E10 petrol – Can I Use It In My Outboard Engine?

09 Feb

E10 Petrol and Outboard Motors

There is a lot written about E10 petrol for use in road vehicles – including the government E10 vehicles checker and the Department of Transports E10 petrol explained guidance notes. The government guidance delivers good information on the bigger picture but just two lines on use in boat engines and outboard motors. Unsurprisingly this is simply a warning to check with your engine manufacturer. 

About E10 Petrol

E10 petrol (fuel containing up to 10% renewable ethanol) meets the same quality standard as unleaded petrol 95 (E5 – fuel containing up to 5% renewable ethanol) but has as greater volume of renewable fuels (ethanol) blended into it to reduce the nations CO2 footprint. It is worth noting that E10 will have little impact on emissions associated with air quality and public health. E10 is all about the nations carbon footprint and its use will be equivalent to removing 350,000 cars off the road in the UK.

E10 Petrol – The Environment

It is fair to say that the vast majority of the boating community want to do their bit for the environment. As a community we really immerse ourselves in the natural world and are arguably more connected to the environment than many others, consequently, we want to protect it for the future. 

However, as with many forms of environmental protection involving people, there are compromises to be aware of, but these are not insurmountable. E10 petrol has been used around the globe for about 25 years, including Australia, the USA and parts of Europe so this is not new. A bit of knowledge and, some appropriate action will offset any ill effects. Modern engines are better prepared but some of the older engines need a bit more attention.

A Word on Petrol Labelling 

The wording on the labelling can be misleading for those unaware of what it really means. The use of two small words “up to” serves to change the definition of the assumptions we all make. Simply assuming that E5 is 5% ethanol and E10 is 10% ethanol is too simple. The labelling reads “up to 5%” or “up to 10%” as it is legally required to be displayed on all unleaded fuel pumps. This means the inclusion of these two small words is to the advantage of those concerned about the ethanol content in their fuel. 

Esso states on their website, “Synergy Supreme +99 is actually ethanol free…. Legislation requires us to put E5 labels on pumps that dispense unleaded petrol with “up to 5% ethanol” including those that contain no ethanol which is why we display them on our Supreme Synergy +99 pumps”

(Source: )

Shell, BP, Tesco and other fuel suppliers do not commit to such details, merely stating their fuel, “may contain up to 5% ethanol”

(Source: LAA Magazine – September 2021 | The Light Aircraft Association – page 33)

E10 Petrol – The Compromises

All this said, as mentioned above, there are still compromises, and these include:

  • Damage to parts due to increased ethanol content
  • More rapid deterioration of fuel
  • Disposal of stale fuel 
  • Lower energy density of the fuel

Damage To Parts Due To Increased Ethanol Content

The inclusion of the ethanol in petrol creates two main issues:

  • Water absorption by ethanol 
  • Corrosion

Water Absorption by Ethanol

The water absorption is an obvious issue – nobody ever wants water in their fuel. However, water is attracted to ethanol so you will get water in your fuel. A modern engine will have a fuel water separator between the fuel tank and the engine, and this will extract the water from the fuel. During normal regular use this will deal with the water. 

The water issue is compounded by inactivity; the most obvious example of this being the winter layup. The combination of dormant fuel containing ethanol, and part full fuel tanks (which are prone to extra condensation where the fuel is not covering the internal walls of the tank) serve to add water to the fuel. The water separates out and sinks to the bottom of the tank and in the worst-case scenario overcomes the water separator and contaminates the engine. To prevent this happening, it is recommended that during early season use, especially after layup, regular checks of the water separator are conducted to ensure that it is not overcome. A full tank for winter lay-up will prevent condensation in the tank but an additive should be used as well.

A fuel additive, as a preventative measure rather than a reactive option, will serve to offset a lot of the issues with the ethanol. There are plenty of option on the market but in our workshop, we use Fuel Set. We use this product because we can buy it cost effectively in concentrated form (10ml to treat 40 litres of fuel) and can use it year-round if deemed necessary. Other products are available, and we have no evidence against their effectiveness so choose one that works for you. The important thing is adding a conditioner or stabiliser to your fuel will give you additional protection from water absorption and preserve the life of your fuel. 


While the effect of water on the internal parts of your engine are detrimental so are the effect of the ethanol mixing with water. It is not the ethanol alone that causes the issue, it is the combination of ethanol with water that becomes acidic. Modern engines on EFI fuel systems are better able to withstand the effects of this acidic mix if taking the control measure mentioned within this article.

The biggest issue is around the older engines still operating on carburettors. The fuel dries out in the carburettor, and this can be corrosive. Needle valves and small springs are prone to rust and the materials that are commonly used to manufacture gaskets, seals and fuel lines are not always manufactured with ethanol-resistant fluorinated polymers. 

 If you escape the corrosion issue you will have small particles of ethanol out of solution or rust or gaskets parts because of the damage caused by corrosion. The moment you try to operate the engine and pull these particles into the carburettor jets you will block them. Most often they will not unblock again, even when using a proper ultrasonic cleaning bath. This will result in the cost of a new carburettor and the cleaning out of the fuel system. Sadly, the smaller your engine the more likely you are to have an issue with E10 fuel.

I need to be honest here…the issues are all the same with E5 petrol, the change to E10 is only going to exacerbate the problem so if you had a problem before you’ll get it again but just a lot sooner unless you take preventative measures. 

To avoid these issues, you must drain the carburettor completely of fuel before laying up or for any extended period of non-use. The best advice is to drain the carburettor after each use; it’s all too easy to think it will be OK and then forget and leave it too long.
An easy way to do this is to disconnect the fuel line and let the engine run dry. When it stalls try and start it again, often it will run on a little before it stalls again. This method also allows you to flush the cooling system with fresh water (especially with portable engines) while the fuel is running dry – a double benefit. 

Rapid Deterioration Of Fuel 

With E10 this deterioration can start as soon as one month after being dispensed from the pump. Again, the water is causing the issue here. While ethanol can absorb water it also reaches saturation point very quickly as saturation occurs at approx 5ml (approx a teaspoon) of water per litre of petrol, and once saturated phase separation occurs. In layman’s terms this is the separation of the water and ethanol mixture from the petrol which then sinks to the bottom of the tank as it is heavier than petrol. This leaves you with two problems:

  1. The water at the bottom the tank is picked up before the fuel and damages parts of your engine if there is no fuel water separator fitted. 
  2. The petrol left in the tank has a lower octane level after phase separation so gives you less efficiency when combusting, thus effecting the performance of your engine.  

A fuel additive, like Fuel Set (mentioned above) can help slow down or prevent phase separation and keep the fuel in good condition. If in doubt use an additive at each fill up to keep the fuel healthy.

It is important to recognise that a fuel additive is a preventative measure and will not reverse the effects of phase separation, there’s no way to reintegrate petrol and ethanol once they have separated.

Disposal Of Stale Fuel 

Even when regularly used a leisure boat engine and fuel tank are not used with the regularity of a car. This leads to periods of non-use that can see the fuel go stale as described above. Once the fuel has gone off it needs to be disposed of professionally. This is expensive and difficult, and if the boat is on a pontoon in the marina a pump out facility is required, and this adds cost.

To avoid this issue, keep the tank topped up, use an additive to condition the fuel and drain the fuel system of fuel during long periods of non-use. Engines using carburettors, especially the smaller portable engines, will suffer the most if not drained down. 

Lower Energy Density

To achieve full power fuel air mixture may need adjusting when running your engine on E10 fuel. Again, the modern engines will be set up for this, but the older ones will not be set up for E10. The reality is that if different size jets in the carburettor are required then the whole system will need resetting to ensure correct combustion in the chamber. You may need to consider the wisdom of spending a lot of money on an old engine when it might be simpler to start again with a more modern engine and retire the old one.
A good workshop will give sound advice about the best course of action; don’t shoot the messenger if they do not have a cheap and cheerful solution!

E10 Compatible Outboard Engines

There is a lot to consider in this article but for many owners this will be simple academic interest as they will be fine to carry on, more or less, as they were when using E5. The following engine specific information is for UK and European engines only. Engines manufactured for other regions of the world may differ, check with your countries national manufacturer’s support for clarification. 

This list is not exhaustive but covers the main operators in the UK:

  • All Suzuki 4 strokes are compatible with E10 petrol. Suzuki no longer manufacture 2 stroke outboards and any still in service are not compatible with E10 fuel. 
  • All Mercury / Mariner engines built since 1996 are compatible with E10 petrol.
  • All Yamaha outboard motors built since 2003 are compatible with E10 petrol.
  • All Honda engines produced for the EU market since 1996 are compatible with E10 petrol.
  • All Tohatsu engines are compatible with E10 (no dates given) – see the Tohatsu Marine UK E10 statement
  • Evinrude E-tec engines (all 2 strokes) are compatible with E10 petrol

E5 Fuel Availability

If after all this, you are concerned about E10 fuel and would prefer to keeping using E5 we have made the calls around The Solent area and have the following guide. 

All petrol outlets from Poole to Chichester, all MDL marinas and all Premier Marinas from 1st March 2022 (see the below amendment) will dispense E10 petrol only. 

E5 petrol is available from the following places, all stated that it would be supplied while it was available from their wholesalers:

  • All Premier Marinas nationally
    • 14th Feb 2022 amendment: Premier announced that they will move to E10 only fuel across the group starting from 1st March 2022. This will be phased in to allow customers time to adjust and signage at the pump will be updated when the change is complete at your location.
  • Lallows in Cowes (also selling E10)
  • Salterns Marina in Poole Harbour

If you have read this far and are concerned about E10 fuel at least this leaves you with a positive finish. E5 is still out there, and you can purchase this from the water edge at plenty of places on and around The Solent. For those of you operating on portable fuel tanks there is always the E5 option, or even ethanol free at Esso, at your local garage as well. 

No matter what fuel you opt to use in future the best practice advice is to:

  • Use a fuel additive 
  • Drain your carburettors after each use 
  • Never use old fuel. 

If you do these things your fuel system will look after you in the way you would expect… just don’t forget the annual servicing and winterising for everything else. 

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