99 burban no start.

Ok heres the deal wife is driving it and has all the normal stuff on AC stero and what ever else and it just dies while doing 40 MPH so i get it towed home .Its cranking really slow so i swap out the alt still no start swap out the started which has be acting up still no start .as i am cranking it the volts are going down it has fuel and spark and cranks much fast with the new parts on .any idea thanks.

Welcome to AF, how much fuel pressure, 60 Psi? Did you say you replaced the alt and starter, why? What do the volts drop to on cranking, very short on meaningful info here.

sorry the volts drop down to 12 volts

well I got it started fuel pump was shot again. I only had 40psi at start up.