Hey, Phil. The IAC valves changed from the spring loaded design in both the 240 and 700 series when they went from LH 2.2 to LH 2.4, starting in 1989 with the B230F and B234F, and in 1990 with the B230FT. There are no longer contacts wearing grooves into the IAC commutator, so no wear grooves (which was previously an issue) and the IAC's now last considerably longer. Both my LH 2.4 940s are happily on their original IACs with no hint of problems. At 400K km on both 940s, I've never even needed to clean them and my idle is as stable as you could ever ask for.

Having said that, I do spend extra time keeping my throttle bodies and PVC systems scrupulously clean at very oil change, as I recommended in my reply to the OP. I've got it down to a matter of minutes taking the throttle body off for a thorough inspection and a cleaning if needed for any hint of grunge, pulling the flame trap for an inspection and cleaning as needed, and always check my TPS click adjustment when I'm done. I learned the hard way early on with my brand new LH 2.4 B234F to not tinker with throttle body adjustments to try to improve idle unless you know what you're doing. The LH 2.2 systems with the bypass adjustment required a bit more finesse to keep them idling as nice as possible. The K-Jet systems were much more of an art to keep them idling nice.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now