Has your fuel gone off?

LAYING A car up will involve leaving a quantity of fuel in the tank and fuel system, but that fuel won’t last indefinitely.

Petrol and diesel fuels are made from distilling crude oil. The make-up of either fuel isn’t just a single ingredient of refined oil, it’s a complex cocktail of chemicals, each playing a part in helping the engine. Typically, the extra ingredients aid start-up, lubrication and clean running of the engine.

Fuels evaporate with age, and it is the volatile additives that evaporate first, the fuel element being heavier. In simple terms, this means the fuel gets more dense with age. In the case of petrol, the octane rating rises with age, meaning a richer air-fuel mix, and more chance of fouling spark plugs, running issues and difficult start-ups.

After just five weeks, petrol is notably different to the correct recipe for consistent running. Engines need an air/ fuel ratio of 14:1, so you can see how far off that balance even a month old petrol will be. Efi systems will automatically adjust, but spark plugs on carbureted engines are more likely to foul due to richer running. If the carburettor is adjusted on aged fuel, the setting will not be ideal when running with fresh petrol.

Diesel fuel ages more slowly, generally lasting 12 months or more. Diesel’s problem centres around a sediment that forms with age, as the fuel reacts with oxygen. The sediment gums up the system, blocking filters, eventually even stopping the engine through starvation. LPG though, is stable and unaffected by storage.

BP suggest emptying tanks before storage. This is difficult and dangerous if not done correctly, so their alternative suggestion is keeping tanks half full with quality fuel. This goes some way to reducing the air space which allows condensation to form. Then, before starting an engine after storage, they suggest diluting the fuel with a third of fresh fuel.