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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.
you hook the gauge up to the schrader valve (looks like a tire valve stem) on the chrome hard fuel line on the passenger side of the car. and dis connect the vacuum line that goes to the fuel pressure regulator and plug the line. this will give you max fuel pressure. and yes you check it at idle with the vac line off the fpr it will show the the max fuel pressure which is what it would be at WOT