How long does it take for fuel to go 'off'?

[fbc]

We've had a car sitting unused for the best part of eight weeks. When it was last driven it ran perfectly, but now it's developed an occasional hesitation under load (to the point of almost cutting out) and an uneven idle. A mechanic has suggested that the fuel has gone off - it's 98 octane - which would explain the engine not being totally happy, and would mean a nice cheap fix of simply filling the tank with fresh fuel.

So I'm wondering if anyone knows how long it takes for fuel to go 'off' like this when it sits unused? I've always known fuels can go off, but I thought it took longer than eight weeks

 

[patrick10]

a dad of a freind of mine has had a rx7 sitting not moving in 12 years. he got out recently and started it up on the same gas that was in there. it ran decent but i dont think gas should go too bad if its been sitting in a tank.

 

[Dr. Woo]

It depends on the climate. Friend of mine had bad gas in his truck after it had been sitting for less than a year.

 

[asthenia]

Depends where you get the gas too. If it's from one of those no-name stations, they bought it at auction from a big oil company as "returns".

Basically, the tanker trucks fill up the big-name stations, then when they go back to the refinery for more, they dump out what was left into a big tank. That sits around for months, until the highest bidder buys the whole thing. Your 8-week old gas could be 6 months old.

Or maybe you just got a little water in the tank...?

 

[fbc]

Hmm based on those responses the fuel going off diagnosis is a bit dead. The fuel was bought from a BP outlet.

 

[tom_kauf]

I didn't think fuel could go off. When it's sitting in the tank, you will get condensation (water) in the tank and maybe an old tank will cause the fuel to mix with crap (rust, dirt etc.)

 

[Z Draci]

Fuel will go bad. Even old fuel in the underground tanks of petrol stations will absorb water and go "bad."

When sitting for years and years, some of it will turn into a jelly and block smaller EFI pipes. In carburetted cars, it'll seize up the moving parts inside the carbs.

When in doubt, just drain your tank and refuel with fresh petrol. It's cheap insurance against some pretty annoying problems later on.

You can buy fuel stabiliser which is poured into the fuel tank when storing your car for a few months. Petrol has some pretty unstable molecules which will collapse upon itsef and react into something else even while dormant.