Gas: Full up or half a tank???

[Setec Astronomy]

Originally Posted by Bebopp

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...

When you draw fluid out of closed tank, you will pull a vacuum in the tank which will prevent fuel draw and cause pump cavitation. The pressure is equalized through vents which let air into the tank to replace the volume of the fluid withdrawn. The air is of course, ambient air, which contains moisture. If the tank is refilled while the temperature of the air is above dewpoint, the "moist" air will be expelled during the filling process (which of course with today's vapor recovery systems, means that moist air will go back into the underground storage tank). If the tank cools below dew point (say, you fill up in the morning, drive off some portion of the tank which is replaced by ambient air, then park the car for the night), the moisture in the air will condense, just as it does on the outside of your car (dew).

[az57chevy]

For the garage queens we need to keep the tanks full. Moisture in the air condenses as it cools and so the more air in the gas tank the more water will condense.

Our gas is so junky you need to use stabilizer. I've had gas in a lawn mower (admittedly a less high tech machine) refuse to ignite after three months of outside storage.

Fuel pumps need cooling and that's what the gas in the tank provides.

OCCASIONALLY you need to run it down to nearly empty to help burn off residue, gunk etc. Otherwise it will build up.

[Setec Astronomy]

Originally Posted by BigJimZ28

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

That's presuming that the fuel pump is damaged by temperatures below the boiling point of gasoline. If the pump is immersed, it can't get hotter (other than local hot spots) than the boiling point of the coolant (which will be somewhat increased by positive tank pressure, if that exists). I presume you think the bulk temperature of the fuel in the tank will be increased by traveling thru the fuel rail in the hot manifold area and back to the tank (plus frictional heating from the pumping), but the ambient is quite a bit lower than fuel rail temp...my gut feel is the fuel in the tank isn't going to get heated much in the final analysis...heck, the tanks are plastic today (which of course reduces the radiation and skin convection due to the lower thermal conductivity of plastic vs. metal, which means the fuel could be hotter today than way back when).

[Accumulator]

Originally Posted by P1et

I have a '97 Dodge that has had the same fuel sitting in it for almost two years now. I haven't started it yet, but am hoping it will be just fine.

The above post, however, scares me a bit!

I let the XJS sit with the same untreated gas for several years once, no problem. But I guess that doesn't mean that your experience will work out as well

Given how long your Dodge has sat, I'd disconnect the fuel pump and crank it for a while (with no fuel delivery) to pre-lube everything before you try actually starting it up.

[Spilchy]

Originally Posted by Accumulator

The gas in the Jag sits there for a *lot* longer than a month...actually a month is nothing for my vehicles, many of them sit for that long at a stretch. I've *never*, not once, had a problem with gas in an auto going bad and I've never used any stabilizer additives.

Yeah, the doctor that I detail the vintage 450 SEL for lets his car sit and never starts it or add stabilizer. The gas has got to be at least a couple years old (refreshing a few gallons like once a year). The car always starts on the first crank after a little priming. We fired it up last weekend to pull out to snap some photos for him so he can send them in with his application for antique plates.

He does use stabilizer for his vehicles at his vacation home that just sit there unused for years like an old Honda Prelude, Buick station wagon and a 1948 Lincoln Cosmopolitan with suicide doors.

As far as my cars, I never allow to drop below a 1/3 of a tank. I just sold my 12 year old Honda Civic that was getting 30 mpg on the highway with no additives or cleaners. The only thing I would do is change the fuel filter a bit earlier.

[ZeusCGP]

I usually fill to 1/2 and refill at 1/4. I like to save weight.

[Lowejackson]

I drive an Alfa so anything less than an indicated 1/4 of a tank probably means an empty tank...or its half full or quite possibly 90% full

[pgp]

I usually fill it when it gets to half a tank, for the above mentioned reasons. I always have gas which is good in the winter, and if I am short on money I can drive a little while. You can't do that when it's empty.

[Setec Astronomy]

Originally Posted by Lowejackson

I drive an Alfa so anything less than an indicated 1/4 of a tank probably means an empty tank...or its half full or quite possibly 90% full

What is that, a "Lucasazione" fuel level sender?

[hockeyplaya13]

Originally Posted by Setec Astronomy

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

Lucky. We had the suburban for 5 years and then had to replace it. The guy told us not to go below 1/4 of a tank, but I don't remember if we went below that regularly or anything so I don't know if that was the cause or not. I'm just saying what he said.

[KnuckleBuckett]

Work in an automotive development lab for 20 years.

Keep it full.

When filling your tank at the station, pump no faster than the mid setting.

[mikebai1990]

Originally Posted by KnuckleBuckett

When filling your tank at the station, pump no faster than the mid setting.

Could you explain the logic behind this?