Can a dented fuel tank's gas gauge be reading correctly?
Dear Tom and Ray:
I'm sending this anonymously because I don't want my friends to know what a bonehead I am. I went out to the driveway to replace the rear brake shoes on my daughter's '98 Honda Civic EX. I positioned my floor jack under the rear cross member, but did not jack it up. I then went into the garage to get my jack stands, and placed them beside the car. Without checking the jack, I started to jack up the car. Something didn't seem right, because the car hardly lifted for the amount I jacked. I looked under the car and saw that the floor jack had rolled forward a few inches and the saddle was pushing up on the fuel tank, which was now dented in! I quickly jacked down the car, but the damage was done. I drove the car through the neighborhood and up onto the interstate to confirm that it still would run fine. It now has more than a thousand miles on it since I dented the fuel tank, and it runs with no problems. My question: Is the fuel gauge still accurate, or is it incorrect because of the dent in the tank?
I have gotten my daughter to refill the tank whenever the gauge registers half a tank. It takes about six gallons to fill it up from there. My daughter wants to drive beyond half a tank on the gauge to see if the gauge is accurate down to empty. I don't think she should do this. What do you guys think? By the way, I did finally replace the brake shoes, but she doesn't let me work on her car any longer.
TOM: Don't worry, Joey. Your secret is safe with us. I mean, how many Joeys can there be with daughters who own green '98 Honda Civic EXs with bad gas gauges writing from Maple Street in Tallahassee, Fla.?
RAY: With the last name Sawyer?
TOM: We made that up, by the way, readers. Joey, I think your daughter has to try the experiment. And you need to be following her with a five-gallon gas container.
RAY: This car was still using a metal tank in 1998. So you crumpled it. But you probably didn't deform it too badly; otherwise, it would be leaking now.
TOM: These tanks can take a pretty good beating. I mean, they're not designed to support a 2,500-pound car on the saddle of a floor jack, but they do get hit by rocks and road debris and snow banks, and they manage to survive all that stuff.
RAY: What you need to find out is how much you've reduced the volume of the tank, and whether you've interfered with the operation of the float, which tells the gauge how much fuel is left. And there's only one way to do that, Joey.
TOM: If you really wanted to be a good dad, you wouldn't even make your daughter participate. You'd tap one of your buddies (who now knows what a bonehead you are because you wrote to us) and have him follow you while you drove your daughter's car.
RAY: Here's what you'll do. This car has a tank that holds about 12 gallons. You know the gauge works fine between full and half-full, because you know that accounts for six gallons. You also know how many miles that gets you. Let's say, for the sake of argument, that you get 150 miles out of the first half of the tank.
TOM: So you want drive another 150 miles, or until the tank is empty. Along the way, you want to correlate the mileage with the reading on the fuel gauge.
RAY: So when you've driven 75 miles, is the gauge reading a quarter-full? If so, you know it's accurate to the one-quarter mark.
TOM: If the car runs out of gas before you expect it to, you'll want to note what happens. Did the gauge drop suddenly from a quarter-full to E? Did it stay on a quarter-full and just run out of gas?
RAY: And with this newfound, scientifically derived information, you'll be able to tell your daughter exactly what to expect, and when she needs to fill up from now on.
TOM: And by the way, Joey, if the car runs out of gas as soon as you leave the house, you'll know what you're getting her for Christmas this year.