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Who Taught YOU to drive?

Running out of gas is just one risk of letting tank get too low

With high gas prices, as at this California station, drivers may be tempted to stretch the time between fill-ups. Not a good idea, car mechanics say. With high gas prices, as at this California station, drivers may be tempted to stretch the time between fill-ups. Not a good idea, car mechanics say. (Paul Sakuma/Associated Press)
By Peter DeMarco
March 27, 2011

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With gas prices once again rising above $3.50 a gallon, you might be tempted to let your tank run extra-low before fueling up. But the longer you wait, the greater the chance you’ll actually run of gas, and that can’t be good for your vehicle, right?

I mean, could you actually damage your engine by failing to fuel up? Is driving with your fuel gauge near empty, even if you don’t conk out, harmful to your car?

When we joke about running on fumes, what are we really talking about?

I explained back in January how running out of gas could conceivably land you a ticket. This week, we look at the mechanical side, as well as other useful fuel tidbits.

Pumped out For the lowdown on running empty, I turned to some trustworthy sources: John Paul, American Automobile Association of Southern New England’s “Car Doctor,’’ and Butch Ferraro and Tony Izzicupo, owners of Salem Street Auto Clinic in Saugus, who have been fixing my family’s cars since I was a kid.

Fuel pumps in modern cars, the mechanics explained, sit in the middle of the gas tank, surrounded by cool gasoline.

When your tank’s empty, the fuel pump burns hotter, inviting wear and tear, and if you consistently drive around with your gas gauge near empty, you run the risk of burning out the pump.

“The first thing I tell my wife is, ‘If you’ve got a half a tank of gas, fill it up!’ ’’ said Ferraro. “It keeps the fuel pump cool.’’

Running out of gas poses little risk to your engine. But over time, dirt and rust particles settle on the floor of your gas tank, like sediments at the bottom of a bottle of wine. Just as you wouldn’t want to drink wine sediments, your car’s engine doesn’t want to drink dirt and rust.

If you’re down to your last drop of gas, it’s going to be contaminated with some of that crud.

“Hopefully the filter will catch it. If it doesn’t catch it, it could clog a fuel injector,’’ Ferraro said. That won’t necessarily cause a breakdown, but your engine won’t run as well.

Diesel’s different If you run out of gasoline, you usually just need to add more to the tank. But if you drive a diesel and run out of fuel, you’re more than likely going to have to tow your vehicle to a repair shop.

“When diesels run out of fuel, you get ‘air-bound,’ ’’ Izzicupo said. “The injector pump gets full of air and you can’t get the car started by just putting diesel fuel back in the tank. Now it’s a process.’’

A laborious process, actually, involving the removal of filters, pressure-blowing the fuel line to clean it out before pouring in new fuel, and repriming the engine. “It’s the worst,’’ Izzicupo said.

On fumes Running on fumes is a nifty expression, but engines don’t operate on gasoline vapors. So, just how much gas is in your tank when your car’s fuel warning light comes on?

Ferraro and Izzicupo said it’s either three or five gallons, depending on the vehicle. Paul was slightly less optimistic: He said you probably have “between 30 and 50 miles’’ of gas left when your warning light comes on.

Of course, if your vehicle has a digital counter instead of a simple warning light, your car tells you just how many miles you have left before it shuts off.

Or does it?

“I’ve never actually trusted it to the point where it’s dropped to zero,’’ Paul said. “Admittedly, as I was pulling into a highway gas station one time, my car was counting down in like 4-mile increments. You had 25 miles until empty, then all of a sudden 23 miles, then 18. I was getting a bit nervous.’’

Car manufacturers base their “miles left’’ calculation on a driving formula — the operator obeys speeding limits, doesn’t brake suddenly, isn’t carrying extra weight, etc. — that you personally might not follow, Paul said. As a result, the “miles left’’ reading is at best an estimate.

And driving as fast as you can to get to a gas station is the wrong thing to do when you’re very low on fuel, Paul said. “That would be the time when I would want to drive as efficiently as possible.’’

Spare fuel? I asked my experts whether there’s anything else you could pour into your gas tank, in desperation, to make it that last mile to a filling station.

While diesel fuel and gasoline both come from crude oil, they aren’t interchangeable because they burn at different temperatures.

A bottle of alcohol, perhaps?

“No. Your engine won’t run on that,’’ said Izzicupo. “Plus, you’d never waste a fifth of vodka like that.’’

Paul, from AAA, remembered a product from years ago called Spare Tank that you could carry in your trunk. When you ran out of gas, you poured it into your gas tank and it “pushed’’ residual gas sitting at the bottom of your tank into your engine.

Spare Tank hasn’t been sold in 10 years, but as it turns out, it’s about to be reintroduced nationally under the new name Spare Fuel, the company’s chief executive, Richard Jones, told me.

He said his product, which was pulled off shelves in 2001 because of a patent dispute, is safe to carry in your car because it doesn’t contain any butane or any other highly flammable components. The company’s website says: “In fact, if poured over a burning match, it will extinguish the flame.’’

Spare Fuel is available online for $21 for half a gallon — enough to get you to a gas station — but before you order some, I’d advise waiting for the state Division of Fire Safety to declare the product safe and legal for sale in Massachusetts.

Fire officials are reviewing whether the container that Spare Fuel comes in meets state codes for spare gas cans, a point that was unclear at press time. “It’s under review,’’ said spokeswoman Jennifer Mieth.

Peter DeMarco lives in Somerville. He can be reached at demarco@globe.com. He also updates a Facebook page, “WhotaughtYOUtodrive?’’

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