I put a new fuel pump in my 99' 4.3L blazer (4 door 4x4) and now im getting about 12 miles to a gallon compared to 15-16 mpg when i had the old fuel pump in, any sugestions on whats going on?

 

Chad,

Did you replace the fuel filter when you changed the pump? Why was the pump changed? Did it just quit working,....was it weak? Did you replace the wiring connector at the pump? Do you know what your fuel pressure is now?
If I had to make a guess I would say your fuel filter needs replacing.(based on the information you provided) I would try that first,.....then if it doesn't improve I would start looking into other areas.(air filter, tune up,etc.)
good luck, keep us posted.

Gator

 

I can't see how changing the fuel pump could decrease your fuel milage......unless you have a fuel leak at a connection now and it's leaking when the fuel pump is working