My 99 2500 gmc starts first thing in morning but as the day goes on it has a trouble starting. If I pump the gas pedal a few times it start but struggles. What could be the reason/ cause for this?

 

What engine? Are there any Diagnostic Trouble Codes in the OBDII system?

 

Had o2 heater bank 2 sensor 1and sensor 2 changed those out and changed the fuel filter out and still does it.

 

Has the 6.0 in it

 

Check the fuel pump pressure using a fuel injection pressure gauge, and also check the fuel pressure regulator - pull the vacuum line off the nipple and see if gasoline runs out of either the fuel pressure regulator or the vacuum line. If you get fuel spilling out then you need to replace the fuel pressure regulator.