Hey dude, go and stand at your front driverside tire over your engine. If you look at the side of the intake closest to you , you will see a flathead screw tucked in a threaded cylinder about 5/8ths of an inch wide. It will bite you on the nose when you find it. Mine was impossible to budge, so you might want to spray some lubricant on it.
Boy oh boy, I'd feel like an idiot if this were true. I'm "on assignment" so don't have my car around, but I don't think my turbo has any sort of idle adjustment that is that obvious. Under "idle adjustment" in all my manuals, it just says "take it to the dealer."
By comparison, N/A ZXs are easily adjusted by a screw that, as you say, will bite you every time you open the hood it's so obvious.
I fortunately have never had to have the idle on my car adjusted. I have found that if the plugs, oil, filters, etc. are kept up -- and the vacuum leaks kept after -- the car has always run perfect.
I've got new plugs, fresh oil change(last week), and replaced all vacuum hoses with new lengths all within the last three months. The idle has always been a little flaky but never like recently. It will not idle regularly, refusing to stabilize above 500 rpm. It revs up and down and will stall when I'm driving and come to a stop at a light. Similar to the way a vacuum leak would behave, but I checked all that stuff and found nothing. I have a new admiration to how quiet the motor is at idle now as I came here all functions of the engine are very quiet when my head is two inches from operating parts. I can here the injectors pulsing.
First check all of your vacuum lines for leaks. If it even looks old, like it may be soso, replace it. Then take some sort of NON-FLAMABLE liquid and spray it around all possible area where an air leak may occur(hoses, intake manifold against head, etc.) If you hear a sudden change in the idle while doing this, you have found a leak. Fix it. Then once you are done with that make sure all your sensors are working properly(may be hard sometime, but haynes has some diagnostics for such things). Oh yeah, don't get water or whatever liquid you use into the TPS connector or any connectors for that matter. And never get any type of liquids other than maybe water on the O2 sensor, you will mess it up. Clean all the connectors that attach to the engine, including the injector connectors. I also use diaelectric(sp?) grease on all my connectors mostly to reduce corrosion. After cleaning them all, start your engine back up and gently wiggle every connector, even the injector connectors. If you here a noticable difference in the engine because of this, you may need to replace the connector. Good low resistance connections are a must for a computer controlled engine. Also, if you replace any connector, use a soldering iron and heat shrink to install the new connectors. Don't just crimp them, it may work for the short term but it will give you grief later on down the line. Now that you have hopefully made sure everything else is running properly, you can adjust the idle. Trust me on this part about checking everthing else out first. Been there, done that. Now the idle speed adjustment is on the throttle body on the distributor side. This is a flat head screw that physically moves the throttle shaft control arm. This should do the trick. I just listed all the other stuff first, because when I first got my turbo engine running, the idle was way to high as a result of me fixing all the leaks and bad connections. Perhaps if you fix all the other possible problems, you may not have to touch the idle. Just a thought.
CA "TS 240ZT"
One other thing to check: I've read that the accodion-like rubber boot that connects the Air sensor to manifold can get tiny cracks in its folds that lead to a surge like you're talking about -- or at least an incorrect air fuel mixture. So go wiggle that thing and see if it makes any difference. Another trick to see if there's a hidden vacuum leak is to take off the oil filler cap when it's running. If the motor runs worse, your vacuum is probably okay (air introduced by the missing cap constitutes a major vacuum leak). If the idle doesn't change, there is still a leak somewhere. That's sure what it sounds like. And they can be a bitch to find, I know. In one of the fuel injection repair videos I have, the guy said that one Z-car motor had a bad vacuum leak that they couldn't find until they took the motor out of the car. Turned out to be the rear main seal had disintegrated and the engine was sucking air in there!
I know what you mean about listening to a well-tuned Z motor running. I love listening to the pulse of the injectors over that purr, too. Hope you hear it again soon!