New fuel pump gas mileage got worst 99 4.3 silverado

[bray27]

Shop Installed a new fuel pump in my 1999 4.3 Silverado, everything is the same as before but immediately after it was installed gas mileage went down. 

Any ideas? Sometimes a rough start sometimes not. 

[Jworks]

I can't answer for the mileage issue but I can say hang on to that truck long as you can .  Good models; and 4.3 is a good engine.  I got 484k miles on one before I sold it.

[bray27]

Not selling it, trying to figure out what would gas worst gas mileage on a new fuel pump

[dna9656]

Ck your fuel pressure regulator, the injectors, and the ECM.

[bray27]

ok, what am i looking for on the ecm? that’s a pretty big thing. 

[asilverblazer]

What prompted the replacement?

 

I'm thinking the fuel trims need to adjust for the new pump/pressure. Might clear up over a few drive cycles.