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

The last 2 stories remind me of Primus stoves...

Feb 21, 2016 at 4:15 PM | Unregistered Commenterdiogenes

Yes, and paraffin blowlamps. French blowlamps seem to have been universally petrol fired - horribly dangerous in my view.

Feb 21, 2016 at 5:57 PM | Unregistered CommenterMartin A

I can understand meths but petrol does seem to be a trifle dangerous

Feb 21, 2016 at 11:13 PM | Unregistered Commenterdiogenes

but I guess the higher fractions evaporate earlier end up with a mixture closer to paraffin than to octane, in fact there is probably some kind of decay curve. And, of course, the oil cos add solvents and stuff to "improve" performance and who knows what they will rsult in, chemically. But after 4 years ort so, the volatile stuff must have evaporated, leaing you with something more difficult to ignite from cold. Maybe you ned to warm the fuel tank before

Feb 21, 2016 at 11:17 PM | Unregistered Commenterdiogenes

Martin A
Fortunately they (the French) seem to have moved onto LPG canister fired ones now.

Feb 22, 2016 at 8:16 AM | Unregistered CommenterSandyS

I've always thought that the problem with brushcutters and chainsaws is down to the oil in the two stroke mix if you don't empty the tanks when not in use. The petrol in the carburetor evapourates leaving the oil to gunge up the works and actually prevent the new fuel getting where it's needed.

Feb 22, 2016 at 8:21 AM | Unregistered CommenterSandyS

Sunbeam Stiletto in the garage with exactly this problem... complicated by the tank in the front and the engine in the rear, and not much room to get at the carburetor... and as a sign of getting older, I am not in the mood in me/the house getting covered in old sticky petrol and half a day of swearing, scraped knuckles, "helpful" interventions of the wife, I might delegate this to the trustworthy garage who still knows how carburetors work. Maybe your topic will fill me with the enthusiasm to get t started. At least nowadays I can work in a garage... I am far too soft compared to my "student with a socket set outside the digs" days...

Feb 22, 2016 at 8:33 AM | Unregistered CommenterMedia Hoar

I thought tis question on petrol deteriation had already been extensively discussed a couple of years ago here..
..but a quick search I couldn't find the old thread.

Feb 22, 2016 at 9:29 AM | Registered Commenterstewgreen

Martin A, I appreciate this thread has been revitalised etc, and you were after scientific papers, which I can not assist with but....

Garden machinery mechanics are well used to the owner bringing in the lawnmower (or whatever) that started and ran "perfectly" all last year, but this year won't work at all.

The same happens with outboard engines.

Manufacturers recommend you run an engine dry by turning off the fuel, before laying up the engine. This enables left over fuel to be put in the car, so it is not stored over winter, and fresh fuel is then bought.

Fuel lines and carburettors etc to "gum" as a percentage of the fuel evaporates, and presumably the remaining fuel can not be quite the same.

Sale of new 2 stroke outboards is now banned, whether outboard mechanics have noticed any difference in trade from non-functional engines I do not know.

Whether Honda (for example) sell Honda outboards and lawnmowers through the same UK company I do not know, but they might be worth asking if you are still after research science to back up what is "well known by everyone"

Feb 28, 2016 at 4:47 PM | Unregistered Commentergolf charlie

Re: the Zephyr: if it has a distributor with balance weights to increase the ignition advance at higher revs, then the quality of the old fuel might not be up to the demands of the advance mechanism. Therefore higher revs just don't work when its slurping cold soup instead of fresh fuel

Mar 18, 2016 at 3:13 AM | Unregistered CommenterThe P

It has an advance mechanism and I had checked with a strobe that it was working correctly. But the Zephyr was coughing and spluttering even at low revs.

It started easily enough but, put the accelerator more than 30% down accelerating slowly from rest, and it would cough and misfire. Since I drained the tank and refilled with fresh fuel, it runs perfectly.

Mar 18, 2016 at 8:55 AM | Unregistered CommenterMartin A

Martin A google search

"Petrol life in vehicle tanks"

for a BP PDF. This may point you in the direction of technical data.

Mar 21, 2016 at 9:48 PM | Unregistered Commentergolf charlie

gc - Thank you for the link to the BP document.

I have to say that the BP document seems a odd on several points. As just one example, it suggests 10% of the petrol will have evaporated after four weeks in the vehicle's tank. Everyday experience says that that is just nonsense.

Some time back I tracked down some information though not comprehensive. Links I found here.

I think there may be several degradation mechanisms: evaporation of light fractions; oxidation; absorption of water vapour; separation of alcohol from other ingredients. Which one is dominant probably depends on the circumstances.

Mar 21, 2016 at 10:54 PM | Registered CommenterMartin A

Martin A, I was looking for something else when I found that!

My limited experience is the fuel in a jerry can from last year, and the petrol still in the tank of a lawn mower from last year. Ditto fuel for outboard engines.

About 5 years ago, I had had a jerry can of petrol in my car for about a year, put some in a lawnmower, and it spluttered. I tipped the rest into my car, si it was diluted 10:1 with no ill effect, and cursed myself for forgetting what "I knew". The lawnmower ran fine with fresh petrol.

I have ASSUMED it is an evaporation of lighter fractions issue, but how do the lighter fractions escape a jerry can? Plastic or metal? Car fuel tanks will have some form of vent, but are better protected from diurnal temperature change than a lawnmower or outboard in a shed. The lawnmower/outboard is more likely to be left for long periods in winter rather than summer.

I am familiar with "diesel bug", but that is mainly biological activity due to moisture. Petrol "decay" is NOT an area that I have experience of, other than, if in doubt, replace it.

We are not allowed to store larger volumes of petrol any more. After the fuel tanker strikes, I expect there is still a lot of dud petrol in some people's sheds.

Mar 22, 2016 at 12:32 AM | Unregistered Commentergolf charlie

gc - A previous comment that I evidently did not fully take in at the time, stated:

The main volatile component in petrol is dissolved butane.
Aug 7, 2015 at 4:22 PM It doesn't add up...

If there is a significant proportion of butane in petrol, then it's obviously simply itching to get out, with its boiling point being around zero degrees C. Anything other than a sealed metal container will not keep it in.

I still think there is more to petrol deterioration than simply the butane fizzing off - absorption of water, separation of components, oxidation, other chemical reactions...

Mar 22, 2016 at 7:57 PM | Registered CommenterMartin A

Martin A, the extent of my knowledge about permeability of materials by gases, does not go much further than conventional 'party' balloons can not be used for helium, and that the cheapest flexible plastic pipe should not be used for the waste pipework of a marine toilet!

The BP article did suggest that stale petrol would cause an engine to burn rich. This matches my (bitter!) experience of a 2-stroke outboard engine about 10 years ago. I was told to change the fuel (it had overwintered in the Med) and everything was ok. I wondered at the time whether 2-stroke oil had caused gumming up of the carb or fuel lines, as the petrol had evaporated, but in hindsight, I think it was more likely to have been a deficiency in the petrol, rather than an overconcentration of oil. I also wondered whether a duff ratio of 2-stroke mix had been mixed the previous year. With the problem fixed with new fuel, no further investigation was necessary, but it still niggles with me!

Mar 22, 2016 at 10:16 PM | Unregistered Commentergolf charlie

This thread's been going a long time! I'm very glad it has. It gives me the chance to post my own thoughts on 'stale petrol'.

Having been engaged in the motor engineering business for over 40 years (retired in 2007), latterly I frequently came across the problem of gummed up small engine fuel systems after being left not run for several months (lawn mowers, chain saws, strimmers etc).

I have read MUCH about only using new petrol, and discarding old stock. Being now a pensioner, this didn't hold much appeal to me. So, having followed dozens of links and read till my eyes are nearly falling out, I came across what could be the solution. Briggs and Stratton market an additive entitled: "Fuel Fit Additive/Stabiliser. 992381" with the following description:

Keeps fuel fresh for up to 3 years

Fuel Fit not only keeps the carburettor clean for easy starting but now also protects against the corrosive effects of ethanol

Detergent ingredients to prevent gum and varnish build-up on engine parts

Metal de-activators which stop chemical reactions caused by dissolved metals in fuel

Corrosion inhibitor that forms a protective barrier on metal parts against harmful effects of ethanol-water mix

This product looks like exactly what I have been searching for. It's available on Amazon for £5.80 for a 250ml bottle. The reviews on Amazon are glowing! So I have ordered two.

As I am an avowed AGW sceptic, I possess a generator for fear of the main grid going down due to unreliable renewables and the problems they cause (see e.g. South Australia recently). In Scotland, windmills are marching across the once-beautiful scenery like there's no tomorrow.

Fortunately I haven't had to use my generator for several months - but I keep/kept a stock of petrol for use in an emergency. So you see my dilemma? I think, and hope, this additive will save me a lot of pain and expense. I hope this helps anyone else who comes to this thread via Google (and of course, the OP - Martin A).


Oct 4, 2016 at 12:52 PM | Unregistered CommenterLuc-Ozade

I found this very knowledgeable article on Ethanol in petrol.

I include this reference because all the indicators are that it was the forced introduction of ethanol (by the stupid green hoard of CO2 demonizers) which has led to the early (some say 30 days) degradation of petrol.

Oct 4, 2016 at 1:22 PM | Unregistered CommenterLuc Ozade

Luc Ozade, I have no experience of the Briggs and Stratton additive, however I have heard of people using it and being happy to do so again.

To reiterate points previously made, petrol does deteriorate. If you can avoid storing petrol for longer than 2-3 months by using it, the better. Any petrol you still have from last year for your emergency generator, would be best poured into your cars petrol tank (mixed with fresh petrol) and used. Fill your storage containers with fresh petrol. Place an additive in if you want. I would still not assume it would be 100% reliable in 3 months time. If a lawn mower fails in the summer, it may not be critical. If your generator fails under the sort of conditions where you need to depend on it for 1 hour or 48 hours, what are you going to do? Nobody is suggesting you should throw away 5-50 litres of petrol. Use it, and buy fresh, before you find out it is unreliable

Oct 4, 2016 at 3:16 PM | Unregistered Commentergolf charlie

golf charlie: I agree, precaution IS the best policy. (By the way, I can't spell 'horde', lol.)

I'm looking at purchasing another generator in the near future, one with an electric start. Nowadays I find pulling the cord start on an engine a trifle painful. However, I'm looking forward to trying out the Briggs and Stratton additive to see whether or not it will make my current generator start first time, second, or at all. The petrol in the tank, and in my containers, is now nearly a year old, so it should be a good test. I shall probably drain the carburettor first. Having tested it, I will probably take yours (and others) advice and use the stored petrol in my car, bit by bit, and put fresh petrol in my containers again. I shall report back in due course.

Oct 5, 2016 at 5:16 AM | Unregistered CommenterLuc Ozade

Luc - Thanks for your comments.

Although I imagine that the B&S juice will provide some genuine benefits, I noticed that quite a few of the four and five star Amazon reviews for the B&S juice are essentially like positive reviews for homeopathy medicines. Eg along the lines

Can't comment on how effective this additive is, but from the other reviews it should be just the ticket to keep my fuel fresh." [four star review]

The review I liked best was

I use this additive for my lawnmower. It's impossible to know whether there is any measurable performance gain but I can feel the placebo effect kicking-in each time I start the engine for the first cut of the season.[five star review]
There seem to be a number of degradation mechanisms in petrol, including oxidation of some ingredients, separation of alcohol from the rest, and evaporation of light components. There can be no doubt that petrol does degrade with time. But obtaining firm information on the nature of the degradations and their rate seems problematical.

Somewhere, not long ago, I read that petrol now contains a proportion of butane - which, on its own at atmospheric pressure, is a gas. Presumably the butane will evaporate quite quickly unless the petrol is kept in a sealed can. Hard to believe that the B&S juice can stop the butane from evaporating, whatever other benefits it may provide.

According to the contents list of Ullmann's Encyclopedia of Industrial Chemistry** Petrol additives can include:
5.1.1. Corrosion Inhibitors
5.1.2. Detergents
5.1.3. Antioxidants
5.1.4. Metal Deactivators
5.1.5. Anti-Icing Additives
5.1.6. Additives for Combating Combustion Chamber Deposits
5.1.7. Valve Seat Recession Protection Additives
5.1.8. Antiknock Agents

Sounds as if the B&S juice is a cocktail of several of those things.

Incidentally, I have noticed that a mower with a B&S engine always takes several pulls of the string to start when cold, whereas mowers with Honda engines (and a manual choke control) invariably start at the first pull.

Here in France, many filling stations sell both "95 octane" and "95 octane E10". I don't know for sure but I imagine that the non-E10 95 octane is ethanol free. Anybody know for sure?

** Available on for £4978.86 (hard cover). Please send me a scan of those pages when you have received your copy.

Oct 5, 2016 at 9:58 AM | Unregistered CommenterMartin A

Martin A thank you for a useful update! It seems that people who take the time to buy and use B&S Juice are less likely to rely on petrol stored for 6months+ anyway.

FWIW In my experience ...... Honda Lawn Mowers and Outboards are very good engines, let down by stupid cost cutting on some cheap ancillaries. I would buy again.

40 years ago, Briggs and Stratton engined Lawn Mowers were regarded as the best. I think the Japanese have overtaken them on quality and undercut them on price.

Oct 5, 2016 at 7:33 PM | Unregistered Commentergolf charlie

I'm another owner of a small 63cc 2-stroke portable generator and I've experienced exactly the same problems as many other correspondents, namely the varnish-like deposits in the carburettor. After a lot of work, and with fresh fuel, the generator now runs but needs some choke all the time, suggesting that I've still got a blocked jet somewhere. These tiny carburettors can't be completely dismantled for cleaning, but I've had some success with a small ultrasonic cleaner, hot water and detergent. More work still to do. For the future, I too have bought some of that Briggs & Stratton fuel stabilizer. Time will tell.

Anyway, the point of this post is to say that I have kept unleaded fuel in both metal and plastic cans, and both types exhibit a regular build-up of vapour pressure, particularly the plastic can containing mixed two-stroke, which on occasions bulged quite alarmingly. I can only assume that some volatile compounds are evaporating out of the liquid all the time.

I'm also reluctant to use any of this old fuel in my wife's petrol car. I do, however, have a small collection of petrol blowlamps which will probably burn the stuff.


Nov 11, 2016 at 10:38 PM | Unregistered CommenterPhil G4SPZ

Phil G4SPZ

Try searching for a carburettor cleaner fuel additive and add it to the fuel, and run the engine until it runs out of fuel. This may remove the last of the crud.

To avoid fresh crud forming, don't store petrol in the machines fuel system. To avoid petrol "going-off", don't store petrol in your cans either.

Buy petrol when you need it in one of your fuel cans, decant what you need into the other can, and add the amount of 2 stroke oil you need. Put the rest of the untreated petrol in the car, so it gets used.

If you find yourself with 2 stroke fuel mix left over, add the appropriate amount of carburettor cleaner, and run the engine until it runs out of fuel. This will be less effort than trying to decrud your fuel system manually again.

Nov 11, 2016 at 11:16 PM | Unregistered Commentergolf charlie

Here in the states, where petrol is called gasoline, most of us store gasoline long term, like for lawn mowers and such, with additives like "Sta-bil". This extends the storage life out towards a year or so.
But petrol/gasoline does go stale, like any long chain carbon based liquid. Olive oil is kept in dark well sealed bottles for a reason.
My bet is that oil storage faces some pretty steep challenges as well.

Nov 11, 2016 at 11:24 PM | Unregistered Commenterhunter