Hi Frank.

I'm afraid I don't have an answer for that starting idle difference between the turbo and n/a cars. But I wonder if the turbo might use a different LH2.4 ecu than the 240. Actually, your turbo sounds just like my '93 Regina n/a (no significant idle rpm increase), with the Regina being in most respects like the LH2.4 system. You probably wouldn't have noticed a the "problem", but for the 240 to compare it to.
Are we sure the 240 is the "correct" one?

You say you, "pinched off the hose to the IAC and the RPM's dropped...". How "drastic" was the RPM drop? I ask because there's nothing in Bentley on LH2.4 throttle plate adjustments (stop screw and link rod) — but the Regina does have a linkage test, as I described above to Bob, where a drop to 500 or even a stall is OK. Otherwise the mechanical throttle adjustment is called for.

I'm not suggesting to fiddle with the linkage on either car, just pointing out what little I can really offer on your question. Probably the best thing would be to start a new thread, where you are likely to get more responses.

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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.