Replaced fuel pump, loss of MPG...

[ApproachMedium]

Seems like a lot of threads here about people loosing MPGs after doing repairs, heres another one! A few weeks ago a buddy of mine and I ventured out to replace my fuel pump since I was pretty sure that the check valve in it was bad and I was tired of cranking this thing over 5 times to get it to start. I ended up getting an Airtex one from Advance auto parts since they had the best price, at $271 bucks over the GM dealer price of $650.

After replacing the pump the truck now starts up right away, as intended. Its since gotten alot better at idle, as previously it would misfire badly, but only at idle. I was carrying consistently code P0303 for misfire on the #3 cylinder and P0420 for the cat. The cat was replaced twice within the past year, second time because I had a faulty fuel injector.

Now I can go a few days with no SES light. When it does come on it throws P0303 misfire #3 and then will go off after a day or two. Maybe just from all the lack of fuel pressure was causing the misfires? I assume my next step should be to replace the spark plugs again, as the #3 is probably fouled up. But what still really has me confused here is I went from about 18-20MPG to now about 15-16MPG. I do mostly all highway driving. It seems the change was directly related to my pump being changed. Could I have not been burning as much fuel because the engine just wasn't getting it with the old faulty pump?

[HenryJ]

Could the difference be a coincidence? Winter grade fuels are being introduced about the time you changed the pump?

[ApproachMedium]

Its possible... Previous years winter blends never did me this bad. I used to be able to go 200 miles full to 1/4 tank, now 200 miles is pretty much the low fuel light coming on.