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Does old fuel lose octane

The fuel in my tank is maybe 3 or 4 months old. I got to take my car out this week cause the roads are clear of salt. I thought that I heard the motor detonating a little on one blast. The only thing I could think that would cause it was old fuel. I did back the timing off 2%.

Anyone know?

Thanks
Dean

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Old 02-22-2006, 08:56 AM
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If anything it probably gains octane as the volatile fractions evaporate off.
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Old 02-22-2006, 09:29 AM
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probably gummed up an injector or other fuel injection component, and may be runnign lean on one or 2 cyllinders... that would cause popping and backfiring


run some techron in yer tank(preferably when it's not fully filled)
then take her out for some rev spanking...
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Old 02-22-2006, 09:35 AM
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octane

yes fuel losses octane over time. this is my first post so i hope this goes through. working at an airport for an individual who collects many street/dirt bikes vehicles and planes we run into this always. if you do not have a cat you can run av gas and store for years. if you have a cat you are screwed because av gas will fry cat ASAP, so you will be buying STABIL for ever and paying thru nose! after having cleaned gazillions of old vintage carbs in solvent tank, and smelling turpentine varnish smell til my eyes are blue, you have a couple of choices.........drain all gas out(pain in the butt and you have condensation ie. rust)- fill tank with unleaded and stabil- or finally if no cat pour 101 low lead av gas from local aeropuerto like i get to do on my '76 chevy 383 score legal class 8 race truck, 85 454 3/4 surburban 4x4 and now my 87 911 carrera cabri-olay, since i took cat off until emissions. reason you can store thangs w/av gas is NTSB and FAA demand stabilizer in ALL AV GAS, due to idiot pilots letting them sit for years due to cleaning shorts out from last screw up(ie.pilot error) and finally getting huevos to fly again! LOL! your next issue will always be batteries staying charged and thats a no brainer. hope this helps for porsches stranded in snowbound areas. living in arizona sucks.........i have to drive to the snow! LOL!
Old 02-22-2006, 10:23 AM
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I had a hard time understanding that post...

But if so, what is the mechanism by which the octane rating is lowered?

All, I can think of is the possibility that (besides the volatization BV mentioned) there are chemical reactions happening over time, _and_ that thos reactions reduce the octane rating somehow.
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Old 02-22-2006, 11:01 AM
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Varnishes and gum is a result of the additives in the fuel. I can see the octane rising because the lighter more volatile hydrocarbons evaporate leaving the heavier and more complex hydrocarbons. Higher octane gas has more complex hydrocarbons that don't burn as easily.
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Old 02-22-2006, 11:11 AM
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octane

hell i am no scientist but using av gas to store a non catalytic vehicle is only solution we have found. be it a 2 stroke dirt bike, 4 stroke dirt or street bike or vehicle. any make any model pre cat converter. keep battery charger on it, and it will fire everytime! one plane was stored here 8 yrs until pilot decided to fly again. he was not even fazed about fuel, and we asked why? and he explained stabilizers 101. yes youll pay the going rate and yes youll play hell getting into airport w/out clearance or amigo who can get you some in approved DOT container. my class 8 SCORE RACE TRUCK/prerunner sits w/44 gals. in its fuel cell of the stuff for a year btwn races and runs like a RAPED APE! no miss so NOTHING! ie. no problemos! no gummy bears in jets. enough said about the wonders of 101 lo lead AV GAS!
Old 02-22-2006, 11:23 AM
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Gasoline degrades during long-term storage by the following mechanisms:

Oxidation: Certain types of HC's in gasoline (olefins and diolefins) can slowly oxidize and form the gummy "varnish" that can plug fuel inectors and carbs. Soluble gums are formed first that stay in solution but deposit as the fuel evaporates. If oxidation is severe, insoluble gum can form as black particles in the fuel.

An interesting side note: California gasoline is more stable that what is sold in the rest of the country because CA regulations require gasoline formulated with lower levels of olefins. Oxygenated olefins increase emissions.

Moisture: Hot/cold cycles will condense water on the inside of the gas tank. This typically will settle in the bottom of the tank and cause rust, which contaminates the fuel system with particles. In cars with vented caps, temperature cycles also cause a regular influx of air (and oxygen) through changes in vapor pressure in the tank.

Evaporation: Over time the more volatile components will evaporate off. The loss of low-boiling components will make the car hard to start, as the volatiles are the first to vaporize and form the combustible air-fuel mixture. Winter gasoline is formulated with a higher percentage of volatiles for easier starting with cold. Oxygenated gasoline, sold in some states in the winter, contains bound oxygen that cannot react with the olefins.

If a gas tank could be tightly sealed, there would be no evaporation and loss of octane. But with evaporation, the loss of volatile compounds decrease the anti-knock index of the gasoline and the energy content of the gasoline increases
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Old 02-22-2006, 11:35 AM
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The short answer is to drain the old fuel, refill with high-octane and use STA-BIL if left parked for extended periods!!!!

STA-BIL works!!
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Old 02-22-2006, 01:22 PM
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Mr Wholberg nailed it.
Octane will be reduced over time depending on temperature and exposure to atmosphere.
The checmical structue of the molecules will change, both because of oxidation and because many refining processes rely on cracking or reforming molecules. These processes create a desired type of molecule that is not stable over time.
Old 02-22-2006, 01:40 PM
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WORD!
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Old 02-22-2006, 01:42 PM
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My car will be sitting up for a month and half or so while I wait for my front torsion bars to come in from Germany. I put some stab-bil in my car and ran the engine at idle for a minute or two.

How long - at idle - will it take for the gas to run through to the injectors?
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Old 02-22-2006, 01:52 PM
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Stuff needs to be added before you fuel up, then drive the car to mix it up....
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Old 02-22-2006, 01:54 PM
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Hey, thanks for the replys and the info. I only had about 1/2 a tank of old fuel. And with the awesome fuel mileage that the turbo gets I refilled it today. I sure hope this nice winter keeps up.

Thanks again
Dean
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Old 02-22-2006, 01:54 PM
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Quote:
Originally posted by redcoupe86
My car will be sitting up for a month and half or so while I wait for my front torsion bars to come in from Germany. I put some stab-bil in my car and ran the engine at idle for a minute or two.

How long - at idle - will it take for the gas to run through to the injectors?
When gas is formulated, enough stabilizer is added for one year, but you don't know how long the stuff was stored before you put it in your car. I think you should be OK for a month and a half. I think most gas companies recommend adding a stabilizer if you are going to park the car for more than 3 months.
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Old 02-22-2006, 02:05 PM
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I think a few of you missed one good point. Keep the tank full! That lowers the volume available for condensate to occur. No condensate = no moisture/rust. The AV guys know this, I think one of them previously did mention it.
Old 02-22-2006, 08:28 PM
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That's a really good point - Fill it up! I filled up my motorcyle completely and forgot to add STA-BIL. I was expecting issues with the carbs, but 6 months later, the engine ran really well all the way through the gas tank.
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Old 09-05-2006, 09:41 AM
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Dean,

-I've also found that this summer with all of the ethanol and other crap in our gas out east, that if I let my sc sit for more than a few weeks it runs like crap until I fill the tank and run some techtron. This year it's been worse for me than ever before, I'm guessing it's the alcohol in the gas that my CIS doesn't like but who knows,.

I did find that on a trip to PA this summer I got much much better MPG on my way home with a tank full of nice country gas VS the crap we get on the coast.

-Adam
Old 09-05-2006, 10:00 AM
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It's been bad enough here that I mix torco race gas concentrate - the plugs always look cleaner when I run the stuff. Just blend in about a 1/4 the bottle. Normally, a full bottle takes the car from 93 octane to 104. On my newer cars, I run a bottle of Redline SI-1 every 3,000 mi. Techron in a pinch.
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Old 09-05-2006, 10:43 AM
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Just make sure you let me know when you plan on draining it so I can came over and have you put it in my tank.

Can't believe you're actually buying what these bozo's are telling you!


Last edited by stlrj; 09-05-2006 at 10:58 AM..
Old 09-05-2006, 10:55 AM
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