Is E10 bad for your car?

01/05/2025 02:00 PM

Since legislation was introduced supporting E10 in 2003 and later going on sale in 2007, E10 has always been a hot topic for motorists. For the most part, E10 is the cheapest option at the petrol bowser.

RELATED: Does your car actually need premium petrol?

The Australian Government was so hopeful for the future fuel that in 2009 there was an amendment made to the Biofuels Act to phase out regular 91-octane unleaded petrol by 2011 in lieu of E10.

By the time 2012 rolled around, the government had decided to axe its plans to phase out regular 91-octane unleaded after the public raised concerns about certain cars not being able to run on the fuel and needing to swap to a premium 95- or 98-octane petrol.

As it stands now, New South Wales petrol stations are mandated to have at least 6 per cent of the total volume of fuel they sell in a quarter be ethanol, while Queensland is mandated to sell 4 per cent per quarter. Other states are not mandated to sell the fuel.

More than a decade on from the government backtracking on its mandate to make E10 the standard fuel, we investigate the question: Is E10 bad for your car?

What is E10?

E10 is a petrol mixture containing 10 per cent ethanol and 90 per cent petrol. Ethanol is a biofuel made by fermenting grains and sugars from products such as corn, food waste, and sugar cane. The fermented sugars and grains are then distilled and blended with petrol.

Most of the ethanol used to make E10 comes from New South Wales or Queensland. It is made by fermenting starch left over after wheat has been turned into flour, or from grain sorghum.

The E10 ethanol/fuel mixture works out to be 94 octane.

Is E10 the same as 91?

No, E10 is not the same as 91-octane petrol. As mentioned previously, E10 is a mixture of petrol and ethanol, while 91-octane is straight unleaded petrol. E10 is 94 octane, while standard unleaded is 91 octane.

We go more in-depth into what an octane reading means for your car in the story at this link, but in short, lower-octane fuels are considered to be ‘less stable’ than higher-octane fuels – and as such, if your car has not been designed to accept them, E10 or even 91-octane fuels could cause damage to the engine in the long term.

Vehicles that have been developed to support E10 fuel can typically accept 91-octane regular unleaded – but, particularly for older vehicles, the opposite is not always true.

Many new cars – primarily a number of European vehicles – are not designed to support 91-octane or E10 petrol, instead requiring premium 95- or 98-octane fuel.

When you have a more volatile fuel, such as 91, it can detonate at different rates in the engine’s cylinders, creating an explosion too early and doing something called ‘knocking’ or ‘pinging’ depending on how aggressively early the fuel has detonated.

A knock event will occur because the piston in the cylinder is not at the optimal height in its up-and-down stroke.

While the piston is still travelling with an upward force, the explosion happening too early will attempt to force the piston back down when it’s not ready.

Vehicles that accept 91-octane fuel have an ignition timing that allows for the volatility of 91. But for vehicles that need 95 or 98 octane, at best, this will cause your engine to lose performance, run rough and vibrate a lot. At worst, it will cause a knocking noise, bend the connecting rod from your piston to your crankshaft, melt the piston or even cause your piston to expel itself from your engine block.

By opting for fuel with an octane rating better suited to your engine, the engine will see a more stable burn due to needing to be ignited at a higher temperature and burning slower than a lower-octane fuel.

Is E10 bad for your car?

E10 is fine in cars that are built to withstand the ethanol mixture. But if your car is not E10 compatible, it can actually cause more issues than the aforementioned pre-detonation.

Ethanol is a more aggressive solvent than petrol, which means that over time it can deteriorate gaskets, O-rings, and rubber hoses if your car has not been built with ethanol-compatible rubber.

Also, because it is mixed with water and enzymes, it is more corrosive than petrol, which can cause issues with metal petrol tanks, non-compatible fuel pumps, and fuel rails.

Lenny Sie, a mechanic at Hunter Street Automotive in New South Wales, said he has seen many issues with E10.

“When putting the wrong octane fuel in your car, provided that a modern-day engine is smart enough, which they generally are, the overall performance of the vehicle would just not be there. The customer would complain that the car is sluggish, non-performing, and hard to start when the lower-octane fuel starts glazing the spark plugs,” Sie said.

“This was one of the biggest problems when E10 fuel came out. We found that, yes, the engine could accept that fuel, but we found all of these detrimental issues like spark plugs glazing and plastic body fuel injectors were not designed for ethanol fuel, and they were locking up and leaking.”

“All because ethanol is a very dry fuel with no lubricating properties.”

The issues are mostly for vehicles built before the 2000s. In the 21st century, most vehicles are built to withstand ethanol fuel. The NSW Government released a free compatibility checker, which you can find here.

There is also the matter of fuel efficiency. E10 is a less fuel-efficient petrol due to ethanol requiring more volume to detonate at the same rate as regular unleaded fuel.

Therefore, while E10 may be cheaper per litre, your vehicle may consume more fuel on the road – so you may not notice any savings when it comes to the cost per kilometre.

It depends heavily on the cost difference between the two fuels. Differing studies suggest E10 will see fuel consumption increase by around 3 to 5 per cent compared to 91-octane unleaded.

To see an increase in savings, you would need the fuel to be more than 5 per cent cheaper per litre, and ensure your car is compatible with E10 so you’re not putting strain on the components.

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