An air filter would have been a good suspect on an older car from the '80s with a carburetor, but today all cars other than Chryslers use a mass air flow sensor. That measures the volume of air going into the engine so the Engine Computer can command the corresponding amount of fuel to go with it.
Instead, a misfire condition is a better suspect. Unburned fuel and air goes into the exhaust where the air is detected as a lean condition. The computer responds by commanding more fuel to all of the cylinders, but no matter how much more fuel it adds, there will still be that unburned oxygen being detected.
Of course there could be another leak in the fuel supply system. You didn't say what was leaking or how you repaired it, just that there was some leak. We have to assume the fuel mileage was normal before you did this repair. If a steel line was rusted through and you just patched a part of it, there's a good chance there's another part of it leaking. You also didn't give any indication on how bad the fuel mileage is so it's hard to determine other likely causes for it to be low.
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Friday, November 8th, 2013 AT 12:34 PM
