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THE CAR:

2004 Forester XS

5 speed manual

117k miles

 

THE FACTS:

I always fill my tank when I am between E and 1/4 tank.

I drive basically the same route every day at the same speeds, etc. (Mostly two lane back roads at about 60mph with few stops)

 

Sometimes I get 18 MPG, sometimes I'm pushing 30 MPG.

 

When I fill up, I fill up all the way. Sometimes it takes 8 gallons, sometimes over 11 gallons despite filling up when the gauge is at the same place every time.

 

I may have a rust hole in the filler neck of my gas tank. I'm not sure. It would explain my check engine light that is on for an evap leak (same thing as loose gas cap). However, no gas seems to leak out when I fill up.

 

THE QUESTIONS:

Is my mileage really fluctuating that much? If so, why?

 

Is my fuel gauge reading inconsistently and therefore making me think that I'm filling up at the same time when I actually am not?

 

Is the pump shutting off at different times because of a rust hole in my filler neck? My gas gauge often reads Full for as much as 70 miles, so I could be less than full, but the gauge is showing full.

 

COMMENTS:

It just occurred to me to start filling up at the same odometer reading instead of the same fuel gauge reading for a little while. Maybe if the fuel economy appears consistent, I can rule some things out.

 

Anyone ever have a similar issue?

 

General thoughts and opinions?

 

 

 

 

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Just some things I'd personally do if I were you:

 

1. Fix that evap leak. It's probably the reason.

 

2. Check the tires with an actual gauge (don't rely on gas station gauge as they are not accurate or can be way off) and make sure they are at the recommended pressure.

 

3. Replace the O2 sensor if you have higher mileage. They can play a huge part with MPG.

 

4. Start filling the tank at a 1/4 and try and avoid going below that. Reset your trip odometer EVERY time you refill the tank. I got in the habit of doing this myself years back. Every time I fill the tank and get back in the car, I always reset it before starting the car. Reason being is I know EXACTLY how many miles it was driven with X amount of fuel added.

 

5. Do a full tune-up. Dirty air filter is especially problematic. Start running synthetic oil. A 5quarts+ bottle costs less than a gas refill. Auto Zone often runs deals where they knock a few bucks off if you pair it with a certain brand oil filter like Bosch, so you can get both for around $35 or less. Your MPG should improve around 1 MPG on average. Change the plugs and wires too if it has them.

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Don't know about newer models, but I never trust a Subaru fuel gauge!

 

Use your trip meter--reset it every time you fill up.  Keep a super accurate log of miles vs. gallons.

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Don't know about newer models, but I never trust a Subaru fuel gauge!

 

Use your trip meter--reset it every time you fill up.  Keep a super accurate log of miles vs. gallons.

 

I do track the mileage using the odometer. That's how I know that the mileage is so inconsistent.

 

I'm thinking that I should investigate my filler neck.

 

I've got a hypothesis that mileage isn't actually inconsistent, but that the amount of gas put into the tank is inconsistent and screwing up the calculation. Without a reasonably good seal when filling the tank, the pump doesn't know when to shut off. I think I have vapors escaping when I fill up so the pumps shut off at different times from one fill-up to the next.

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"Fuel vapors" are just vapors. Years ago in one of my carb'd mustangs, it had a tiny hole punched in the fuel cap to assist in easier fuel suction at the mechanical fuel pump or something. Car still got 12 MPG city and maybe 16 highway. Unless gas is actually leaking from the tank, your "fuel vapors" escaping aren't 2-5 gallons "evaporating" into thin air. Read my post further up and follow the advice.

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