I have a 2004 Chevy Corvette with about 87,500 miles, automatic, no OBD codes, 5.7 LS1, and a 3.15 rear end, bought used from a dealer about 10 months ago. As far as I know it's completely stock and it runs well but it gets really bad gas mileage, sometimes as low as 11 or 12 in the city but usually around 13-14 and about 19 or 20 on the highway and only about 22 or 23 on a long trip. Other C5s that I've driven get well over that, typically around 18-19 in the city and well up in the 20s on the highway and even in the low 30s on a long trip. I rarely drive it hard, so it's not my driving habits, and it gets pretty much the same mileage no matter what type or grade of gas I buy.
This particular year had a gas tank connector problem which causes a leak between the two tanks but a local dealer tells me it was fixed under the service bulletin and everywhere I've taken it says they don't see a fuel leak even though according to numerous sources, it didn't actually spill out but evaporated from the connector. I don't smell gas and never see any gas on the ground.
When I first bought the car at the beginning of the year I didn't notice the gas mileage being so low but noticed it after I had it for a couple of months. Another thing worth mentioning, no matter what the onboard DIC indicates are number of mileage remaining, I never get anywhere that. On a full tank of 18 gallons I typically will get about 220 even though the DIC indicates I should get about 340-350 miles, and on my last C5 I would actually get over 300 on a full tank in the city and over 400 on a trip.
I've had a number of things checked on it and no shop can find anything wrong mechanically but I literally can't get anyone to run a pressure check on the fuel system to see if there is a pressure leak. They keep telling me that it CANNOT be a fuel pressure problem or I would smell gas and no one will actually perform a pressure check.
Any idea what it might be, and is it true that it's not possible for it to be a fuel pressure issue since I don't smell or see leaking fuel?
Start by checking all the tune up items like spark plugs. Also check fuel trims to see if it’s running rich.
Sorry, I should have specified. I did a tune up which helped very slightly (it was getting slightly worse previously) and the fuel trims are running within normal ranges. I've taken it to several shops and everywhere I've taken it to says they can't find anything wrong, except that nobody will check the fuel system pressure.