trouble with fuel
 

i have a 2003 gmc sierra 1500 5.3 4wd my fuel is not holding pressure and i changed the intake gasket wires and plugs and fuel filter i dont know wheather it is the pump or the pressure regulator how do both of those work. i know the pump itself pumps fuel and the pressure regulator regulates the pressure but how would i know which one to change out?

A quick check of the fuel pressure regulator is to pull the vacuum line that goes to it and see if there is fuel in the line. If there is then the regulator is bad. The pump can also bleed down pressure. The injectors can also leak, but you will probably have a starting issue - longer crank time.

thats exactly what i have how would i check the injectors to see if they are leaking?

When the truck finally starts does it smoke at all? If the injectors are leaking there will be a puff of smoke as the leaked gas is burned. Did you check the pressure regulator for gas in the vacuum line? From past experiences I would say it is your fuel pump as that is the culprit most of the time from my experience. I haven't had to put injectors in a later vortec (4.8,5.3,6.0) motor yet.