Before you start twiddling with stuff, check for codes from your computer. If no codes, then look for a vacuum leak. Possibly the PCV valve popped out or something.
If that's not the problem, try unhooking the neg battery terminal for 15 minutes and then rehook it back up. Start car and let it idle for 10 minutes, all accessories off. Then shut it down and restart and let idle for another 10 minutes. All accessories on.
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351W 89 Mustang GT Convertible