I'm not sure how many people are familiar with the term "sour gas" but it refers to gas that gets too old, and basically as I understand it, old gas builds up peroxide (I'm not sure by what chemical mechanism) which can damage elastomeric engine parts, leading to corrosion.

Today someone asked me when the last time I filled my tank was. I had no idea. I have about 12,300 miles on my HCH, which I bought at the end of March of 2004. This means it takes me approximately 1 month to go through a tank of gas. Does anyone know how long it takes for gas to "turn" ? I have no idea how long is too long, and wasn't successful in google-ing the answer. I also apologize if this question's already been answered here before. I've started to think maybe I should only fill up half-way to keep my car lighter and prevent my gas from turning bad.

My next question is even tougher: does anyone know if ethanol and biodiesel also "sour" ?

I have no evidence one way or another, but I suspect it takes quite a while for the reactions to take place to any significant degree, upwards of 6 months to a year. I was told in auto shop that if you want to store a vehicle long-term (years) it's best to have a full tank of gas and the use of an additive 'stabilizer' would prevent the gas from going sour. Propane does not go sour, but I'm not sure about diesels or ethanol, but I would imagine ethanol could deteriorate into some kind of acid, probably acetic (vinegar), but that's just speculation.

It's been almost 3 months since I filled my Colt, so if the problem was significant I would smell it, although I did use an ethanol blend since the stupid thing drinks high octane. Luckily for me I've avoided all the crazy hurricane pricing and now it's almost back to what I paid months ago.

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'97 Prius Simulation Prius inner workings

I have read that the reformulated (oxygenated) gas starts to break down in under a year. The non-reformulated gas lasts a lot longer than that from what I understand. Unless your car is sitting unused for a long time, I wouldn't worry about it. If you don't have to buy reformulated gas in your area, you have even less to worry about.

All fuels 'go bad' at some point. Mostly from the heavier hydrocarbons separating out and forming a thicker liquid that gumms up the works. If you've ever seen a varnished-up lawnmower carbeurator, that's what caused it. This also effectively lowers the octane, making it less suitable for engines. At the same time, the 'good' hydrocarbons are vaporizing out if its not airtight.

The rule of thumb for regular gasoline in ~1 year to be safe. Probably rather longer, but a year is easy to remember.

Reformulated gas, especially with ethanol instead of MTBE, can have a shorter shelf life. Entrained water (there is always some) will cause the ethanol to preferentially separate with the water, and not remain mixed with the gas. Most gas is well under 1% water, but levels of 0.5% will cause this.

Gas stabilizer additives will work, but even they only do the trick for awhile. If you're not using any gas engine for more than a few months, drain the tank.

Diesel will normally last longer, but it is possible to skunk up diesel- an example is in sailboats where the engine may not be used all that much and the fuel can sit for several seasons.

we were told when we bought our generator that we should put in stabilizer for the long years wait - about 9 months.

Gas goes "bad" after a few months. Small discplacement high compression engines are more prone to be affected by it. If you run a tank a month, that's not a problem, but if you go beyond that I would buy a bottle of "Stabil" (fuel stabilier) and also buy a bottle of gasline antifreeze to remove any water that may condense in the tank.

Both water in the fuel and stale gas will affect your car negatively.. the stale gas over time will damage your fuel system and the seals associated with it.

take the advice or not... ask any motorcyclist and they'll tell you the same.

With Guns, Trucks and Beer, Who Needs Women?

Years ago when I had a Subaru Outback, I had to park it for about 3 montsh and was worried about fuel 'souring'. I added a fuel stabilizer as suggested by some. It's an additive that is supposed to prevent souring. On returning from the trip, the check engine light came on and the car went into limp home mode. It cleared itself after siphoning the fuel out and putting in new fuel without the additive. Twice after that did I have to park the car for about 3 months, and I did not use any additives at that time, and had no problems.
In the old days it was recommended to top off the tank before long term storage, to reduce condensation. With the Prius fuel bladder that is not needed, and I would think with the Prius II it would be better to park with very little fuel, so right after resuming operations you can top off with fresh gas.

sure.. let make up "new" rules...

fine.. dont listen to me.. .talk to a mechanic or an engine builder and they will tell you exactly what I posted.

With Guns, Trucks and Beer, Who Needs Women?

dumb question but instead of waiting for your gas tank to empty, why not refuel every 2 weeks? that way your gas remains "fresh"

If you ever want a whole tank full of stale gas without having to park your car for three months, just buy a tank of gas in Ocean City in January. Years ago, I had a friend who lived in OC. I visited him just after Christmas and filled up the tank there. I *barely* got home! the car seemed to be running on only 2-3 cylinders and would only go 50 miles per hour with the pedal to the floor.

The problem fixed itself after two tanks of "fresh" gasoline. But it was quite an eye-opener.