How full to keep the gas tank?

I have heard several theories on gas tanks....
.....never let the gas tank get almost empty.
.....always fill up before the tank is 1/4 full.
.....always use a fuel injector cleaner every dozen tanks.
.....fuel injector cleaners are worthless.

Anyone got any theorys/advice?

--

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VOTE HIM OUT! November 4, 2004

 

||....never let the gas tank get almost empty.
||....always fill up before the tank is 1/4 full.

Almost all modern cars have a fuel pump immersed in the fuel tank, with the
motor above the pickup point. When it gets low, the motor can be partially
uncovered. Since the motor is cooled by the fuel running through it and around
it, it can run hotter if the fuel level is very low. Habitually running the
tank to the last drop can shorten the life of the pump.
The best way to kill a pump, is to park on a steep slope when the fuel level
is low, uncovering the pump and pickup. It won't start. By the time it dawns on
you to let the car roll down onto the flat, you have already damaged the motor
by letting it run without coolant/lubrication during the dry cranking attempts.
It will probably start once you get it on level ground, but it's days are then
numbered.

Texas Parts Guy

 

Not a good idea to run the fuel pump dry.

I'll just say a full tank is generally good. More air in
the tank may increase condensation. I think there's also
a greater likelihood of varnish in the tank with more
oxygen.

I do, but a good quality gas is probably "good enough".

They work to varying degrees. Techron Concentrate is
supposedly quite good, although others swear by Redline
SI-1 or BG 44K.

 

OK. I hardly ever drive the car to the point where the GAS light comes on,
and if it comes on when I am driving, I fill up.

Do you think that I am safe, or should I fill up when the tank is 1/4 full?

--

http://www.bushflash.com/thanks.html
"Bubba got a blowjob, BU$H screwed us all!" - Slim
http://www.worldmessenger.20m.com/weapons.html#wms
George "The AWOL President" Bush: http://www.awolbush.com/
WHY IRAQ?: http://www.angelfire.com/creep/gwbush/remindus.html
http://www.toostupidtobepresident.com/shockwave/chickenhawks.htm


VOTE HIM OUT! November 4, 2004

 

||
||
||Rex B wrote:
||>
||> ||....never let the gas tank get almost empty.
||> ||....always fill up before the tank is 1/4 full.
||>
||> Almost all modern cars have a fuel pump immersed in the fuel tank, with the
||> motor above the pickup point. When it gets low, the motor can be partially
||> uncovered. Since the motor is cooled by the fuel running through it and
around
||> it, it can run hotter if the fuel level is very low. Habitually running the
||> tank to the last drop can shorten the life of the pump.
||
||OK. I hardly ever drive the car to the point where the GAS light comes on,
||and if it comes on when I am driving, I fill up.
||
||Do you think that I am safe, or should I fill up when the tank is 1/4 full?

Without knowing the specifics of your car, and seeing the way the pump is
positioned with respect to the sensor for the light, I would not have a clue.
Err to the side of caution, and try to fill up at the 1/4 mark.
Texas Parts Guy

 

Let's assume that the fuel pump and motor assembly design is such that gasoline
is circulated within the assembly casing and around the motor, to cool the
motor, in the gasoline's path from assembly inlet to outlet. (I haven't
confirmed this yet, so I'm going to make this assumption for now.) In this case,
all that is necessary to ensure this alleged 'motor cooling flow' continues is
for the assembly's inlet to be below the gasoline level in the tank.

For reference on this, drawings of a fuel pump in a fuel tank for a 1996 Civic
appear at

http://www.honda.co.uk/owner/CivicManual/pdf/11-87.pdf

http://tinyurl.com/2flnq (Majestic site)

I'm not buying it. Have any citations that discuss the motor-pump assembly
operation?

attempts.

The condition you describe is the same as running out of gas.

So you're claiming that a few starts of the engine (and so fuel pump-motor
assembly) on an empty tank will destroy the assembly?

I suppose starting a car on an empty tank may reduce the life of the fuel
pump-motor assembly, but I'm doubtful, for now, that the reduction is
noticeable.

Citations welcome.

I think the original poster is perfectly safe continuing to drive such that the
gas warning light hardly ever comes on. If it does come on, and he/she fills up
right away, I remain confident that the system is designed so that no damage
occurs.

It would be a pretty silly design for the gas warning light to come on right as
damage starts occurring. Some margin of safety surely is built in.

I do like y_p_w's theory about a full tank tending to prevent condensation etc.
buildup. I've seen this discussed before. E.g. from the Car Talk guys re winter
driving:

http://www.cartalk.com/content/features/WinterDriving/
---
Keep your gas tank close to full, for a couple of reasons.
In the summer, you can take a chance and run down to fumes. But in the winter,
if you do get stuck or stranded, the engine will be your only source of heat.
And you don't want to have to worry about conserving fuel and saving the planet
right at that moment...you want to stay warm. (And make sure you keep a window
open a crack if you're sitting there with the engine running. We can't afford to
lose any listeners from carbon monoxide asphyxiation.)

The other reason for a full tank is that warm daytime temperatures will fill the
empty space in the tank with moisture, which will condense during the cold
night. This water will sink to the bottom and, sooner or later, rust out your
tank.
---

They amend this somewhat at
http://cartalk.com/content/columns/Archive/2003/March/02.html , noting that they
have seen less and less gas line freezes (due to condensation buildup, due to
too much air in the gas tank and not enough gas) over the years.

(Tom & Ray owned a garage together for a number of years. One of them still
works a full-time job at his garage.)

 

I'd sure welcome any citations people have on fuel injector cleaners improving
engine lifetime and performance.

I think popular sentiment leans toward Chevron Techron. It's only about $6 a
bottle and preferably is used about every oil change.

After 152k miles, my 1991 Civic was getting great mileage (40+ MPG) as long as I
kept the PCV valve clear. It would clog up a bit about every two tanks of gas. I
never used a fuel injector cleaner or any other additive. I buy only regular
gasoline and am not picky about where I buy it.

Someone else reported recently that after 200k miles his Acura's PCV valve was
the original and had never fouled. Into the car he puts only "Esso/ExxonMobil
gas, whic is laden with MMT. Regular in the winter, premium in the summer."

For the first time a few weeks ago I put a bottle of Chevron Techron in my car's
full gas tank. I'm not certain yet but after one tank of gas my rough impression
is the PCV valve is staying cleaner longer.

 

Man, I would love to be able to get a few starts and maybe a little mileage
out of an empty tank

"> So you're claiming that a few starts of the engine (and so fuel
pump-motor

Starting a car on empy, have you thought about submitting this as an
alternative fuel source

Hugh "The phrasing was good for a few chuckles" Graham

 

Hugh,

I am happy to make you aware that the starter motor, crankshaft, and DC-powered
fuel pump turn, and the spark plugs fire, when starting a car with no fuel in
the tank. If they didn't, there'd be no cause for concern.

Thank you for the chuckles. ;-)

 

||> Habitually running the
||> tank to the last drop can shorten the life of the pump.
||
||I'm not buying it. Have any citations that discuss the motor-pump assembly
||operation?

Here's a start. A google search gets lots of hits.

http://www.alldata.com/techtips/2002/20020920d.html

Texas Parts Guy

 

||
||I do like y_p_w's theory about a full tank tending to prevent condensation
etc.
||buildup. I've seen this discussed before. E.g. from the Car Talk guys re
winter
||driving:
||
||http://www.cartalk.com/content/features/WinterDriving/
||
||The other reason for a full tank is that warm daytime temperatures will fill
the
||empty space in the tank with moisture, which will condense during the cold
||night. This water will sink to the bottom and, sooner or later, rust out your
||tank.
||---

This is written about boats, which should have more problem with condensation
than cars. Boats do not have sealed systems to the extent that cars do.

The Myth of Condensation in Fuel Tanks
by David Pascoe

Frequently we hear it said that the cause of water in fuel tanks is due to
condensation. I have long doubted this assertion but the issue has come up so
frequently that I was finally motivated to try prove to the point. The basis of
my belief or assumption is that:

There isn't enough air volume within a tank to hold much vapor.

On average, tanks are half full, further reducing volume

The amount of water vapor in air is very small, even at 100% humidity

Conditions aren't right to cause condensation in a fuel tank

Research produced the following values for the maximum amount of liquid water in
air at the following temperatures:

30C/86F 30 grams/cubic meter
20C/50F 17 grams/cubic meter
10C/13F 9 grams/cubic meter

There are 28 grams per ounce, so 30 grams = 1.07 oz; 17 grams = 0.6428 oz.

A cubic meter equals 264 gallons of liquid volume, therefore:

A 200 gallon tank = 0.76 cubic meter.

At 86F, an empty 200 gallon tank could contain 22.8 grams of water vapor, or
0.81 oz.

At 50F, an empty 200 gallon tank could contain 12.92 grams of water vapor, or
0.46 oz.

Note that this is the maximum amount of water vapor that a completely empty tank
could contain, in neither case a full ounce of water.

In order to condense water out of the atmosphere a surface must be much colder
than the air. The problem for the condensation in tank theory is; how do we end
up with a fuel tank that is much colder than the air? One way would be to have a
very cold day that suddenly warms up dramatically, but when does this ever
happen? The weather can turn cold very fast, but does not suddenly get very
warm.

Aluminum is second only to copper for rapid heat transfer properties; it will
therefore adjust to atmospheric temperature changes quickly. Gasoline and diesel
fuel, like water absorb [sic] heat and cold slowly. Thus one might expect to see
sweating on the outside of a tank as the day warms up from cold mornings, but do
we? Well, I can say that after 35 years of inspecting boats, I've rarely seen
tanks sweating. Note: Sweating may be likely to occur with boats in very cold
waters when warm days are encountered.

This issue first came up a number of years ago over a question of whether
internal engine rusting could be due to condensation caused by sudden
temperature changes as from day to night and vice versa. Since that time,
inspection of hundreds of engines showed that rust only occurs on the underside
of valve covers due to water contamination of the oil. Very few engines have
rusty undersides of valve covers, thereby proving the point that ice cold engine
blocks in the morning don’t sweat at it warms up during the day. If that is
true, then how could it be true that fuel tanks sweat?

My answer is that they don’t and these calculations prove it. My initial
assumptions were correct. You do not need to store or lay up your boat with full
fuel tanks. If you are getting water in your fuel, it is getting there some
other way.

http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm


Texas Parts Guy

 

Thanks. This site seems to focus on running a fuel pump without fuel, which I
agree is "not good" for the pump. I'm more curious about your claim that the
pump's motor is not adequately cooled when the fuel tank falls below, say, 1/4
tank.

I did google and saw several sites claiming pretty much as you do on this. And I
found just one internet forum site where someone argued along the lines I do,
with his own experience to back it up.
http://www.canadiandriver.com/discuss/messages/68/14277.html?1087480360 (see
Articsteve's comment).

I have a 1991 Civic (fuel injected) and it has a fuel pump in the fuel tank. I
bought the car new and have almost always run the car until it was down to 15%
or more of the fuel tank's capacity, filling the 11.-- tank with at least 10
gallons most of the time at fill-ups. No problems that I'm seeing. The car has
153k+ miles on it.

Also, your explanation doesn't make sense to me, as cooling flow around the
motor comes, you seem to claim, from the pump itself. The pump will move fluid
as long as its inlet is submerged. Plus, even if the motor becomes uncovered at,
say, 1/4 tank, I'm not buying that the relatively stagnant surrounding air can't
adequately remove its heat. It can't be a very high horsepower motor, after all.
(Some electric pumps are located outside the fuel tank, so submersion isn't at
all needed for cooling of them. Are these pumps' lives shorter? Not that I've
ever read.) I'm not even sure that the assembly becomes uncovered until very low
fuel levels--much lower than 1/4 tank, I bet. Otherwise, it seems like the
owner's manuals or various service manuals would warn about this. Mine does not,
for one. Nor does Chilton's.

But again, I see all the sites that make the claim you do. I certainly don't
want to argue with folks who want to err on the safe side, especially since
there are other good reasons not to drive until nearly empty.

 

It does (or did, the link someone else posted says this is an
increasingly rare occurrence) and I experienced it with my carbureted
Porsche 911. The winter of '76-'77 was brutal in the northeast and in
the spring I developed a problem in which the engine would stall after
going up a steep hill on my way to work. It always restarted promptly
afterward (on the level). I was told to check the gas tank for
condensation and drained a lot of water from it. Immediately back to
normal operation after doing this.

The owner of a Porsche dealership in Ohio and an active member of the
Porsche Club said they were seeing a lot of this in his service dept.
and were draining a quart or more of water from some cars. I don't know
what the standard Porsche gas tanks were made from back then. Remember
that in those days gas tanks were vented to the air.

It doesn't happen overnight, takes months to accumulate.

If this did occur in modern Hondas, would the water affect the injectors?

 

"More oxygen" is not going to affect the rate of oxidation of fuel. A
higher partial pressure might do that. But the partial pressure is not
likely to vary with the amount of fuel in the tank under normal
circumstances.

But obviously there will be more varnish precursors (olefins, etc.) if
there is more gasoline. Therefore the conclusion is to have less, not
more, gasoline if you want less varnish.

Not much of a consideration anyway for good quality gasoline.

O.

 

As others have suggested, this is not much of a consideration these
days with sealed fuel tanks and with covers over filling pumps to
protect against water from rain while filling.

O.

 

On Wed, 23 Jun 2004 20:30:07 GMT, (Rex B) wrote:

||On Wed, 23 Jun 2004 16:25:30 GMT, "Caroline"
<>
||wrote:
||
||
||||> Habitually running the
||||> tank to the last drop can shorten the life of the pump.
||||
||||I'm not buying it. Have any citations that discuss the motor-pump assembly
||||operation?
||
||Here's a start. A google search gets lots of hits.
||
||http://www.alldata.com/techtips/2002/20020920d.html
||
||Texas Parts Guy

Here's another:

http://pages.sbcglobal.net/gorf/bmwtips/maintfuelpump.html


Texas Parts Guy

 

Now I'm totally paranoid about ever again driving my car with less than "a few
gallons of gasoline" in it. ;-)

After 13 years and 153k miles on the 1991 Civic, with no fuel pump problems, I
reckon I'll just keep risking it. I've always wanted to have to replace a fuel
pump, anyway.

(Seriously: I agree with Rex that folks should err on the side of what they feel
is safest for their auto-mobiles. Plus, hey, a few gallons in Honda's biggest
car is pretty low on gas. It's still over about 20% on my car.)

 

Did he suggest starting it on empty? He said "very low".

 

||snip
||> Here's another:
||> http://pages.sbcglobal.net/gorf/bmwtips/maintfuelpump.html
||
||
||Now I'm totally paranoid about ever again driving my car with less than "a few
||gallons of gasoline" in it. ;-)

I think it's more something to be aware of, than something to worry about.
We routinely drive our VW Beetle till the low fuel warning gong sounds, then
fill it up. And we do slosh the fuel quite a bit
My CRX reads dead-empty long before it's truly low, so it's a non-issue
there. I don't take a chance, but it fills up with only 7 gallons or so.
But I'm a little more careful with my Ford pickups, as a I believe them to
be more vulnerable in this respect (need to change one now). I'd extend that to
GM products, as the pump on my Astro van failed not long after I tried to start
it with a low tank while parked on a hill (had to let it roll back).
This is not based on fact, but on a small anecdotal sample, plus completely
unsubstantiated prejudice
Texas Parts Guy

 

I was thinking along the same lines: I bet certain makes and models are somewhat
more prone to this problem than others.

From all the discussion at sites on the net, it seems to me there must be plenty
more with experience like you had with your Astro Van.

Plus, something(s) has (have) to explain the difference in fuel pump system
problems in Consumer Reports' annual survey.

Ha ha... I know what you mean.

Of course Honda's distributor design is flawless. ;-)