I have a '90 240 (stick). I believe that the "idle setting" of which
you speak is the stop screw on the throttle body. This is NOT an idle
adjustment! It merely holds the butterfly as closed as possible
without allowing it to ride in the aluminum alloy body which can cause
wear. The idle is controlled by the computer and not adjustable. You
will see that when you start the car. The idle hits about 1900 at
first, settles to 1500 for about one or two secinds, then settles at
the normal idle speed. (at least to say, mine does that). Also,
without actually connecting a quality instrument I would not rely on
the precision of the dash tach without first checking it.

Or, I could be wrong- it's happened before!  ;-)

            __  __
    Randy & \ \/ /alerie's
             \__/olvos
'90 245 Estate  -  '93 965 Estate
  "Shelby"     &      "Kate"