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Discussion > Is there any hard data on the deterioration of stored petrol?

Is there any hard data on the deterioration of stored petrol?

There seems to be a very wide - almost universal - belief that petrol rapidly deteriorates in storage. Available factual information seems essentially nonexistent, although observation and experience suggests that the effect may be exaggerated.

In what way does stored petrol deteriorate? (Loss of volatility? Change of octane rating? Change of other burning characteristics?) What is the practical effect of such deterioration? (Loss of power? Damaged exhaust valves? Gummed up carb?...)

I have a practical need for specific information on this question. I imagine it is also a question of interest to people with backup electrical generators.

Nov 5, 2014 at 12:41 PM | Registered CommenterMartin A

Martin-A, no hard data, I'm afraid. I'm not a petroleum chemist, per se, (though I increasingly wish I had chosen that career.)
My guess is that the question may boil down (pun intended) to the level of impurities introduced in varying manufacturing processes, nature and origin of chemical feed-stock, subsequent storage conditions, the specification grade (purity) of the product, and the purpose for which it is intended.

The presence of air/oxygen, water, and sunlight may accelerate the degradation of minor components of petroleum distillates over time. The vast majority of components in petroleum distillates are, of course, chemically stable indefinitely. Added impurities tetraethyl-lead, ethanol, etc, etc, and impurities within the impurities may more easily give rise to by products leading to insoluble precipitates. (I'm taking a guess that that may be your problem.)

The presence of particulate metal (compounds) within a storage tank may also accelerate polymerization and precipitation of otherwise stable components.

Nov 5, 2014 at 1:35 PM | Unregistered Commentermichael hart

Stored petrol in a sealed container does not deteriorate, however as soon as you put it in a petrol tank feeding an engine it will start to go off. It does this as all tanks need to be vented to allow the petrol to be fed by gravity or a pump and as the petrol is used it needs to be replaced by air.

The low temp volatile fraction is the first to evaporate off.

In a modern fuel injected car this is normally not a problem as the injectors will have a fine enough spray for the mixture to ignite but in a carburettor this spells trouble, hence why on lawn mowers and classic cars and bikes you need to drain the petrol before the winter lay up.

The gumming up comes from the ethanol, it allows bacteria to grow, wipes out pipework, glass fibre and plastic tanks plus has its own deposits too. E10 is now being introduced so expect issues to increase.

Nov 5, 2014 at 1:59 PM | Registered CommenterBreath of Fresh Air

Ethanol is hygroscopic, I remember reading that powerboat owners in Florida had had problems when the ethanol content of petrol (gasoline for them) was increased and they didn't empty the tank. As ethanol corrodes plastic and rubber I'm not certain how this would affect storage in the ubiquitous red and green plastic cans used to store petrol these days. Here in France metal Jerry cans are still available but rust might be a problem for those (off topic I guess)

Nov 5, 2014 at 3:06 PM | Unregistered CommenterSandyS

Yes it is hygroscopic, I use meths to get rid of wetness after I flush clean a tank with water, but the powerboat owners are suffering again because the tank is vented and the moisture in the air is being taken up by the ethanol and new air brings more moisture due to the vent. Annoyingly the ethanol releases the moisture as water which sits in the bottom of the tank and the if steel it rusts, if not steel it feeds the bacteria producing deposits. Close the vent and once the moisture is taken from the trapped air no more moisture can be extracted as the air is now dry and stays dry which stops these issues.

Ethanol is a pain and should be banned

Nov 5, 2014 at 3:54 PM | Registered CommenterBreath of Fresh Air

I have a carbed '93 mini that last summer had been off the road for a couple of years. It was _very_ reluctant to start, it would sort of fire but then run like a dog, coughing and spluttering and would hardly pull itself to the end of the drive. I suspected the petrol so topped the half full tank up with a couple of gallons of fresh petrol.

Then I noticed two of the plug leads were on the wrong way round, I think I was the victim of my brother in law's or friend's practical joke. When I sussed this the thing fired up and ran as well as it ever has (or as well as an A series engine with 75K on the clock could be expected to).

It ran brilliantly at least to the end of that tank of petrol, so I'm dubious about stories of stored modern petrol going 'off' very quickly.

Nov 5, 2014 at 3:54 PM | Unregistered CommenterNial

Thanks for the advice.

Up to now, I have had no problems with aging petrol. My mowers (Honda motors with carburettors) start second pull of the string after wintering with petrol in the tank. I have a couple of cars in the garage that get used infrequently but they always start first touch of the starter and run smooth and fast. (Always 98 octane - never the ethanol-laced stuff) One of them can go more than 6 months between runs so, like Nial, I am dubious of stored petrol going 'off' quickly. And, in the metal petrol tank of a car, it's not going to evaporate to any significant extent.

I'm now tinkering with a Ford Zephyr Mk1 of unknown history, so it's possible the petrol in its tank has been there for years. On anything more than half throttle, it was hesitating, misfiring and spitting back through the carb. After verifying the ignition, I'd checked pretty well everything there was to check on the carb, replacing dodgy economy valve diaphram there, dodgy carb gaskets, and so on. That cured one problem (black smoke) but the misfiring and carb-spitting was still there.

Deteriorating petrol seemed a possibility (I was running out of other things to try). I just put 10 litres of 98 octane in the tank and the misfiring seems to have diminished. So that _may_ have been the problem. We'll see what a few more litres does.

Thanks again for the information.

SandyS - you buy alcohol à bruler and acetone in plastic bottles at the supermarket. I don't know what the plastic fuel cans are made of, but I think you can be certain that they will hold anything you can buy at a filling station with complete security.

Nov 5, 2014 at 4:13 PM | Registered CommenterMartin A


I've seen the float bowl and needle valves in a mini carb (su) get pretty gunked up when the fuel within evaporated over a couple of years.

You could take your carb off and clean it, but you might find the fresh new fuel will gradually dissolve and flush out any remaining gunk.

Nov 5, 2014 at 4:35 PM | Unregistered CommenterNial

The pilot jet on my Amal carbs is notorious to get gummed up, I have several 16 thou drills which I use every spring to clean them out before the first start of the season. New petrol will loosen some but not all of the crud, carb cleaner used to be 100% on crud but not the new crud from the bacteria, ultrasonic cleaning is now the fool proof cleaning method.

Nov 5, 2014 at 6:21 PM | Registered CommenterBreath of Fresh Air

Thanks Nial. I had the carb (Zenith) to bits, soaked in acetone for 48 hours, squirted carb cleaner through all the channels, checked the jets were right etc. Even before all that, it seemed pretty clean internally.

Tonight I got the wherewithal to siphon everything out of the tank so I can see how it does with 100% fresh fuel.

Nov 5, 2014 at 6:26 PM | Registered CommenterMartin A

This morning I emptied the Zephyr's tank and put 10 litres of fresh 98 octane in. Once warm, the car ran without any misfiring or spitting through the carb. [Any suggestions what to do with 12 litres of unusable petrol?] It seems pretty certain that the problem was the fuel.

So the deterioration of petrol with age is not merely an urban myth, it seems. I'll still be keeping my eyes open for any information on what is the actual mode of the deterioration. Evaporation of the most volatile ingredients perhaps.

Thanks again for the suggestions.

Nov 6, 2014 at 12:27 PM | Registered CommenterMartin A

[Any suggestions what to do with 12 litres of unusable petrol?]

Stick it in the tank of a modern petrol fuel injected car, make sure there is some fresh petrol in there already, the injectors will create a mist fine enough for the old fuel to ignite. With carbs the petrol is not a fine mist as it exists the jets and requires the low volatile part to provide the initial flame which then burns the thicker bit. Modern cars also have much better sparks which helps too.

My local Garage owner had his first non firing injected car this year due to old fuel, he had only ever seen it on carbed cars (he has an MG and an old Landy), hence the tip to not put it in a empty tank.

Premium unleaded has a much better chance of being Ethanol free so I know use this on all my carbed bits of kit.

Nov 6, 2014 at 6:24 PM | Registered CommenterBreath of Fresh Air

I have just had a leaf-blower serviced. The plastic fuel line had all but disintegrated and the debris had fouled the carb. According to the shop, ethanol in fuel is the main culprit. It is a nightmare for many small power tools and the like as they often have plastic carbs, tanks, etc.. Apparently the latest machines use ethanol-tolerant plastics but the rest are on borrowed time.
There is some hope....I believe the EU has scaled back its biofuel mandates so we are only at E7.5 instead of E10 due to the growing recognition of the absurdity of growing food for fuel.

Nov 11, 2014 at 3:34 PM | Registered Commentermikeh

I've still failed to track down any hard information on what goes wrong with petrol. My experience of an engine starting very easily and then running smoothly but then misfiring more and more as the throttle is progressively opened does not make sense to me.

There are suggestions that petrol goes bad because of:

- Absorbing water (especially fuel containing ethanol)
- Losing the volatile components through evaporation
- Chemical changes that change its burning characteristics.

There are said to be products that reduce the tendency to go bad. There are also suggestions that bad petrol can also cause good petrol to go bad - like an infection.

There is a reference that looks as if it might contain firm information but it is paywalled:

Automotive Fuels: Werner Dabelstein1, Arno Reglitzky1, Andrea Schütze2, Klaus Reders1. Its contents list includes "Additives for Increasing Storage Stability - Antioxidants" which suggests that oxidation is one cause of degradation.

I think that petrol containing alcohol is not a new thing - I remember my dad explaining to me that "Cleveland Discol", sold from the 1930's, contained alcohol.

One interesting suggesting was that, to get rid of water in a car's petrol tank, you can just add half a litre of iso-propyl alcohol.

More petrol info here:

Nov 11, 2014 at 6:36 PM | Registered CommenterMartin A

> I've still failed to track down any hard information on what goes wrong with petrol. My
> experience of an engine starting very easily and then running smoothly but then misfiring
> more and more as the throttle is progressively opened does not make sense to me.


Do these symptoms not suggest a mechanical blockage/ gummage problem rather than a chemical problem? You're getting enough fuel through for idle/ low throttle running but as soon as you need more the mixture leans and you get the popping and banging?

Nov 12, 2014 at 9:24 AM | Unregistered CommenterNial

Nial - I cannot say for certain what the problem was. Thanks for the suggestion. My conclusion (97%) was that it was the fuel, even though it's hard to understand why bad fuel would make it misfire as the throttle was opened.

The car would start misfiring at a certain throttle opening (or perhaps a certain rpm; can't say as the car does not have a rev counter) irrespective of whether the car was standing in neutral or was pulling up a slope. I assumed it was an ignition or carb problem until I'd pretty well run out of things to check/replace in those two areas.

I could not be sure, with an engine-driven mechanical fuel pump, that the fuel pump was delivering properly but, when I stopped the engine abruptly, the carb always seemed to have the correct depth of petrol in its float chamber.

The jets seemed absolutely free and clear - although I put carb cleaner and compressed air through them anyway (and through all the passages in the carb). [There was a carb problem (solved) causing over-rich idling (economy valve diaphram leaking) but solving that problem had no effect on the misfiring.]

As soon as I put new fuel in the tank, the misfiring reduced noticably.

Then as soon as I put more new fuel in, having drained the tank as completely as I could, the misfiring ceased altogether.

I'd been trying to track down the problem for some weeks previously, since I had acquired the car, so the chance of something else correcting itself spontaneously at that instant seems remote. The previous owner had also tried to solve the problem in the months he had the car but without success. When I told him of my success he joked that had he thought of changing the fuel, he could have sold the car for a better price.

I suppose I could drain the tank, bung the old fuel back in, and see if the problem reappeared. That would pretty well pin it down for certain but I don't plan to do so. I can't yet try the engine at full throttle (clutch needs replacing) but we'll see in due course if it performs correctly at full throttle.

Notwithstanding BOFA's advice, I think I'll get rid of the suspect petrol by burning it off in an old cast iron barbeque, rather than put it in the tank of my "collectors car of the future" which does use fuel injection.

Nov 12, 2014 at 5:46 PM | Registered CommenterMartin A

"Petrol in a sealed metal can does not go off, provided it is purchased in the UK or any other more or less civilised place. It will be OK for years. About the only additive in fuel is the detergent which doesn't evaporate. In these lead-free days, octane number depends on the molecular make-up of the fuel, and all you need to do is stop evaporation. Even the smallest molecules in there (such as hexane) can't get through metal. Plastic, as used for fuel containers, is itself made up of long-chain hydrocarbons (usually high-density polyethylene) which have similarities with the short-chain petrol hydrocarbons, so small petrol molecules worm their way into the relatively large spaces between the plastic molecules and get out the other side. Losses can be several percent over long periods (months) but thick plastic helps reduce this. You can't beat metal with its compact atomic structure for storing petrol."

"Petrol is OK in a can with an airspace; a tiny amount of the more volatile fractions of the fuel will evaporate to fill the airspace, and that's the end of it. It becomes a 'closed system in equilibrium'. Molecules go back into the liquid phase at the same rate as they leave it and enter the gaseous phase. Even a three-litre airspace in a five-litre can will only contain a very small amount of fuel vapour, equivalent to one or two CCs of fuel depending on the temperature."

Dec 17, 2014 at 3:09 PM | Unregistered CommenterJohn

John - thanks for that, though I'm not sure where it came from - do you have a reference?

I'm not convinced by the story about "petrol molecules worming their way" out of plastic cans to the extent of several percent loss over a period of months.

Dec 17, 2014 at 4:03 PM | Registered CommenterMartin A

This link gives a good description of the problem and solutions:

also, boat owners, before laying up for winter, always (well the wise ones do) top up their fuel tanks to the top, to stop condensation causing water to collect at the bottom of the tank.

In the link above, the description of what petrol is can be confirmed by putting a small quantity into a tall narrow jar, sealed well! and left. You will see it stratify into its different layers after a few months.

As for getting rid of old petrol, I put a pint per week into my wife's petrol Corsa where it burns off with the bulk of the unleaded.

I have no problem with the 'lighter' products of petrol 'worming' their way out of a can. The lightest material we know hydrogen is very difficult to store, with metal leaking like a sieve (ish).

A paper here: discusses how hydrogen in a pipeline causes the very expensive steel used to construct the pipe to become brittle.

I feel that the 'lighter' petroleum products will have an easier time permeating through my red or green plastic can in the garage over winter.

Dec 19, 2014 at 5:31 PM | Unregistered CommenterSteve Richards

Steve - Thank you. Hydrogen is notorious for its ability to diffuse through almost anything so I would not use it as a guide to what petrol might do..

However my scepticism about petrol worming its way through approved plastic containers was not justified:

Health & Safety Booklet HSA 18
Plastic containers with nominal capacities up to 5 litres for petroleum-spirit:mrequirements for testing and marking or labelling


11. The nominal capacity of the containers must not exceed 5 litres. The nominal capacity
is the maximum volume of liquid the container is intended to hold at 20± 2°C.


19. The loss in weight of the contents of a closed container should be not more than
5 g when a full container is kept for 4 hours at a temperature of + 75°C in accordance
with the test method described in paras 41 and 42.

A gram per hour (even at 75°C) is quite a rate of loss.

Dec 19, 2014 at 7:57 PM | Registered CommenterMartin A

I have found that stale petrol will cause lack of power and misfiring,because of the lack of the volatile components of the fuel,the engine will run as if the mixture is too weak.

Aug 3, 2015 at 4:57 PM | Unregistered CommenterRaygas

The main volatile component in petrol is dissolved butane. It is added to improve startability especially in winter conditions, and also because it improves the Motor Octane Number of the fuel (which indicates its performance at higher revs, where as Research Octane Number measures performance at a slow idle). A wider introduction to the topic of gasoline blending for octane is here:

All those components will tend to alter chemically in a variety of ways as petrol is stored. Refineries like to avoid storing it for more than 6 months without re-blending (indeed, they usually prefer to blend prior to dispatch from stored components), even with tanks with a low air surface to volume ratio. Olefins are particularly reactive with air. The composition of petrol may vary significantly according to the processes used to make its components at the refinery - reforming atmospheric distilled naphthas produces branched alkanes, while catalytic cracking of heavier fractions produces more in the way of aromatics and other cyclic compounds, along with extra LPGs that may be alkylated to make iso-octane and other alkanes.

Aug 7, 2015 at 4:22 PM | Unregistered CommenterIt doesn't add up...

Idau - thank you for the link. To be studied with interest.

I have a car (1955 Zephyr Six) which (it now seems) had probably not been significantly driven for around four years at the time I bought it. It always started very easily but blew lots of black smoke at all times and would cough and splutter on more than 50% throttle.

Repairing the carb (knackered economy diaphram) got rid of the smoke. Only after rebuilding the carb and checking the function of the ignition system did I eventually discover that replacing the petrol completely cured the coughing and spluttering.

Aug 7, 2015 at 10:26 PM | Registered CommenterMartin A

I had found it impossible to start my lawnmower engine after it had been standing a long time, after having almost pulled my arm out of it's socket! I stripped down the carb and found that the jet was partially blocked. After cleaning and replacing it I was still unable to start the mower. I then drained the tank and replaced with new fuel and, bingo, first time start.
I have a 60 year old BSA motorcycle which is ridden only infrequently and then only in fine weather (I know, Big girls blouse!). I have found this to start and run easily on low throttle setting with full choke. On the road it runs like the proverbial bear's arse at the higher revs in the higher gears and no choke and cuts out at traffic light stops on low throttle unless I put the choke back on.
The fuel used was very old and had been standing for a long time and this was the fuel I had transferred into my lawn mower.
I shall not be riding the bike until the better weather when I shall be using new fuel, but if it runs much better it will indicate that old stale fuel is not to be used.
Like previous comments I will get rid of my surplus old fuel by gradually introducing it into my modern car, with hopefully, no side effects.
By the way, I had found the bike carb all gummed up when the bike had been laid up for years previously.

Feb 21, 2016 at 12:29 PM | Unregistered CommenterBaldeagle

"Like previous comments I will get rid of my surplus old fuel by gradually introducing it into my modern car, with hopefully, no side effects."

Somebody suggested I got rid of my stale petrol that way. I was not sure that a set of 12 burned exhaust valves would not be a side effect. The gamble did not seem worth the risk, so I burned it off bit by bit in an old cast iron barbecue tray.

Last week, I started my 2-stroke brush cutter using last year's fuel from a 5 litre can. It started readily but coughed and spluttered when I opened its throttle. I found that if I got it up to speed by partially opening the throttle under no load, then once it had warmed up fully, it would run without spluttering. This seems to confirm that a fraction that ignites easily had either evaporated or been oxidised. (Reminiscent of stories from the 1940's of starting a car on petrol and then switching over to paraffin once the engine was hot.)

Feb 21, 2016 at 2:05 PM | Unregistered CommenterMartin A