I note he doesn't allow for the heat transfer from the IP to the fuel back to the tank. I don't know how that will change the equation, I just know that 80 degrees (his highest temp) isn't close to how hot a tank can actually get.

 

The article says the tank will have 3/4 of an ounce of water in the fuel tank. That is PER TANK FULL OF FUEL! That's enough water to freeze in the fuel line, (that means your engine will stop) or cause the injectors to be damaged by steam, if the water makes it past the filter. I just don't see a benifit to running the tank less than full. The facts are there IS water condensing into the fuel in an empty tank.

Other considerations are;

1. Less work for the LP. A full tank is easier to pump from than an empty one. We all know about our weak LPs.
2. More security & readiness. If there is a need to get out of the area during an emergency you are already filled up and don't have to look for fuel. I realize this is a not likely, but try telling that to the folks after the hurricanes or 9/11. Be prepared, is my motto. You don't find many firetrucks, ambulances, police cars, ect. with 1/8 full tanks. Running out of fuel in the winter (if you get stuck) can be fatal.
3. Less air. The fuel is not sloshing around as much in a full tank. Air in diesel is easily suspended for long periods of time. This is a problem for LPs & IP's, ANY generation CTD.
4. We are not driving boats. (My personal favorite)
5. More weight for traction. 1/4 tank = 66 pounds, a full tank is about 270 pounds.

 

That guy that wrote the article has eaten way to many sea biscuits. As fuel is removed, moisture laden air, usally salt from water in our climate, enters thru the tank vent at say 90 degrees on a summers day. The combination of cool evening air and air conditioning on the outside of the tank causes the moisture inside the tank to condense on the sides and fall to the bottom. That is where the algae starts to grow. The algae is anerobic. In other words it survives without air. A few years ago I towed in a 54 footer with total power failure do to algae. He had gone thru a dozen secondary fuel filters and all his primary fuel filters in a 24 mile trip. Do whatever it takes but keep water away from diesel fuel tanks. Oh by the way, They where Jimmies You know Detroits.

 

Here is another boat expert....

http://www.boatus.com/boattech/casey/33.htm

 

The article was based on a 200 gallon tank. What he seems to be saying is, if you burned a tank if fuel in 86F 100% RH, pulling that air into your tank, then the tank wall temp drops to 50F, you could get 0.35 oz (0.81 - 0.46 oz) of water in a 200 gallon tank. Adjusting that to a 34 gallon tank, there would be 0.06 oz, per tank of fuel. It would take 17 full tanks of fuel to add up to 1 oz, with these conditions.

But that high of a level of water vapor in air would be rare, if ever happening;
“The maximum absolute humidity at the surface of the earth ... is 27 g/m3” Bosch Automotive Handbook.
And that would be an extremly nasty and unusual day, and unlikely to cool off to 50F that evening.

I have no idea if that is a little or a lot of water as far as diesel's go.

What I get out of this is that it isn't an issue of how full you keep your tank, but more of when you fill your tank. If you make a long drive during the day, you would be better off filling up at the end of that day rather than the next morning, that would force out the moist air before it cools down. Filling at 1/4 vs 3/4 of a tank won't make a difference if it is done after a night cool down.

The tank temp isn't going to be a factor in how much water vapor gets sucked in, that's just outside air temp and humdity levels. But it slow down the condensation by keeping things warm longer, and maybe it could re-vaporize water. Then a fill might get some of that water out even after it had condensed in the tank.

 

Keeping a tank near or at full is wise for one main reason.

No water condensation build-up on the interior tank walls.

Take a guess where all that water condensation goes when you have a tank that's mostly just air-space. Yes, you start to dillute your fuel with H2O, and you create a great breeding ground for algae, and of course water in your fuel.

I mentioned this on another forum. When a vehicle sits for some days with a low fuel volume in it's tank, the naturally heating and cooling of the day/night temperature cycle will cause water droplets to form on the inside of the fuel tank wall. The condensed water will gradually work it's way(via gravity) to the fuel at the bottom of the tank, and dillute your fuel, and also create a great petri dish for growing things.........i.e. algae.

This goes for both gasoline and diesel engines.

My dad forewarned me about this phenomena when I was first starting out in motoring as a teenager. He also stayed to the principle that a full or nearly full fuel tank has no room to create condensation, as the interior walls of the fuel tank are immersed in fuel and not exposed to the air. Full fuel tanks = less air volume = less potentional for condensation= less h2O dillution= less chance of algae.

 

I also fill up when the tank is empty. I've towed about 230K miles(30K in an 02,153K in an 03,55k on my 05) in the last three years, I seldom ever use additives and I've never had a fuel system problem. There is no technical reason not to run the truck until the low-fuel warning comes on.

 

If you tow so often, you are probably refilling the truck shortly after you hit the low fuel light. No problem there. When it comes to trucks that sit longer though, humid air displacing the fuel and then condensing in the tank with temp changes is a reality. I'm not saying it's a frequent cause of problems, but if we're talking best practices, it certainly is at least marginally better to have your tank full than empty. On an older mercedes diesel I had, the fuel filler neck and cap area became rusty because of condensation and had to be replaced.

 

I think keeping the tank full is better than letting the tank run nearly empty before refilling. If a long trip is in progress it does not matter if the tank is left until the fuel level is near empty before fueling up. When the run is over for the day, it is best to fuel up rather than leaving it until morning. If my tank is 1/4 down and the truck is going to sit idle for a few days, then I will top it up before parking. I like to keep as much air out of the tank as is practically possible.

I have observed that when my portable compressor with a 20 gallon air tank is used, just to air up one tire, that there is moisture in the tank next day when I open the drain valve at the bottom. Moisture collects in the tank even on warm sunny dry days when air is compressed into the tank. My close relation rusted a hole in his gas powered swather fuel tank. He did not leave any gas in the metal tank because someone would always steal the gasoline.

As fuel is used moisture laden air is sucked into the vents to fill the vaccum. There may be only a very small amount of water condense into the tank each time, but over the years it will add up to a detrimental amount.