You should reset the computer each time you replace a sensor. I had a horrible time with the EVP on mine until I reset it. There's no "break-in" for sensors. They only wear out, and that's every 30K-40K miles for EGOs (theoretically...).
You may wanna check out the corral's tech article on resetting idle on MAF 'Stangs. Click
here to find out exactly how to do it. Other than that, just be sure the IAB is
clean, and that your EGO connectors are positively connected and there's no exhaust leaks. Hope this helps ya.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 