Does petrol really go stale?
Petrol from the pump is a mix of many organic components with different properties that determine the performance of the fuel. If left in an open container it will, in time, completely evaporate. But as it evaporates, the composition of the fuel will change as different chemical components evaporate at different rates. This evaporation also happens in fuel cans and tanks and the process of degradation starts the moment the fuel is purchased.
Petrol will generally last for around 3 weeks at summer temperature in a vented fuel tank, after which time the performance will suffer, and it will be best to add fresh fuel to restore the performance. Petrol in a sealed container will last for more than 6 months before the performance suffers too much.
Fuel stored in underground garage tanks is unlikely to degrade much because of the rate of replenishment, except in remote, small garages which these days are rarely found.
How the petrol changes in the machine’s fuel tank.
The lighter, more volatile components evaporate first. These are the chemicals that provide valuable octane benefits on starting from cold. These are very volatile and compose most of the fuel/air mix during initial startup, but when they are depleted by evaporation the mixture becomes leaner, causing poorer detonation, pre-ignition and piston damage, especially in small high revving two strokes such as strimmers and chainsaws.
The fuel that remains when the volatile parts have evaporated has a higher density, as well as a higher octane rating, but as it is not as volatile, cold starting is impaired. Because carburettors meter fuel by volume the mixture now becomes richer because of the extra amount of fuel in the denser liquid. This richness will cause plug fouling and unburnt carbon deposits will block exhaust ports. The lack of volatile octane will suppress full revs.
So, in short, degraded fuel will result in hard starting and lack of top end revs and power, but the machine will run once started.
Formation of Gums and Other Deposits
After several months storage at summer temperature the petrol will start to form carboxylic acid and gums. This degradation will continue as the fuel ages and the carboxylic acid which forms will slowly attack the soft materials in the fuel system. The first to suffer will be the rubber and plastic fuel hoses, which will lose their flexibility, followed by primer bulbs and carburettor diaphragms. Finally, even the aluminium of the carburettor will be eroded.
The ‘gums’ or ‘varnish deposits’ are solid materials which settle on the inside of the carburettor, blocking the microscopic bores and jets, thereby affecting the operation of the carburettor.
Stale fuel is the number one cause of stiff metering diaphragms and the primary cause of carburettor replacement. Gums and varnish will quickly stick the piston rings of a running engine causing expensive failure.
If this engine is lucky enough to start it will run on a separated fuel /oil mix and rapid engine damage could occur, but more commonly the damage is done during storage while the phase separated fuel corrodes the inside of the carburettor. Damage of this nature will not be covered by warranty.
Stale petrol is nothing new
We had the problem of stale fuel long before the addition of ethanol, it’s just that today’s fuels turn stale much quicker than in the past, in fact, depending on temperature, light and humidity, the fuel can be unusable in as little as 30 days. The ethanol speeds up the process for two reasons, it has an affinity to water, and the fact that ethanol is oxygen rich, speeds up the oxidation process to acetaldehyde (ethanal) finally into ethanoic acid (aka acetic acid, which is a carboxylic acid).
How do we overcome the problem?
- Keep your fuel fresh and store it in sealed containers in a cool dark and dry place (under the bench on a stone floor, rather than on the bench in front of the shed window). Store your machines in a similar manner.
- Purchase only enough fuel for 30 days use, and never, ever use fuel purchased over 60 days ago. You may get away with it, but the chances are that you will have problems.
- Use a fuel additive such as Briggs and Stratton Fuel Fit or Hayter’s Premium Fuel Treatment. If this is added correctly to fresh petrol at the time of purchase, then your fuel should stay usable for up to 90 days. Remember that additives will never be able to rejuvenate old, stale petrol whatever the manufacturer claims on the bottle.
- You could drain your machine before storage, some manufacturers even recommend this, but this may cause the carb metering diaphragm to shrink and go brittle. It never rehydrates properly.
- You could change the type of petrol that you purchase. Shell Nitro +, BP Ultimate and Esso Synergy are 3 brands of petrol that we know of that do not contain ethanol (Others may be available!). This petrol will still degrade over time, but the degradation process will be much slower and take place to a much lesser extent than the standard 91 RON or E05 variant available.
How to recognise stale petrol
As petrol ages it will change noticeably in colour, smell, and viscosity. New petrol, fresh from the garage, will have a pleasant ‘tang’ and will be almost clear, with just a slight colour, but as it ages it will develop an unpleasant smell more akin to paint or varnish which will linger on materials or skin for much longer, whilst the colour will deepen considerably. The old fuel will also become much thicker, more akin to very light oil or diesel fuel. Really old petrol will have a very heavy and unpleasant smell and can be very dark in colour, whilst being very thick, more like a liquer.
What is the best solution?
The best solution by far is to use an alkylate fuel such as Motomix or Aspen. These fuels are derived from petrol but go through a special ‘alkylation’ process which removes 90% of the unwanted chemicals, leaving around just the 10 essential components. This fuel performs as well (if not a little better) than standard petrol but is chemically stable and will take much longer to degrade. Fuel lines, tanks and carburettors will not be attacked by this fuel and the machine can safely be stored for years with the same fuel with no detriment.
In addition, the clean burning nature of the fuel will ensure that the internal workings of the engine stay clear of any gums, varnish, or carbon residues.