99 Tahoe 5.7 vortec LT 4X4
​​​​​​ I replaced the fuel pump and since doing so when I start it runs fine when it's cold runs great but when it gets the temperature it starts spitting and sputtering and popping it just doesn't run right.

 

Just some clues not a solution: When it's cold it runs in closed loop or from preset values in the computer. When it reaches temp it runs off the sensors and makes adjustments by what they tell it. But if it happened right when you changed the fuel pump then it's probably something you did. Look at all the places you were at. Did you leave something unplugged? Did you bump something and knock it loose? Things like that. Most likely it's throwing a code which would be a really good clue.

 

Don't like to us the word "scares", but when someone gets done replacing parts and winds up with new problems, a shop/tech has to start from the beginning to check everything the last one in there did. Many times we have no idea where they worked and what they might have disturbed.
Good rule to follow ,, "Always recheck your work".

 

"Many times we have no idea where they worked and what they might have disturbed."

But after we find the problem they tell us "Oh yeah, I took that apart" or something along those lines. LOL.