I didn't actually post this problem to the forum, but I read a lot of other posts on the subject. The 1.9 has a problem with very low idle, car has to be run for a couple of minutes on the first start of the day just to get to run well enough to keep from cutting off, and even when it stays running the rpm's sit at about 500 at idle.

I saw a lot of approaches - IAC, MAF, vacuum, throttle body, various sensors - I was throwing parts at it and getting nowhere. I tried all of them as well, and no change.

By accident, I noticed a small set screw behind the cable pulley on the throttle body. It isn't immediately obvious that it's adjustable, as the allen key end of the screw is covered with blue paint, apparently put in at the factory (I saw the same in two other S40's in my local junkyard).

Once I realized it was adjustable, it was just a matter of getting the right allen key and adjusting it until the car was idling properly.

When my car first started acting up, the change came on fairly quickly, just a week or two from running decent to barely starting at all, so a factory set idle stop screw wasn't the first thing I thought of, but I've been running over two weeks now with the adjustment and it's purring like a kitten.

Here's a link to the screw location
https://www.brickboard.com/IMAGELIB/volvopic.htm?js_link=1&id=13131

That would mean that the set screw came loose. Did you notice any of the paint cracking at the base of the screw head? Or considerable wear at the screw point?

If the screw came loose, it could happen again.

Good find, by the way.
--
Keeping it running is better than buying new

I didn't see any cracking on the paint (loctite?), and there looking at it now there doesn't seem to be any indication on the threads where the screw position shifted, but I've been running it for a few weeks now.

I was wondering if something inside the throttle body itself may have worn down from use that made the position of the stop screw out of alignment, but I'm not curious enough to tear the throttle body apart. Maybe the screw position is something that needs to be checked from time-to-time.

In any case, of all the approaches I saw in regard to this problem on this and different forums, no one had identified this as a possible source. Honestly, I just got lucky on this one.