When I start up that mistang it goes from 1000 to 900 and keeps surging up and down sounding like if has a cam . Once I give it some gas it levels out but higher than 900 . After awhile it will go down to about 900 once it's heated up . Since that I've replaced a cop that could have been bad with a brand new one and still doing it . I have no clue why it's doing this on start up .