In no way am I trying to argue against what you're saying . . . just throwing some stuff out in the open.

The AAV is fully open at -13 degrees F, and fully closed at 140 degrees F. So realistically the AAV is usually part-open (not fully open) when the engine's cold (because we rarely see -13 degrees F ambient temps), and then progressively closes as the engine warms up.

My theory is that as these parts age, the adjustment function (i.e. the part of the AAV that actually opens and closes with respect to variations in temperature) starts diminishing in operation.

In other words, when the AAV is SUPPOSED to be, say, 25% open, it actually isn't (it's some other state up or down). Thus, regardless of what temperature the engine is when you set the idle, since the AAV isn't adjusting properly, your idle will never be right.

For the original poster's car, I'd say that the AAV is actually more closed when cold then it should be. He sets the idle at cold, and then as the car is heating up, the AAV closes prematurely, and therefore starves the engine of air at idle.

Again, that's just my theory, so feel free to shoot holes in it.

For my personal car, I think that my AAV never actually closes, which would explain why, when I set my idle at hot, it's ridiculously high when cold. Anyways, I should get the new AAV in a matter of days, so I'll report back.
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74 164E auto -- looking to convert to manual M410