Im startign to see this in my 2004 again. It falls fairly quickly to "half" in a hundred miles, sits there for a while and falls to empty quickly. The trip odometer is more accurate (350 miles per tank in winter,
I also see my fuel gage fall more quickly in colder months, even though the mileage often stays the same. I keep an eye for gasoline "adjusted" for winter. But I also think part of the explanation is that in summer, cool gasoline is pumped from underground into one's gas tank. Then it heats up and so expands. This keeps the fuel gage reading higher, compared to cold months, for awhile.
I've been recently noticing the same behaviour in my '04 Civic LX regardless of the weather. And I just found a TSB about it on the EBSCO online database, something to do with incorrect software specifications. The solution is to replace the fuel gauge assembly. This applies to Civics with the "Visteon" gauge assembly.
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Click on "Auto Repair Reference Center" Select your year, make, model and trim level. Click on "Service Bulletins and Recalls" Click on "Fuel Sys, Driveability, Filters (Air & Fuel)" Click on "INACCURATE FUEL GAUGE READING" to open a .pdf file with the TSB.
Looks like I'll be making an appointment with the dealer, and hopefully they won't have to keep it and drive too long to see the symptoms themselves.
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