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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
Possible if the problem was in open loop (cold start up). You did say that the problem happens no matter if it's hot or cold right?
exaust leaks do cause idle problems in computer control cars, but not so much in a mass air enviorment in closed loop.
How bad is the header leak? Is it just a little or did it completly blow the gasket out of the header flange?