My '92 Isuzu V6 is getting 11mpg (more or less) after fuel pressure regulator change, PLEASE HELP!?

Ok, so we have an Isuzu Rodeo with around 270K miles (more or less), it had a wreck in mexico and a bunch of stuff they didn't fix, for example, they took the black can that's like for the fuel vapor and air and that's how we brought it back over here to the states, ok so like 1yr, to 1 1/2 yrs later,we finally put that black can on again and the fuel pressure regulator started to leak gasoline, and the mpg was still pretty good, now that it has been changed and the canister has been installed, the mpg went down to like 11, on a full tank of gasoline, we do not even get 300 miles to the tank!!! please help!