Here'* a thought. The computer has no idea where the IAC position really is, it only knows the signals it sends it. Correct me if I'm wrong but the computer closes the IAC all the way when the car is shut off and 'knows' this when its restarted so it at least has that much of a reference. It knows "Its closed, if I send it 10 pulses like normal the idle should be 1000rpm." Well if when you installed it it wasen't completely closed (fully extended) it'll think it is, send it 10 pulses like normal, and low and behold its idling at 1500rpm. I'm not sure how much the computer can adapt to that but if it can't figure it out it will continue. It'll send 10pulses on startup and 10 pulses on shutdown and never fully close it.

I was thinking about that the other day when I pulled mine to clean it...just a thought though

A very good thought. Although I don't know how it specifically works, I think it might go something like this:

OK, were gonna start... so I'll move this thingy out 10 pulses...
Varoooooom.
Hey. WTF... I count 1500 RPMs so I'll move in a bit...
OK, it'* at 1200. I think I'll move it in a little bit more...
Ahhhhh. I've reached my preset idle speed.

I don't think it should be a physical reference but a calculated one. It seems that, by some earlier posts, the PCM can adjust to changing conditions.

Can a PCM really say WTF????

Yes mines said it a number of times

Good thought but think about this:

The computer 'KNOWS' the IAC is closed on startup
It pulses it 10 to open it how far it thinks it needs for 1000rpm
It gets say 1700rpm says "oh sheit" and closes it some
1600rpm, closes more
1500rpm, its taken those 10 pulses back and 'KNOWS' its closed
computer: "Ahh dumb driver must have his foot on the pedal" or "oh damn there'* a hole in the (dirty) throttle body" or any number of other 'excuses.'


Now the adaptive part would be if it can figure out that those aren't the problem and figure out that the IAC isn't all the way closed. BUT the computer needs to make assumptions based on what it 'knows' and if it 'knows' the IAC is closed it won't question it. I think it'll just assume its a problem elsewhere.

I think this is somewhat proven by any rough or high idle caused by any air leak other than the IAC. If you drill a small hold in the TB valve the PCM COULD close the IAC more to compensate but it probably won't, it'll just sit confused on whats going on. Same thing with dirty TB'* causing a rough idle, the IAC could, in theory, compensate but it doesn't. Whats your thoughts?

mine likes to say FU.. Too bad our ECU'* can't compensate for everything. I guess that'* part of life, sometimes I like a stupid ECU, that way you can screw with its mind, and at least mine doesn't retard the timing with brake torquing!


-justin

I think the other sensors such as the TPS could alert the PCM to another source of the high idle. But in the case of a vacuum leak, you're right. There is no compensation and the car continues to idle high.

So the question still lingers: IAC movement based on physical calulations or practical calulations. I'm lacking the gold stars to say one way or another.

Well lets figure it out from what we know. We know the PCM has no idea where the IAC actually is, it only knows it sent it a signal to move x amount. We know the PCM won't intentionally damage a sensor. We know the PCM isn't extremely smart and must make assumptions based on its own actions.

So with that I would think its safe to say if the IAC isn't all the way closed not only will the computer not know it but it won't try to force it closed because it thinks its already fully closed. It isn't going to potentially damage the motor by trying to force it closed further.

Now the TPS does tell the computer the throttle position, thats why if you use a scantool while you drive you'll notice the IAC moving around to adjust. But like you said, it doesn't compensate for other leaks and I don't think it will for a mis-adjusted IAC plug.

This is getting interesting

Try unplugging your MAF and see if it runs any better. Mine had the same rough idle, then a high idle, and if I unplug the MAF sensor it idles perfect... but it hiccups when you drive and mileage goes down.

Is the IAC adjustable prior to installation? When I removed the one from the 90, the pintle was attached to a threaded stud that could be screwed inward or ourward to a certain extent. I do not recall seeing this threaded stud on the 93. I also remember reading a TSB regarding the IAC that said to measure from the end of the pintle to the sensor body. If it was not 23 millimeters (I think), the instructions were to push it in until it was. This was for new IAC installation.

What actually got me thinking about this was the other day when I pulled mine and cleaned it. While scrubbing hard I noticed that I had pushed the plunger back into the housing. I made sure to pull it back out before re-installing it but that got me thinking about it..

I was a little off on the guess:

INSTALLATION:

1. If installing a new IAC valve, measure the distance between the tip of the pintle and the mounting flange.

o If greater than 28 mm, use finger pressure to slowly retract the pintle. The force required to retract the pintle of a new valve will not cause damage to the valve.

2. Lubricate the IAC valve O-ring with clean engine oil.
3. Install the IAC valve assembly.
4. Install the IAC valve attaching screws, and tighten to 3.0 Nm (27 lb.in).
5. Connect the IAC valve electrical connector.
6. START the engine and allow it to reach operating temperature.

NOTE: No physical adjustment of the IAC valve assembly is required after installation. The IAC valve is reset by the ECM when the ignition is turned ON, and then OFF.


Can you elaborate on the NOTE?