Can idle mixture just "go wrong" for no reason?
 

I am trying to figure out if my car's very low idle is a result of the idle mixture just suddenly falling out of spec, or a symptom of another issue. So basically my question is: If your idle was set dead-on at 950 RPM for 3 years, can it suddenly go out of whack for no reason and need to be readjusted, or is a low idle a sign of something else completely? I'm asking this in concert with my other question a couple days ago about the car only coming to a decent idle after miles of driving. It's gotten to the point now where the idle seems to never get back to 950 RPM.

Could be a vacuum leak, or bad wire connection, clogged fuel filter.

Well, there is always a reason! It just may not be obvious. Fuel deposits, corrosion, or foreign matter on the ultra fine screen inside the control pressure regulator often seemed to do it to me back when I had a CIS car. Could be fine for many months or years, then bang. After buying several of these at what was then a couple hundred bucks a pop, I figured out how to take it apart, clean, and reassemble.

Intake leaks are the likely culprit or bad fuel flow due to a clogged filter, crap in the gas tank screen or in some cases a failing 02 sensor.

Is it CIS? There's a few things that can cause the idle to go bad.

did your freq valve stop working? is it in the system with a computer, etc. or 78/79 style without that stuff?

yes. air leaks and fuel pressure, frequency valve as JW stated, and that is controlled by a relay under the seat and powered by fuse 18.

the WUR can cause the FP to change or a loss of power to the WUR. if it takes a VERY long time for to run correctly, i would look at the power to the WUR first.

If you still have points, as I do w/ my replacement SC engine, check those. Same thing happened to me recently.

Hey, sorry I didn't give enough background info. Yes, it's an '83 SC motor with CIS, but I don't run an 02 sensor, and I believe the AAV is blocked off. Don't quote me on that, but one of the valves that isn't really necessary was blocked off long ago by a PO and I never messed with it since. The car ran perfectly for years, but within the last 9 months or so the idle has been doing all sorts of strange things, from huge 700-2000 RPM bouncing surges, to now sitting at 400 RPM and almost dying. I think the WUR is to blame, because I've driven the car a good distance, it seems to settle at 950 no problem. So is there a quick way to check that the WUR is getting power?

a test light or volt meter at the air reg or WUR plug while it's running. take the plug off and stab both terminals. both units are on the same circuit. one of the terminals needs to be hot. the other is ground obviously.