Condensation in fuel tank
 

I have heard that you should keep your fuel tank as full as possible,
to reduce the exposed surface on the interior of the tank, on which
condensation (ie. water) can form.

Is there any truth to this?

Also, is it ever dangerous (other than the risk of spilling petrol
in the process!) to fill up the tank completely, to just before where
the cap goes? I'm referring to the tanks where this is physically
possible, of course. In other words, is there ever any need for
'a bit of air in the system' ?

Condensation in fuel tank
 

On 30 Oct 2005 20:32:13 -0800, "Old Wolf" >
wrote:

>I have heard that you should keep your fuel tank as full as possible,
>to reduce the exposed surface on the interior of the tank, on which
>condensation (ie. water) can form.
>
>Is there any truth to this?

Actually, you are keeping =air= out of the tank, which carries
moisture ( water vapor ) with it.

>Also, is it ever dangerous (other than the risk of spilling petrol
>in the process!) to fill up the tank completely, to just before where
>the cap goes? I'm referring to the tanks where this is physically
>possible, of course. In other words, is there ever any need for
>'a bit of air in the system' ?

When the gas pump clicks OFF, that's it. No *topping up.* You need
that extra air space in the tank to allow for expansion.

Lg

Condensation in fuel tank
 

Old Wolf wrote:
> I have heard that you should keep your fuel tank as full as possible,
> to reduce the exposed surface on the interior of the tank, on which
> condensation (ie. water) can form.
>
> Is there any truth to this?
>
> Also, is it ever dangerous (other than the risk of spilling petrol
> in the process!) to fill up the tank completely, to just before where
> the cap goes? I'm referring to the tanks where this is physically
> possible, of course. In other words, is there ever any need for
> 'a bit of air in the system' ?

Filling to the level of the cap would be a bad idea for several
reasons. If it's that close to the top any expansion will force fuel
out the vents and be a hazzard. Even filling the tank beyond the
automatic shut-off or so can cause emission system quirks and problems
that may delay you from starting and can turn on indicator lights. Much
of the fuel sold today already has alcohol added as an oxygenate and
that will absorb some moisture. And running off the top half of the
tank makes your in-tank electric pump's job easier, but you don't have
to go crazy over filling.

Condensation in fuel tank
 

"Lawrence_Glickman" > wrote in message
...
> On 30 Oct 2005 20:32:13 -0800, "Old Wolf" >
> wrote:
>
> >I have heard that you should keep your fuel tank as full as possible,
> >to reduce the exposed surface on the interior of the tank, on which
> >condensation (ie. water) can form.
> >
> >Is there any truth to this?
>
> Actually, you are keeping =air= out of the tank, which carries
> moisture ( water vapor ) with it.
>
> >Also, is it ever dangerous (other than the risk of spilling petrol
> >in the process!) to fill up the tank completely, to just before where
> >the cap goes? I'm referring to the tanks where this is physically
> >possible, of course. In other words, is there ever any need for
> >'a bit of air in the system' ?
>
> When the gas pump clicks OFF, that's it. No *topping up.* You need
> that extra air space in the tank to allow for expansion.
>
> Lg


Most gasoline tanks I am aware of include an extra isolated space to handle
fuel expansion, either in the way the tank is shaped, or in a some cases, an
actual add-on remote reservoir of some sort (my 1975 280Z had one of those).
The only danger from over-filling the tank (aside from spilling gas on
yourself, or the ground) is forcing gasoline into the charcol canister and
saturating it with liquid gasoline. The "click-off" point for the automatic
nozzles hardly means anything. As far as I can tell they are almost random.
I've not had a car in ages that you could not add significanly more gas
after the first "click off" point. Unless you stand there and spend a
significant period of time dribbling gasoline into the tank, I don't think
there is much risk of filling the tank so full that you need to worry about
expansion of the fuel forcing gas out into the charcol cannister. The last
car I owned where this was even a possible problem was a 1974 Jensen-Healey.
The tank was mounted high between the passenger and luggage compartments,
with a central filler neck. The neck went straight into the middle of the
tank but included an internal lip of sorts that left an air gap of more than
an inch between the lower edge of the filler pipe and the top surface of the
tank. If you tried really hard, and jiggled the car a lot, you could add gas
and partially fill the gap. But even then I never had any problem with
forcing the fuel out into the cannister.

Ed

Condensation in fuel tank
 

"Al Bundy" > wrote in message
oups.com...
Much
> of the fuel sold today already has alcohol added as an oxygenate and
> that will absorb some moisture.

Very true.

The problem is that once the saturation limit is reached for
moisture in the alcohol fraction, precipitation will occur. Not only will
a large portion of the water settle out but it will bring a good bit of the
alcohol out with it. At this point, it is very difficult to put the wet
alcohol and gasoline back together again.

A good dispersant can be a helpful additive to gasoline, especially
during the colder winter months and in high humidity situations. A lot
of additives are based on alcohols alone, but the better ones contain
surfactants which help keep any moisture moving.

Condensation in fuel tank
 

Old Wolf > wrote:
>I have heard that you should keep your fuel tank as full as possible,
>to reduce the exposed surface on the interior of the tank, on which
>condensation (ie. water) can form.
>
>Is there any truth to this?

In the case of airplanes, yes it is important. In the case of cars, I
don't see it being all that big a deal, and if it is, you can always use
a little methanol in the tank. Now, if you are normally leaving your
car a week or two between drives, it might be an issue then.

>Also, is it ever dangerous (other than the risk of spilling petrol
>in the process!) to fill up the tank completely, to just before where
>the cap goes? I'm referring to the tanks where this is physically
>possible, of course. In other words, is there ever any need for
>'a bit of air in the system' ?

Modern cars with tank ventillation through a charcoal canister can be
screwed up by overfilling. You can force fuel into the charcoal
canister, and now everything starts smelling like gasoline all of a
sudden.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Condensation in fuel tank
 

Condensation isn't a worry in hot weather. I don't bother to fill the tank
full in summer. But since I don't drive the car much in winter I fill the
tank in the fall when we start to get frost (condensation) and keep it
filled until spring.


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Condensation in fuel tank
 

"William R. Watt" > wrote in message
...
>
> Condensation isn't a worry in hot weather. I don't bother to fill the tank
> full in summer. But since I don't drive the car much in winter I fill the
> tank in the fall when we start to get frost (condensation) and keep it
> filled until spring.

Some places condensation can be a problem even in summer, but is usually
much worse in winter.
When the water freezes and clogs the filters, lines, etc, it can really be a
PITA,
since it is too late to get the 'chemical' to the spot where it is needed.
Alcohol
in the fuel has solved a lot of that sort of problem, although there are
still
places where it is not used.

Condensation in fuel tank
 

defintely a problem in marine fuel tanks. We actually employ fuel separators
to keep the water
in a separate bowl.

If your in a warm climate, I wouldnt worry too much but there's truth to it.
Its severity varies.

my .02



> wrote in message
m...
>
> "William R. Watt" > wrote in message
> ...
> >
> > Condensation isn't a worry in hot weather. I don't bother to fill the
tank
> > full in summer. But since I don't drive the car much in winter I fill
the
> > tank in the fall when we start to get frost (condensation) and keep it
> > filled until spring.
>
> Some places condensation can be a problem even in summer, but is usually
> much worse in winter.
> When the water freezes and clogs the filters, lines, etc, it can really be
a
> PITA,
> since it is too late to get the 'chemical' to the spot where it is needed.
> Alcohol
> in the fuel has solved a lot of that sort of problem, although there are
> still
> places where it is not used.
>
>

Condensation in fuel tank
 

"ed" > wrote in message
news:WNB9f.1853$0d.533@trnddc06...
> defintely a problem in marine fuel tanks. We actually employ fuel
separators
> to keep the water
> in a separate bowl.
>
> If your in a warm climate, I wouldnt worry too much but there's truth to
it.
> Its severity varies.
>
> my .02

Absolutely. We used to provide chemical inhibitors for those situations,
ed, to keep
bacteria, mold, etc depressed in the wet fuel. Chemical treatment and the
use of water
separators could prevent trouble.