Don't buy any new parts, but try swapping your idle control solenoid with another one from another EFI Mustang. If you're not sure what it is, it's attached to the throtle body facing the front of the engine, & it has 2 bolts, it kind of looks like a shift light & it's angled towards the driver's side. A new one costs around $150 canadian so before you buy one try swapping one from another 5.0. I hope this helps.
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89 coupe, 351W, 351W heads, 30lb injectors, 190lph pump, 80 maf, 70 tb, ? cam, cobra upper, gt 40 lower(14.6at95mph)
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