thanks for all the advise yesturday I found out why the check engine light is on and the car is idling up and down I was trying to set the fuel pressure on the regulator and then the car started the up and down idle thing so i looked at the pressure gauge it was jumping from 20-60 back and forth as the idle was going up and down thats probably the problem with hesitation its loosing fuel pressure anybody know why this would happen its a 93 mustang gt please help!!!!!! it has holley adjustable fuel pressure regulator it also has a hypertech chip I didn't put it in could that be faulty I don't know I just throwing stuff out now