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ed11563


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Gas Myths: How Close to Empty is it Safe to Get?
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Gas Myths: How Close to Empty is it Safe to Get?

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Dear Car Talk

Jan 21, 2016 (Archives)

5 Comments

myths, gasoline, gas tanks
Dear Car Talk:

It used to be that we were told to keep our gas tanks at least half-full to prevent water condensation. Is this still good advice, or have there been changes made to newer vehicles that allow us to drive on empty?

-- Sabrina

Well, we know there haven't been any changes made to water. But yes, there have been changes made to the vehicles.

Back in the Pleistocene Epoch -- back when I had hair -- gas tanks were vented directly into the atmosphere. The gas cap actually had a little pinhole in it. And as the engine sucked gasoline out of the tank, air would come in through that hole and fill that space that had been occupied by gasoline. Otherwise, your gas tank would crumple up and implode. 

That outside air could get into the tank at all times, even when the car was parked. And it often had moisture in it. The fear was that the moisture would condense, and either you'd build up so much water that the car wouldn't run, or you'd cause the gas tank to rust.

But those are no longer concerns. First of all, as the fuel pump pushes fuel out of a modern car's tank, air to replace that volume is provided via an internal canister that's part of the evaporative emissions system. 

And, even though that stored air might contain a little bit of moisture, it lets in air only when it is needed. So the tank just isn't exposed to nearly as much moisture.

Second, most modern gasolines are blended with alcohol, which holds water in suspension and allows it to be combusted along with the gasoline (what we used to call gas-line antifreeze, or "dry gas," is just alcohol). So there's little chance of water affecting performance.

And finally, most gas tanks are now plastic, so they don't rust. 

So can you now drive "on empty"? Well, I don't think you'll get very far on empty, Sabrina. But feel free to get as close to empty as you dare. 


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ed11563


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    mnpackerfan • a day ago

    I've heard that the fuel in the tank helps to cool the electric motor in the fuel pump and that running too low causes the fuel pump to fail. I still prefer to drive on the top half of my tank.

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    bz1grn • 10 minutes ago

    Most fuel modules, to handle cases when the fuel level gets low, have a filling system that makes certain that the module reservoir is always full. This is accomplished through a "fill jet" that works to continuously overfill the module reservoir and an umbrella valve to keep fuel inside the reservoir if the fuel level outside recedes due to g-forces.
    Consequently, although the fuel level in the tank may be low, the reservoir will always be full and provide "cooling" fuel around the fuel pump.
    Don't assume that the fuel level in the tank correlates to the amount of fuel around the fuel pump.
    Additionally, additives and impurities in the fuel can accumulate on the resistor card that is the important part of the fuel level sensing system. If you never operate your fuel tank in the lower half, you don't give the float arm a chance to wipe that crap off so that if you DO need it, you might get funny readings on your gas gauge.

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    elgreen99 • 33 minutes ago

    Actually, there's one very good reason not to run your tank *completely* empty. I've run into a number of cases where high-pressure electric fuel pumps common in today's fuel-injected cars burned out because the tank was run dry. It turns out these things are lubricated and cooled by the fuel. No fuel = no lubrication or cooling = *former* fuel pump. That said, I don't worry about keeping my tank *half* full because, like most newer cars, my cars both have "low fuel" lights that come on when there's about 2 gallons left in the tank. As long as I'm not seeing that light, there's enough fuel to keep the fuel pump healthy.

    Oh, and the rust thing? Most cars today have polyethylene (plastic) fuel tanks, which of course, don't rust (DUH). Regarding the alcohol part, though, I wouldn't count too much on the alcohol blended into the fuel to keep the system water-free, though. It absorbs water both as it's being distilled from corn liquor and as it sits in the tanks at the gas station, so unless it's fresh, it's probably already water-saturated. Still, it does seem that today's cars with their high pressure in-tank fuel pumps do a fine job of whipping up any water that makes it down to the bottom of the tank with gasoline to make a burnable mixture when it's squirted into the intake manifold or cylinder by the fuel injector. I've not seen any water-related fuel system problems in ages, other than in cars that have been submerged in floods (doh!).

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      bz1grn elgreen99 • 6 minutes ago

      The "sock" that lets fuel into the module reservoir is designed to allow only an acceptable amount of water into the fuel module reservoir and fuel pump.
      Cars don't let fuel sit undisturbed in them for long periods of time like boats do.

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    MrMagoo • 5 hours ago

    I like to think of my tank as being half full, not half empty.

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    johnbarry • 12 hours ago

    I get nervous when the gauge gets low (I ran out of gas once and that was NO fun at all) so I usually fill my tank when it is half empty. Certainly no lower than a 1/4.

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    Mustangman • a day ago

    You can run the car to empty but don't. You don't want to be stranded and any crud in the gas tank may damage your fuel pump. Don't worry about running "dry" burning up the fuel pump. The car will stop running long before that happens. Run it down until the yellow "low fuel" light comes on or if you don't have one, 1/8th of a tank is fine.



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Fort Worth Mom


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When "Belle's", FEED ME light comes on, I fill her up.smile

My old Mom freaks, if she is below half a tank, and will run to the gas station, to fill it up.

She's old. If it makes her feel better, so be it.wink



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NAOW


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I usually wait until the light comes on.. unless it's cold out. Then I risk it as long as possible until the weathers nicer (eta: or until DH drives my car and fills it up for mebiggrin). I hate standing out in the cold filling up my gas tank.



-- Edited by NAOW on Friday 22nd of January 2016 07:16:45 PM

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I usually get gas when my light comes on. Not really on purpose, I just don't think to get gas. One of the pluses of small town life, everything is so close, I really only get gas every couple of weeks. So I honestly forget to check.

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lilyofcourse


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I try to fill it every 2 weeks. It'll be down to about 20 miles til empty.

But today, I got it down to 5 miles to empty.

My Jimmy was a champ.

I literally coasted to the gas pump on fumes once. I died at the pump.

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ed11563


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NAOW wrote:

I usually wait until the light comes on.. unless it's cold out. Then I risk it as long as possible until the weathers nicer (eta: or until DH drives my car and fills it up for mebiggrin). I hate standing out in the cold filling up my gas tank.



-- Edited by NAOW on Friday 22nd of January 2016 07:16:45 PM

 How much more would it cost to get "full service" when it's freezing or pouring outside? It might be worth it.

 



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karl271


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I try to keep the tank filled; it is seldom under half a tank. (Maybe I should be dating FWM's mom LOL) I started doing this after the earthquake and tsunami here in Japan about 4 years ago. The lines at the gas stations were unbelievable and gas was rationed for quite some time. So I decided I wanted to be able for us to take off or at least use the car in any emergency.

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NAOW


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ed11563 wrote:
NAOW wrote:

I usually wait until the light comes on.. unless it's cold out. Then I risk it as long as possible until the weathers nicer (eta: or until DH drives my car and fills it up for mebiggrin). I hate standing out in the cold filling up my gas tank.



-- Edited by NAOW on Friday 22nd of January 2016 07:16:45 PM

 How much more would it cost to get "full service" when it's freezing or pouring outside? It might be worth it.

 

 There are no full service gas stations here. I would make good use of one though if any offered that.



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lilyofcourse


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They are few and very far between.

We have 1 left in town.

I used it often when the kids were little.

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The one type of engine you don't EVER want to let get completely dry is a diesel engine. Unlike a car, where you can just put gas in the tank, and presto change you can drive off, a diesel engine has to be restarted in a very specific manner, requiring the knowledge of a diesel mechanic. Diesel engines are great workhorses, but don't ever, ever let them go dry.



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ed11563


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Momala wrote:

The one type of engine you don't EVER want to let get completely dry is a diesel engine. Unlike a car, where you can just put gas in the tank, and presto change you can drive off, a diesel engine has to be restarted in a very specific manner, requiring the knowledge of a diesel mechanic. Diesel engines are great workhorses, but don't ever, ever let them go dry.

I thought about getting a diesel during the Arab Oil Embargo. Diesel fuel is pretty much #2 heat oil (with some additives and lots of taxes added), and I had a tank full of #2 heat oil in my basement. 



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Lawyerlady


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I have driven 10 miles on "0 miles to empty". I'm not brave enough to try past that.

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Momala wrote:

The one type of engine you don't EVER want to let get completely dry is a diesel engine. Unlike a car, where you can just put gas in the tank, and presto change you can drive off, a diesel engine has to be restarted in a very specific manner, requiring the knowledge of a diesel mechanic. Diesel engines are great workhorses, but don't ever, ever let them go dry.

 I learned that lesson the hard way. Dad was none too impressed when he had to rescue the tractor from the middle of the field 



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weltschmerz


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I always try to keep my tank half full. Do you know what happens when you have a cup of gas in your tank and it's minus 35 out?

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I dont care so much about the engine. I just dont want to run out of gas. Especially if you do any highway travel as you never know when that could get backed up for hours in a wreck or bad weather.

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ed11563


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weltschmerz wrote:

I always try to keep my tank half full. Do you know what happens when you have a cup of gas in your tank and it's minus 35 out?

You get stranded and freeze to death? cry

 



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