Ethanol blended petrol and classic cars

Ethanol in petrol causes problems in older vehicles and the introduction of E10 regular 95 octane fuel as the standard grade in September 2021 is likely to make matters worse

Fuel cap on a classic car

by Phil Bell |

E10 fuel arriving in the UK

From 1 September 2021, regular 95 octane unleaded petrol in the UK can contain up to 10% ethanol and will be labelled E10, replacing the current E5 95 octane unleaded that can contain up to 5% ethanol.

Ethanol and classic engines

Ethanol is added to petrol to make it burn more cleanly, but is has a number of side-effects, particularly on classic engines and their fuel systems.

  1. Because each molecule of ethanol contains one oxygen atom, it makes the petrol it is added to burn more leanly, which can affect performance and engine temperatures.
  1. Ethanol degrades several flexible materials commonly found in older fuel systems, including seals and pipework, leading to leaks. For a list of ethanol-incompatible materials and suggested replacements, see the fuels page on the Federation of British Historic Vehicle Clubs website ( and scroll down to the ethanol section.
  1. Because classic cars tend to be used infrequently and are commonly laid up for long periods, particularly over winter, they are more prone to the next two ethanol-related problems. The first is that ethanol is hygroscopic, meaning that it absorbs water from the atmosphere. Until closed-circuit fuel systems arrived with modern fuel injection, fuel tanks and carburettor float chambers vented to the atmosphere. Absorbed water vapour will settle out in fuel tanks, causing corrosion in steel tanks and fuel lines, and rough running as the water disrupts the correct atomisation and combustion of petrol.
  1. The second infrequent use-related problem is that ethanol oxidises to create acidic by-products that corrode aluminium and brass components found in fuel systems and engines.

How can I avoid or minimise ethanol-related problems?

  1. Leaner combustion can be compensated for by enriching the fuel/air mixture.
  1. Replace fuel system components with those made from ethanol-compatible materials where possible.
  1. Use super unleaded petrol from 1 September 2021.
  1. Use your classic regularly enough to ensure that the fuel in it is regularly replaced by fresh petrol.
  1. Use a specialist corrosion inhibitor petrol additive.
  1. When storing a car longer term, for example, over winter, drain the fuel tank, lines, filters and carburettors.

Are more modern classics safe to use E10?

Most but not all cars built after 2000 can use E10 blended fuels. To check your car, visit

Protection-grade super unleaded

The All Party Parliamentary Historic Vehicle Group has ensured that super unleaded (97 or higher octane) petrol will continue to be limited to a maximum of 5% ethanol for five years, and it will push for availability to be extended in the future.

Do any brands or grades of petrol contain no ethanol?

Although super unleaded petrol can contain up to 5% ethanol, there is no government requirement for it to contain any. Some makes contain none, although few will state the fact and ethanol-free petrol is not labelled at the pumps. Trying to identify ethanol-free petrol is further complicated by the fact that a brand of fuel may be supplied at different blends depending on the region of the UK, some with and some without ethanol. For example, when this article was written, Esso stated, ‘Esso super unleaded petrol (Synergy Supreme+ Unleaded 97 and Synergy Supreme+ 99) is ethanol free (Except in Devon, Cornwall, North Wales, North England and Scotland).’

Other useful pages

• MoT and road tax exemption

• White/silver-on-black number plate rules

• ULEZ exemption

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