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Well, unless you ran over a skunk or had a rodent crawl up your exhaust pipe and die in your muffler, I think the cats are a likely cadidate for the smell. If it smells like rotten eggs or a Chrysler, yes it's the cats, too. You'll have to take them off and inspect them, but I'd do it soon because if there's damage to the converters, they can get EXTREMELY HOT! Do you have the air pump and all of its plumbing hooked up? Is the air tube still going to the center of the cats? This could be another reason if not. Hope this helps, and again, welcome to The Mustang Works!!!
p.s. Check out the rest of the site (it's HUGE!!!) and our NMRA coverage from this past weekend, you'll love it!!!
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch