EEC-IV code reader from Auto Xray, us mustang guys still use them as that is our OBD equivalent. I would check to see if there is any water in the MAP sensor and make sure you have good vacuum and check the hoses. You could have a vacuum leak somewhere.
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Ryan
1990 Mustang GT (In Process)
1993 LX Coupe (Daily Driver, but just choose not to)
2005 Excursion (Big Pig)