Dear Tom and Ray:

My wife hounds me constantly to keep the gas tank above half-full because (1) sediment which builds up at the bottom of the tank will be sucked up into the carburetor and cause engine trouble and/or (2) water will condense inside the tank and that will cause engine trouble. Are either of her arguments valid?
Ralph

RAY: Not really, Ralph, and she probably knows that. She's just too nice to tell you that you're getting absent minded in your old age, and you're going to run out of gas and strand yourself.

TOM: Filling the tank doesn't protect you against sediment at the bottom of the tank. No matter how full it is, gasoline is ALWAYS being drawn from the bottom of your tank. Think about it. If the pick up tube (which sits in the gas tank and sucks out the gas) were at the TOP of the tank, as soon as the tank dropped to, say, three quarters full, you'd be out of gas!

RAY: So the best way to protect your fuel system from sediment is to change your fuel filter on a regular basis. And we recommend changing it every 15,000 miles.

TOM: As for condensation, your wife is right that moisture in the air can condense inside the gas tank--more likely where it gets warm during the day and cold at night--but fuels in those areas of the country are blended with some amount of gas line antifreeze which absorbs the water. So there's little chance that the water would cause any engine trouble.

RAY: The bigger concern is that condensation will eventually cause the inside of the fuel tank to rust. But unfortunately, there's not much you can do about that. You can't keep the tank full all the time, unless you don't drive the car.

TOM: That's what I do. I fill up my '63 Dodge Dart, and park all winter, just to keep the gas tank from rusting.

RAY: Nice try. He parks it all winter because he can't start it.