Erratic idle

[Deltigre]

I've seen some threads but none that seem to fully replicate my issue.

1991 318is, 21x,xxx mi. Admittedly a Bimmer newbie (see post count).

Bare bones of the issue:

• NO check engine light.
• Overall while not idling, engine performs admirably.
• Cold start, car starts nicely, no issues maintaining idle for short amount of time.
• After it warms up, it seems to switch the idle control valve on and off when idling (this is an assumption based on the nature of the problem, where it arbitrarily switches between a smooth idle and a poor, low RPM one). Brakes and steering seem to negatively affect the dip in RPMs (I assume it's the power assist brakes and load from the power steering pump), but it only barely maintains idle through the dip induced by the switch when sitting still. I haven't tested the issue enough to figure out if the steering/brake load kills it completely because mostly that happens sitting at a crowded red light (so I just tap on the throttle to keep it going). I can probably test it in neutral in my driveway at some point.

I suspect the ICV shouldn't even be triggered when the engine is warm - its job is for cold starts, right? From what I've skimmed online so far, the ICV seems to be tied to the vacuum system (would explain the brakes affecting idle) and electrical through the ECU from at least the oxygen sensor and the throttle position sensor?

From my limited knowledge it seems the ICV shouldn't even be turning on when warm and something is amiss, like the O2 sensor.

The vacuum hoses are currently lightly cracked around junctions and will probably be replaced soon, but none seem to be actually compromised at the moment. This is not thorough, as the intake manifold has not yet been removed, so anything under there is a giant question mark (though I suspect it's in similar shape).

Overall, how does the ICV function? Reduction in air pressure to increase fuel mixture? Is it possible to safely (to me and the engine) stop the ICV while the engine is running by doing something like unplugging it, just to test (if that's even necessary, and will produce the intended result of not choking the engine while OFF)?

Looking to see if there's a better idea for the issue or if I'm going to have to go do random stuff like clean the ICV and replace the hoses and O2 sensor (and possibly the ICV, though that seems unlikely since it actually appears to be doing its job).

[Deltigre]

OK, so I did more research and a little testing.

Like some other posts I found, it's directly tied to the A/C compressor kicking in, the relay I was hearing was the A/C. Turn off A/C, it doesn't dip so low.

Now that I know what it is, is there any way to counteract that issue?