Oh, most definitely it would create problems such as these. You'd be getting unmetered air past the MAF sensor, leaning out the mixture. ANY air entering the engine not going through the MAF is considered unmetered. If your fuel pressure regulator isn't hooked to manifold vacuum (in some cases even if it is), then that would further lean it out. Sounds very plausible to me. Hope you find the leak soon.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch