Does petrol go off?

[ThreeSheds]

I have heard many claims that petrol will 'go off' over time - some people claiming that even after only a couple of months, old petrol will cause poor running and one person even told me that it wouldn't burn... This is at odds with my experience, in fact I happily used petrol in the past that had been sat in the RRC tank for nigh-on two years with no ill effect.

Now I have just read an article in SuperBike by a learned gentleman who appears to back up my opinion that old petrol is unlikely to cause any problems, so I located a copy on the web and here it is:

Originally Posted by SuperBike Mag, Feb 2010:

Does petrol go off over time?

All sorts of problems are blamed on petrol that's "gone off" either in a bike's tank or in a separate petrol can. Is this really a problem, and, if so, how should you combat it? We've heard the same about brake fluid, so is that the case too?

Once again, it's over to the cleverest bloke we know, John Rowland, Fuchs Silkolene's top automotive research chemist to cut through the internet hype and give us the facts.

"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."

"As for a vented fuel tank like most bikes have fitted, there will be some minor vapour loss of the more volatile fractions, such as hexane, but nowhere near as bad as losses from a container exposing a large sufface area of fuel to the air - such as a frying pan."

"Volume changes in the tank airspace due to daily temperature changes cause the tank to 'breathe', but one breath every 24 hours isn't so bad. Even a few months of this with a half full tank isn't a big deal. I get the impression from some bike owners that they think the fuel tank gasps away like an elephant running a marathon. In reality, it's more like a hibernating dormouse. Imagine a 16-litre tank with an eight-litre airspace, starting at 10C it cools to 0C. Using Charles' law, we can calculate the tank would draw in 300cc of air, and blows it out again when it warms up to 10C. But, petrol vapour is heavy, and the air drawn in will tend to sit on top of it, so, when air is lost when the tank warms up, it will be mainly the top layer with only a small amount of fuel in it."

"As for petrol evaporating from the carbs, some fuel, particularly from non-mainstream suppliers, can leave a tacky deposit when it dries out, but this was more of a 1990s problem. You don't hear of it happening today where civilisation (as we know it) exists. Even so, it's a good idea when putting a bike in storage to add a drop (5cc/litre) of good two-stroke oil to the fuel, run the engine until it's good and hot, shut down and cover with a thick blanket (cuts temperature fluctuation). Then, if the fuel dries out, a protective oily film will be there which is easily washed off when the bike is run again."

"Brake fluid (avoid the cheap stuff) will keep for years in a sealed, full, plastic bottle. Even a part-empty bottle is OK if the top is on tight. People worry too much about this water-absorption business. OK, it happens, but it's a slow process. Anyway, the boiling point of damp brake fluid is around 190C, so, unless you're out to bust the lap record at the IoM, it's not going to let you down."

In summary, fuel and brake fluid are unlikely to go 'off' unless stored incorrectly for years rather than months.

So there you go. If anyone has any old petrol that they think is useless - give it to me please

Roger

[zim]

I've had petrol go off. Whether it's from time and / or elements of the weather. It has a different smell to 'fresh' petrol.

And v8's don't run well on it !

G

[Orgasmic Farmer]

Try as hard as you like you will not start out lawnmower after the winter until you drain the tank and put in fresh petrol. Then off it goes not worries. The science might say that but reality prooves otherwise. May be something to do with the very small volume relative to area exposed??

[Mark90]

One point he fails to mention is that when a significant amount of the highly volatile fractions have evaporated the petrol becomes harder to ignite, it has 'gone off'. This is not likely to become significant with large quantities stored in sealed cans over a reasonable length of time. But with smaller amounts stored relatively open, ie a carb, then the more volatile fractions will evaporate first, and quite quickly. What's left becomes harder to ingite with time as more and more of the higher fractions evaporate. The effect is most noticable on fussy starters. With my XT225 if I just switch the ignition it off, turn off the fuel tap and leave it for a couple of weeks it can be a bugger to start on the fuel that's been sat in the float chamber. If I turn off the fuel tap about 1/3 mile from home so I get back with an empty float chamber. Then when it comes to start it I open the tap and fill the float chamber with fuel from the tank and it starts fine, even if left for months. So yes the fuel does go off, but not as quick in the tank as it does in the carb.

Regarding OF's lawnmover, we have similar experince with our petrol pressure washer which is just used during the race season, that needs fresh fuel after being left for usually about 4-6 months. It is a fussy starter and it has a small cheap plastic tank with screw cap that doesn't fully seal, if it's not upright you do get visable sepage, so it's no suprise that the volatiles do escape.

[landy andy.]

In my experience it will go off if left in a tank or carb, as they have vent holes.

It does not seem to go off in sealed air tight containers, such as jerry cans.

Andy

[Soren Frimodt]

I fix Motorcycles and scooters for a living and see this problem all the time.

Petrol does go off, and as mentioned above it doesn't seem to have that much effect on larger vehicles such as cars. But it seems as though the smaller the engine the more fragile it is. For instance; if I have scooter with a tiny bit of water in the petroltank it won't run properly. But if I put this water/petrol mixture in my V8. which is also on carbs, it just rumbles on. Same goes with old petrol. I have cleaned/flushed many imported motorcycles that simply would not start on old petrol. So the science may say otherwise but I've tried a million times.

[Night Train]

Maybe that explains my chainsaw starting issues when it has been left for a few months.

Perhaps it is a good excuse to get driving any petrol powered projects more frequently.

[mmgemini]

Ah Yes Progress

Maybe the dangerousd stuff sold now goes off. In my youth barn finds generally started on what was in the tank and ran fine on it.

[redneck]

When I work on marine engines I come across this all the time, outboards are left sat in a locker for months on end, the owner goes to use it and is surprised it won't start, usually blaming the engineer who last looked at it. We change the fuel and off he goes a happy customer, till the next time

I have also had brake systems lock up due to water absorption (it was sat standing for about 3 years) and the fluid was a high quality racing fluid.

Never believe a scientist, several I have worked with would struggle to tie their shoelaces (hence the sandals) :D :D :D

[Joe 90]

in my experience petrol does go off and when it does it smells of bad wiskey, the sort that makes you go blind . but we all now that petrol is the fluid of the devil were as diesel is fluid of the gods now dont we.

[landroversforever]

Petrol would always go off in the tank of our old lawn mower. Pain to start, so change the fuel, and would fire up on half a pull of the cord.

[ThreeSheds]

in my experience petrol does go off and when it does it smells of bad wiskey, the sort that makes you go blind . but we all now that petrol is the fluid of the devil were as diesel is fluid of the gods now dont we.

I beg to differ - it is patently obvious that God's gift to women is me, whereas God's gift to me is the V8 PETROL ENGINE

(actually diesels are pretty good too but I just don't understand them when they go wrong so I tend to think of them as black art and witch-craft)

Roger.

p.s. Interesting that nobody supports our learned research chemist - I wonder if he is solely working from theory and without any practical knowledge?

[callum]

maybe its a question of how sealed is sealed. perhaps in a well sealed metal container it does not go off, but it would seem from the evidence above, in less than perfectly sealed containers it does.

[Mark90]

perhaps in a well sealed metal container it does not go off

That's the crux of the matter, the container must be well sealed. Petrol doesn't go 'off' in the same way milk goes off. But if the container isn't 100% sealed then the higher volatile fractions will evaporate, making what's left harder to ignite - that is basic 'science'. In the real world how many containers are 100% sealed to petrol vapours, or how many smell of petrol even when 'sealed'?

[vexedfoxy]

I always assumed that it does 'go off' because some parts of it evaporate faster than others, always drain the tank, I have a small collection of stationary engines that always start better with 'fresh' petrol.

[missingsid]

I currently have a tank full of 10 year old petrol, the V8 started up with no problem but once I removed it and washed out the tank it is not totally happy right now. This is probably due to water in the fuel.

I am at a crossroads do I keep adding new fuel hoping to improve what's there or drain it and start again?

IIRC old fuel also leaves behind a resiny type residue which glues up carbs.

As for the ability to burn, my Z1000 had 2 year old fuel in it which smelt funny and would not start. Once drained out and fresh added it was fine so I poured a little of the old fuel on the concrete path and lit it.

Very slow burn with lots of black smoke and a black residue on the path which is still there now.

Perhaps this is the problem, no fast burn for power and lots of soot to much up the plugs?

Is there any way of getting rid of any water absorbed in the fuel?

Marc.

[Soren Frimodt]

I currently have a tank full of 10 year old petrol, the V8 started up with no problem but once I removed it and washed out the tank it is not totally happy right now. This is probably due to water in the fuel.

I am at a crossroads do I keep adding new fuel hoping to improve what's there or drain it and start again?

IIRC old fuel also leaves behind a resiny type residue which glues up carbs.

As for the ability to burn, my Z1000 had 2 year old fuel in it which smelt funny and would not start. Once drained out and fresh added it was fine so I poured a little of the old fuel on the concrete path and lit it.

Very slow burn with lots of black smoke and a black residue on the path which is still there now.

Perhaps this is the problem, no fast burn for power and lots of soot to much up the plugs?

Is there any way of getting rid of any water absorbed in the fuel?

Marc.

Exactly how I test it on our metal-welding bench at work! Slow burn and poor ignition equals old fuel! Also if you have a feeling theres oil in your petrol, ie somebody "accidently" put motor-oil in it, and the engine therefore keeps flooding the sparkplugs. Just let a bit of it on fire and if you are left with a pool of sticky white bubbly oil, thats your problem

[sotal]

I've got a motorbike in the garage which hasn't moved for going on 4 years, I start it every 6 months or so and it still starts up perfectly every time, it's a 600cc.