Gas: Full up or half a tank???

[P1et]

Originally Posted by JohnnyDaJackal

I have a question for the Autopian engineers and car guys. I'm wondering what's more beneficial, to keep your gas tank at full (i.e. once you get to half a tank, fill it up again) or to ride the bottom half of your tank (i.e. only fill it up to around 1/2 a tank)???

Ever since I got my new car, one of my peeves is the gas gauge is off, especially at the bottom half, so I usually fill it up once I'm at or around 1/2 a tank. I don't like to chance it since my tank isn't the biggest either. I remember when I first started driving one of neighbors was advising me to always do that because it's bad to run your tank empty because of dirt getting in. I really don't know about the dirt, but I feel it wouldn't be to good on the engine to run it almost dry. Well just a couple nights ago my best friend, who is also the most argumentative person I know, was proposing that it's best to never fill it up to half, because anymore and it's extra weight on the car. I understand what he's saying, but I still feel running it low can cause negative effects to the engine. From what I understand it's good to keep at least 1/4-1/2 a tank in the winter months because of the possibility of gas lines freezing up; but I'm wondering about the other months.

So my question is which is the preferred method and why?

By the way, I've seen your pics on POTN and they are quite impressive!

[Setec Astronomy]

Johnny, most cars have non-linear gas gauges. Pat Bedard did an editorial in C/D on this years ago, he talked to some auto engineers who said it's because people don't want to get depressed by the gauge going down fast after they fill up. I'll agree that RSX's are pretty bad in this regard.

The amount of gas in the tank has no bearing on the likelihood of picking up sediment from the bottom of the tank, as that will always be at the bottom of the tank anyway. If you run the tank way down and you have a lot of water floating on top of the gas, yes, you can draw some water into the fuel pickup, and if you're unlucky you might stop the car with it in a place where it could clog a fuel line if it froze (and yes you could get stalling or hesitation if you injected a slug of water into the engine).

As noted before, the water usually gets into your tank with air as the fuel level drops in the tank. Under the right conditions, the moisture in the air will condense onto the walls of the tank and run down onto the fuel, where it will stay. You can also get water from the tank at the station, if they aren't monitoring the water level on top of the gas and run down to the pump suction level, or if you fill up during/after a tank truck fill when the gas is all stirred up with the water and previously mentioned sediment (never get gas if you see a tank truck near the station).

I'm kind of curious what kind of "dri-gas" effect is now prevalent since the MTBE additive has been replaced by ethanol and whether that has a water-absorbing effect like methanol and IPA.

To address a couple other points, when I had my "mold-mobile", I would only drive it about every six months, so the gas in there could be a couple of years old, and like Accumulator, I never had any problems...but as noted, probably not a situation to emulate. I also tend to run my tank down kind of low, mostly from a time efficiency standpoint to make less trips to the gas station, but also a bit of the weight issue in the back of my mind....I've also never had a gas line freeze.

[accordmaniac]

I have the same problem, my Civic since new has read 1/5 1/6 of a tank full when I start driving and after several minutes it reads empty and the light comes on. I've usually filled up when the light comes on and sometimes it takes a couple tries to start when it reaches that low. It is annoying to fill up half tanks so I like waiting till I need to fill it up.

[JohnnyDaJackal]

Thanks for the replies, I guess it really doesn't matter. I will probably continue to top my tank off though because as I said before the second half of my gas gauge seems to go down quite fast, and I like to monitor my mpg everytime I'm at the pump.

Originally Posted by P1et

By the way, I've seen your pics on POTN and they are quite impressive!

Thanks, I really appreciate it. I'm surprised you recognized them though because I really didn't post that much, and whatever I got posted didn't get to much attention (there are lots of good photographers on that site).

[Setec Astronomy]

Originally Posted by JohnnyDaJackal

Thanks for the replies, I guess it really doesn't matter.

Well there's no two ways about it, keeping the tank fuller will lessen the amount of condensation (water) in the tank, whether that makes an operational difference or not is another story. An occassional shot of Heat or Dri-Gas can't hurt.

[hockeyplaya13]

If you go below 1/4 of a tank it will shorten the life of the fuel pump and they aren't exactly cheap. But I'm sure you could do the math to figure out how much gas you would save from not carrying that extra weight and it would probably be more than the fuel pump.

[Bebopp]

Originally Posted by Setec Astronomy

If you run the tank way down and you have a lot of water floating on top of the gas, yes, you can draw some water into the fuel pickup, and if you're unlucky you might stop the car with it in a place where it could clog a fuel line if it froze (and yes you could get stalling or hesitation if you injected a slug of water into the engine).

Gasoline floats in water, so the water will be at the bottom of the tank, not the other way around. As soon as you get water in the tank, it's the first thing that gets sucked in the engine so having either half or full tank doesn't matter. You need to avoid getting water in the tank in the first place.

Ethanol actually bonds with water, and therefore gasoline with ethanol can "absorb" a certain amount of water.

Gasoline by itself has a very low freezing point, anything from -90F to -200F. It varies because of the different compounds in the gasoline. If there is water not fully absorbed in the gasoline, this will crystallize at 32F - however, water is rarely a problem in gasoline tanks.

I don't see any problems with keeping fuel in a car tank for long periods of time, unless temps drop below -90F.

[Setec Astronomy]

Originally Posted by hockeyplaya13

If you go below 1/4 of a tank it will shorten the life of the fuel pump and they aren't exactly cheap.

I regularly go below 1/4 tank and I had my last car for 16 years/125K miles with no fuel pump replacement

[Bebopp]

Originally Posted by Setec Astronomy

Well there's no two ways about it, keeping the tank fuller will lessen the amount of condensation (water) in the tank, whether that makes an operational difference or not is another story. An occassional shot of Heat or Dri-Gas can't hurt.

I don't see how this can happen - the water in gasoline is already "condensed", i.e. it's already in the gasoline, and the only way for it to condense in the tank walls is for it to evaporate from the gasoline. This means you have to heat up the gasoline to get the water to evaporate, then the evaporated water can condense to go back in the gasoline... this clearly doesn't happen.

Once there is water in the gasoline, and you pump it in your tank, it will stay there until it gets sucked into the engine, or it gets mixed with the ethanol.

Water in gasoline is rarely a problem in city environments - maybe in remote areas where storage or transport isn't perfectly sealed...

[Bebopp]

Originally Posted by hockeyplaya13

If you go below 1/4 of a tank it will shorten the life of the fuel pump and they aren't exactly cheap. But I'm sure you could do the math to figure out how much gas you would save from not carrying that extra weight and it would probably be more than the fuel pump.

How will low fuel in the tank lessen the life of a fuel pump?

[Setec Astronomy]

Originally Posted by Bebopp

Gasoline floats in water, so the water will be at the bottom of the tank, not the other way around. As soon as you get water in the tank, it's the first thing that gets sucked in the engine so having either half or full tank doesn't matter. You need to avoid getting water in the tank in the first place.

Sorry, I was having a brain fart (sometimes I get too much stuff in my head and it pushes some other stuff out...or it could be I was at work and had half my brain occupied). The fuel pickup, whether it is in a vehicle or a stationary tank (gas station) is above the bottom of the tank to preclude water ingestion unless the water reaches a certain level. That's why gas stations use something like this: Water and Gasoline Finding Paste when they are sticking the tank to make sure the water level doesn't get to the height of the pickup (at least that's what they did last time I worked in a gas station...in the last century). Stirring up the tank by filling it or jostling it complicates things.

Fact of the matter is, unless you continuously top off your tank, you will get air in your tank, which will have moisture in it, which will condense under the proper circumstances (this of course includes the stationary tanks as well). Fuel system engineers know this, and design to tolerate a certain amount of water in the system. Periodically using an absorber will reduce the accumulation of water in the system, as will periodic draining (usually reserved for extreme problems). Honestly, I can't remember the last time I heard of someone having a water-in-the-gas problem.

[BigJimZ28]

Originally Posted by BigJimZ28

the gas keeps the fuel pump cool the more gas the longer the pump will last

Originally Posted by Bebopp

How will low fuel in the tank lessen the life of a fuel pump?