If the check engine light comes on, check the codes before you do anything else. Now your idle probably is screwed up since you messed with the idle screw. You're TPS is now probably off, too.
Get a code reader if you don't have one, or create a wire with a spade on each end and jumper the test lead to the diagnostic connector. Read this article
Corral.Net: EEC-IV Testing.
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351W 89 Mustang GT Convertible