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Running on Empty

Hi ,

On today’s show 6-20-09 , a caller asked if routinely running a vehicle low on gas was hazardous to the vehicle ( particularly the engine ) Tom and Ray answered for the most part no , except the fuel pump in the tank may heat up more and that might shorten it’s life …maybe ?

Another consideration , familiar to me , is that when the gas tank is near empty , there’s more air space , and if temperatures fluctuate very much , condensation is possibly going to form inside the nearly empty tank and add water to the fuel , which can negatively affect the engine ; whereas a nearly full tank , having less air space is less likely to allow for condensation . Perhaps ,this was more of a potential problem in the past , since now many gas tanks are no longer made of metal ,( and the metal was part of reason for the condensation ), or now that a lot of gas has alcohol mixed in at the pump , any condensation might burn off … as adding " dry gas " ( alcohol ) has been the usual cure for a little H2O in the tank . Anyone else familiar with this , or have additional info on the matter ?



Running on empty or close to it is DUMB, for a number of reasons. In warm weathr, the fuel pump, which is cooled by the gasoline in the tank, keeps circulating the same small amount ofgas which keeps getting warmer and warmer. This definitely shortens fuel pump life.

In cold climates, we may have a situation where your car is parked overnight in a cold place, and you go to the office and park underground in a warm garage. Moist air will be drawn into the tank, which then develops ice crystals when the car comes back outside. The same happens if you have a warm garage and park outside at work in the winter. So, the less empty space in the tank the better. Plastic tanks behave the same as metal tanks, it just takes a little longer for them to change temperature.

Many of us were raised in a cold climate, and religiously add alcohol to our gas in the winter, to prevent this icing. It’s no fun if your gasline freezes up during a cold spell; the car willl have to be towed into a warm garage. Agree that with gasohol, the water will get absorbed, but many of us use regular gasoline without ethanol.

So, try keepiong you tank at least half full, and add a little bottle of “gasline antifreeze”, as it’s called every second tankfull in the winter.

There is seldom if ever any condensation in modern gas tanks. Gasoline is very volatile and in a sealed container (gas tanks) it builds up positive pressure preventing any moist air from being drawn in. A complex emissions compliant venting system (carbon canister) is used to capture these vapors as they continually boil off. Positive pressure is maintained even if the temperature drops markedly.(Gasoline contains both propane and butane)

More urban legend stuff is the need to keep in-tank fuel pumps submerged so they will be properly cooled…In most installations,the pump is located near the top of the tank and is seldom submerged…

This is one of those ideas that’s fun to debate endlessly, but ultimately means almost nothing. It just really doesn’t matter very much. To me, keeping things from getting close to an emergency is much more comfortable, so fill it when it’s down to one quarter. Over the life of the car you might stop for gas a few more times, but so what?

Thanks guys ,

So maybe , potential condensation in the tank is not as much a problem with newer vehicles , anyone know when the newer designed fuel systems became the norm … in the 1980’s possibly ?

How long has this question been asked. Don’t you think the manufactures know how to build a fuel pump that can handle heat and cold and being submerged for any length of time?

Dear wentwest ,

I agree , there indeed may exist greater hazards with running low on fuel than the possibility of a little H2O in the mix , as was mentioned on today’s show - running out of gas mid way over The Golden Gate … but my question was aimed at gaining information about any probable mechanical disadvantage of routinely running on a near empty tank .


One would hope so .

Condensation is a very big issue in my Cessna 172 airplane, and the tanks even have a drain valve which is operated during every pre-flight routine to check for , and remove water in the fuel.

  • But that’s airplanes. -

You can easily assume, therefore, that condensation will occur in any fuel storage container. ( and does )
But in automotive applications you’ll never know it’s there and, except for arctic conditions, it will never be a functional issue.

The electric fuel pump fuction, however, is the major reason NOT to run so little fuel.

And on the monetary side of things, you only spend any extra filling it up …ONCE. then your ‘’$10 a week’’ or so tops it off and keeps it full. ( had this discussion with both kids. )
If there’s multiple drivers each responsible for their own fuel usage. Always have a full tank to guarantee accurate fuel consumption replacement.
Each user begins use of the vehicle with a full tank and returns it with a full tank thereby honestly replacing the fuel they used.

With all respect to my friend Caddyman, who I like and respect, todays systems are deigned to maintain a slight vacuum in order to prevent gas fumes from being pushed out/past the charcoal canister.

The condensation issue is a different one. In the old cars the fuel pump was a mechanical pump on the engine and sucked gas from the tank. The gas in the line was at a low pressure, a relative “vacuum”, because of this. And carburators were really low pressure float bowl systems with eir inside and an opportunity for expansion via the venturi orafice, which was basically a vent. This allowed ice crystals in winter and ready boiling in summer (fluid boils at lower temps at lower pressures). Thus, we’d get “gas line freezeup” and “vapor lock” more readily. Lots of folks used to add “dry gas” to prevent these problems.

Today’s gas is fed under high pressure (typically 40psi to 65psi depending on the vehicle) as pure gas right from the tank to the injectors, and the injectors are shut tight when not energized, there is no “vent”. There is no opportunity for ice crystals or boiling.

In short, “dry gas” is no longer necessary, although a lot of folks still like to add it and that’s okay. Understand. however, that “dry gas” does not absorb water. It breaks up the water’s surface tension to allow it to mix with the gasoline and be carried harmlessly with it. A few molecules of H2O mixed with the gas is harmless as long as it has no opportunity to freeze or boil.

Condensations is the result of the heat being drawn from moisture laden air by a colder surface fast enough that the excess moisture that the now-cold air at the surface strata can no longer retain condenses on the cold surface. Since plastic does not transmit heat nearly as well (quickly) as metal, platic surfaces have less of this effect. The air passing over a plastic surface tends to lose less heat and carries more of the moisture away with it, until the temperature difference becomes greater than is necessary for condensation on metal.

Some condensation on the inner walls of the tank is also harmless. Gasahol has the same effect as the old “dry gas” and any condensation will simply be absorbed. For those without ethanol, any water vapor that condenses and does not get absorbed will simply fall below the gas, well below the pump pickup tube.

Okay, now that I’ve rambled on meaninglessly, let me state emphatically that running a gas tank low is dangerous. If you run out of gas you could get hit from behind by another vehicles. This happens more often than people realize, even to cop cars with lights flashing. And then there are the prowling crazys out there just hoping to stumble on some poor victim stalle don the side of the road. That happens more often than we like to think too.

So, running the tank low is no longer bad for the car, but is extremely dangerous for the driver.

Appreciate the thorough reply !

Thanks. I should have clarified that the gas does cool the pump, and running out can fry it even if it isn’t a submersed one. But I was too busy rambling.

So , if one feels their engine begin to sputter due to little gas in the tank , they should generally pull over and shut it down asap … right ?

AVGAS contains no butane or propane and has VERY low vapor pressure. This is to prevent vapor-lock in crude aircraft fuel systems which can date back to the late 1940’s and early 1950’s These tanks are NOT sealed and are vented to the atmosphere. In order to start these engines in cool/cold weather, carb heat must first be applied to get the avgas to vaporize…Also, many aircraft tend to sit for weeks unused. Their fuel tanks in the wings are exposed to heating by the sun, something automobiles avoid. So yes, condensation CAN be a problem in aircraft fuel tanks. But that’s NOT what we are talking about…

A thought… Todays in-tank fuel pumps pose no risk of explosion (along with the open-contact fuel gauge sender) because there is NEVER any air in the tank, or at least not enough air to form a combustible mixture. Virtually ANY negative pressure in todays fuel tanks would cause them to collapse. Normal vapor pressure for todays gasoline is 5-6 psi in a closed container. In today’s systems, once the filler cap is on, the ONLY way the tank is vented is through that carbon canister. It’s a one-way door, out only. For sure, there will be a negative pressure relief valve built in, to prevent tank collapse in the event of some unforeseen circumstance causing negative tank pressure, but that would be a rare event indeed, like someone filling up with av-gas…


Combustion isn’t the issue. Release of hydrocarbon molecules into the air is the issue. The slight vacuum keeps that from happening to comply with the EPA’s prohibition against it.

As the pump pumps out the gas, a slight negative pressure is maintained in the tank simply by the gas being pumped out. Lack of that slight vacuum is what indicates a leak in the EVAP system and trips the CEL light. Yes, it’s the resistance of flow through the charcoal bed that enables this to happen. Remember the pump is pumping fluid out of the tank, not in. Nothing is pumping anything back in, and that would be necessary to creat a positive pressure.

From a hydrocarbon molecule’s perspective the charcoal bed acts sort of like a filter. Carbon naturally attracts carbon, and the carbon in the hydrocarbon is naturally attracted to the carbon charcoal’s surfaces as the fumes pass through the charcoal bed. The oxygen, nitrogen, and argon pass through, the hydrocarbon sticks. The purge valve, activated by the purge solenoid, allows the hydrocarbon vapors to then be drawn out into the engine when it’s started and burned. The fact is that a tank can create a positive pressure under three conditions, (1) when the temperatire of the fuel and air inside rises, (2) when the tank is filled, and (3) when the fluid is agitated. It’s the charcoal bed’s job when that happens to prevent the hydrocarbon molecules from escaping.

The charcoal bed is “activated” to enhance its effectiveness. “Activating” the charcoal only means bathing it in an acid bath to make it pourous. A chunk of coal full of pores has far more surface area than a solid chunk of charcoal, and it’s the surface area that matters…the more of it there is, the more hydrocarbon it can attract and hold. The porosity also helps the oxygen and nitrogen (the air) to move through more readily as it deposits the hydrocarbons.

All gas tanks except those full to the brim pose the risk of explosion. Explosion is very rapid combustion and resultant expansion. Combustion is he process of the hydrocarbon molecule being in contact with oxygen and having sufficient heat energy applied (activating the molecule) and tearing itself apart, the hydrogen bonding to oxygen and the carbon bonding to oxygen. Only the hydrocarbon in direct contact with oxygen can do this, however, so only the surface and the fumes are combustable. If you were to have a gas tank full to the brim and put a match to it it would flame le a flamethrower but not explode. If you had on with 1" of gas you’d blow yoursel sky-high. ------PLEASE-nobody out there try to test this. Just trust me on the theory.------

Th pressure isn’t the problem. The fumes are.

Honestly, we are all happy you have a Cessna 172 airplane. But this is CAR talk, not Aviation Weekly. Our cars are not subjected to the pressure and temp. extremes that YOU experience.
And $10 a week, that’s like 3 gal.
I really wish I could live in your world and my wife and kids would return the vehicle with full tanks. I don’t know anyone who does that. Does anyone else?

Thank you for the explanation.
I believe that the engineers have taken many different scenarios into consideration and have come up with the best solution to the moisture in the fuel problem, probably years ago.
Today’s engines and computers account for most situations AND drivers habits.

The fuel draw down is never fast enough to cause a vacuum to form. The gasoline in the tank will maintain its vapor pressure as the fuel level slowly drops.

Gasoline storage tanks regardless of size seldom if EVER contain enough oxygen to support combustion. Only in the movies do cars explode…A match placed in an open fuel filler results in a small flame at the tank opening. As the flame burns down the opening, it simply goes out…Not enough air to burn any further. PLEASE do not try to prove or disprove this theory!!

I can think of ONE exception. Back in the 1950’s, outboard engine manufactures, in order to eliminate the need for a fuel pump, used PRESSURIZED fuel tanks, pressurized with AIR. When the boats they were used in caught fire because of the inevitable fuel leaks and the fire enveloped the pressurized tank, spectacular fires and deadly explosions resulted…