Title says it all. After replacing my water pump (damn you Dex-cool ) I decided to change the fuel filter since I had no idea when it had been done last, if at all. However, almost immediately after my mileage dropped. I used to get 16-17mpg consistently, but it then dropped to 13-14.

Stick with me here. The first tank after the change was a measly 13, then my dad needed my truck for a time, so the second tank (I'd driven it some, then him) was a poor 13. However after the second tank (my dad was driving it daily and I was using another truck) he managed to get 18mpg for two-three tanks. I got it back, got 18 from the tank he put in it, then 14, and 13 after that. Now, I know you're thinking I must be too aggressive on the throttle. Thing is though I eased up a lot to try to keep the 18 going (I rarely get past 2500 rpm on accel).

Leak? Not sure. I can't smell gas, don't see any spots, but I haven't put a lot of effort into looking yet (it's cold in Utah). If it is leaking it's not at the filter (I was a little rough with the lines which is why I'm wondering if I eff'd something up).

 

Start with a fuel pressure and leakdown test. With the key on and engine off, it should be between 60 and 65 and not go below 55 for 10 minutes. Post back your initial psi and 10 minute later numbers.
Are you letting it warm up longer than he is? Do you drive more city than him?

 

You put your fuel filter on backwards...

 

He running good gas with no ethanol and you running cheap mixed crap? Even with 10% crap in my trucks the mileage drops.

 

Do you know if Holidays mix with ethenol?

 

Funny I get 22-23 MPG every day 2x4

 

No clue about holidays, we don't have em down here. I do know they post yellow stickers on the pumps here saying may be up to 10%. My blazer would labor knock like crazy on it and get crappy mileage with the 2.8 and 3.4 unless I ran mid grade. With no mixed I could run low grade just fine. My 91 doesn't seem to care. I have to pay about 3 cents more a gallon for good gas but being able to run a lower grade without issues made up for it.

 

http://www.pure-gas.org

 

Not possible.

Perhaps it's a ****ty filter though?

In regards to the ethanol, the 10% is all I've ever put in it. Never really paid it any mind, but I'll look into it some more. I had some issues I brought up in another thread, but that turned out to be bad plugs. So I changed those out, but the mileage hasn't changed much. I'll give the fuel pressure test a go when I have some time.