Does petrol go off/bad when your car isn't used for a while?

[DNYALL]

Hey, so I saw on a television ad some product for making your cars petrol last longer when your car is left for long periods of time. According to the ad, your petrol go's off/bad after sitting for a month without being used...

I have never heard of this befor. Although the length of time quoted on the ad seems a bit soon, still got me thinking about my civic which might be lucky to be started like once in three months at times. So will petrol go off eventually if left for a while? How long untill i should think about draining the petrol then re-filling with fresh petrol?

My car hasn't moved for about three months right now and has half a tank. recommend draining it and re-filling or would filling with new balance out the old and new? I was planning on re-filling with new but if anyone knows any more info, that would be great :thumbsup:

[bennjamin]

ofcourse it does.



According to Shell Australia’s Technical Advice Centre Manager, Warren Scott, you can generally store fuel in a sealed container for around six months before it starts to deteriorate. This assumes it’s kept relatively cool and away from direct sunlight.

“But if the container is not properly sealed or it’s repeatedly opened, [fuel] will start to evaporate and you lose some of the light components in the fuel,” Mr Warren says.

Those light components include highly important butane. As these components are lost, the fuel loses its volatility. Mr Warren says this makes the car difficult to start.

In warm conditions, fuel is especially prone to oxidisation which causes lacquer and gums to form. As a result, the fuel turns dark in colour, loses its clarity and smells like paint stripper.


Gary Whitfield, Senior Fuel Chemist at BP Australia, says that in the right conditions, fuel can last up to 12 months. But once the container is opened, the fuel will start to degrade.

“Once the seal is broken, a vapour space is formed in the container and the fuel will start to deteriorate more rapidly,” he says.

Okay, so what about fuel that’s left in your car’s tank for a long period of time?

Mr Whitfield claims fuel in a car’s tank can last for up to four weeks before it becomes considerably denser. When this happens, the engine is effectively over-fuelled (without any change in injector pulse-width) and there’s an increased chance of carbon deposition onto spark plugs and inside the combustion chamber. This can cause driveability problems such as stutters and misses.

“Regular unleaded fuel will lose a small amount of octane over time but in our low-sulphur and high detergent Ultimate fuel it might increase very slightly. It might increase 1 RON in four weeks – but it’s not something I’d bother chasing,” he says.

“Keep in mind that typical variation in BP Ultimate is 98 – 100 RON while normal unleaded ranges between 91 – 93 RON.”


Mr Warren says that in most cases, it’s only when fuel is left in the tank for several months there’s the risk of engine damage. This is much more likely to occur in cars running dual LPG/petrol systems.

“By this time the fuel will have gone dark and gummy and can clag the fuel pump, filter and injectors,” Mr Warren says. It can also leave a lacquer deposit on the inside of the fuel tank which is difficult to remove.

In addition, it’s important to be aware that fuel brews – specifically fuel volatility - varies on a month to month basis. Standards vary from state to state but, generally, volatility is reduced during summer months to reduce evaporative emissions. Mr Whitfield says this typically makes the engine slightly harder to start but there is no change in octane.


Lachlan Riddel, proprietor of Gold Coast based tuning company ChipTorque, frequently dyno tests cars which have not previously been driven for several weeks.

“On the fuel mix we have in Queensland, I think fuel in a car’s tank is starting to go off in around three weeks,” Mr Riddel says. “We can tune a car, put it away for three of four weeks, come back to it and it will be either closer to detonation or less likely to make the same power.

“I think local fuel is brewed to reduce evaporative emissions in warm weather and, maybe because of that, fuel really goes crappy in eight to ten weeks. By that stage, the fuel is extremely prone to detonation.”

Interestingly, Mr Riddel hasn’t noticed the engine becoming harder to start but the effect on octane is pronounced.

Similarly, the General Manager of Melbourne’s Advanced Vehicle Operations, Steve Coates, believes fuel tends to go off in a car’s fuel tank quite quickly once it’s a couple of weeks old.

“After a couple of weeks not driving a car, I find that it will be closer to detonation than it was at the time of tuning on the chassis dyno,” he says.

“And the brand of fuel seems to make a difference as well – I find that Mobil and BP products are probably the best after being stored but Shell Optimax is really good when it’s new."

Clearly, there are some important factors to keep in mind when storing fuel in a container for several months or when parking your car for a few weeks.

Our recommendation is not to run the engine at high load with fuel that’s been in the tank for any more than three of four weeks and it’s a good idea to add at least 25 percent fresh fuel to the tank every few weeks. This will help ensure there are no fuel-related driveability problems or detonation and will reduce the chance of lacquer building up inside the tank.

quoted from http://www.answerbag.com/q_view/557467

[DNYALL]

oh dear, ok so looks like i'll be emptying my tank asap :/ Thanks for the info man.

[mooshie]

That is a great post Ben.

Exactly the reason i only put half a tank in at a time (weekend car) and why I get fuel at the busiest station that I see.

[dougie_504]

Good idea is to run your car empty pretty much until the fuel light comes on, but the doesn't go stale too fast. The tank in somewhat pressurised but it can't hurt to run it out good.

[aaronng]

Good idea is to run your car empty pretty much until the fuel light comes on, but the doesn't go stale too fast. The tank in somewhat pressurised but it can't hurt to run it out good.

On old cars, when the tanks are run to empty, the crap at the bottom gets sucked up and blocks the fuel filter.

[androo]

On old cars, when the tanks are run to empty, the crap at the bottom gets sucked up and blocks the fuel filter.
I don't think this is really that true, because the fuel is sucked up from the bottom of the tank anyway...

[DNYALL]

Ok so what is the best way to empty the tank of un-wanted fuel. Don't want to run it through my injectors etc. I haven't gotten under the car yet to suss it out. No time. But is there a drain plug of some sort, or would it be easier to just siphon the fuel out oldschool method?

[bennjamin]

I personally wouldn't bother - just fill up with 98 and you will pull up the old stuffs RON rating alittle.
The fuel filter is there to filter out any possible crap anyway....just replace the filter afterwards with a fresh item if paranoid

[gumus89]

Problem is when you have too much space above the fuel. I didnt read the huge article but it no doubt said that.
So if you are leaving a car for a while, fill it up completely. That way the lighter components wont evaporate off.

[DC2-PWR]

Intresting thread, always wanted to know this.

[integragsi96]

sorry for hijacking so pretty much if i leave my car for lets say 3 weeks and hasnt been fired up is ok?

[SHOGUNOVDDRK]

sorry for hijacking so pretty much if i leave my car for lets say 3 weeks and hasnt been fired up is ok?

Depends on teh fuel you fill up with.

Will probably evaporate a bit over the time.

Just have the car on really low fuel before you leave it and when you go to fire it up have a fresh can of fuel waiting for it.

Though I'm sure Ben will expand on this.

[fatboyz39]

Just Fill up, drive the car normally till the tank runs out.

[grifty]

i had an old carby corolla sitting for 6 months, during the 6 months it was never started, when it came time to start it it took about 5-6 cranks to get it started but the fuel was alright since it had a full tank.

so 3 weeks shouldnt be a issue

[integragsi96]

Depends on teh fuel you fill up with.

Will probably evaporate a bit over the time.

Just have the car on really low fuel before you leave it and when you go to fire it up have a fresh can of fuel waiting for it.

Though I'm sure Ben will expand on this.

i think it is about 1/3rd left of a tank of vpower i think.
ill be firing the car up sometime this week i hope

[xclusive_eg8]

My civic sat with half tank of BP Ulitmate for 6 Months with fuel filter off and started 1st go with engine back in and connected. Yes it will go off if just say it's kept in a regular bucket or something but in a actual tank you got no issues, can't comment on actual octane rating after that long but hey it works

[r3ckless]

I run a 1/4 tank n my car gets started once a weekend... Main reason for this is i only run 98 ultimate... The fuel will lose its octane ober time.

For the obsrssive
Compulsive... Changing fuel
Filter and spark plugs yes??

[DNYALL]

I personally wouldn't bother - just fill up with 98 and you will pull up the old stuffs RON rating alittle.
The fuel filter is there to filter out any possible crap anyway....just replace the filter afterwards with a fresh item if paranoid

Ok cool, I reckon it should be right :thumbsup:

[Bludger]

I've enquired about mower servicing tips at a mower shop and they told me that supposedly ULP has shelf life of only a few weeks or a month.

They recommended using premium fuel to fill up the fuel can because it has a much longer shelf life, about 6 months ish.

fact or fiction I don't know, I've just been told.

[Bludger]

Depends on teh fuel you fill up with.

Will probably evaporate a bit over the time.

Just have the car on really low fuel before you leave it and when you go to fire it up have a fresh can of fuel waiting for it.

Though I'm sure Ben will expand on this.

storing the car with low fuel will corrode the tank imo.

I would store with a full tank

[DNYALL]

hmmm mine has got BP ultimate 98 but only about 1/5th of a tank at the moment. should be right i reckon.

[integragsi96]

storing the car with low fuel will corrode the tank imo.

I would store with a full tank

hmm ok well car will be on the road in a couple of days so first thing is putting in fuel then goin to road worthies