WD-40 to get at the motor bushing, but remember to flush out any residue later if you do this. Again, the procedure is described in the FAQ under "Engine:OBD Diagnostic Codes" / "Fuel Injection and Ignition Diagnostic Procedures". As part of the Mode 3 exercise test, the IAC valve is forced through its full range a number of times. Let the Mode 3 exercise test run through quite a few cycles to give the IAC valve a good workout. For the IAC valve, you should hear a good "clock-clock" sound and feel a fairly solid vibration when you place your hand on it. Hopefully this full travel test will help unstick the IAC valve. Might as well do the Mode 4 exercise test while you're at it.
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day
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