Putting the wrong fuel in a car

As a driver, misfuelling your car is something you want to avoid at all costs. In this blog, we discuss the symptoms of misfuelling a petrol or diesel car and what to do if it happens. Ultimately, driving with the wrong fuel can lead to very serious engine damage. If it happens to you, it’s important to fix it as soon as possible.

Although it may seem like a silly mistake to make, don’t beat yourself up, it’s more common than you might think! According to the AA, they deal with 16,000 fuel mishaps a year, the equivalent of more than 45 times a day on average.^[1]

What happens if you put the wrong fuel in your car?

When the wrong fuel is put in a car, it means the engine will not be able to process it and will not run as it should. It can also lead to problems with other parts of the car, such as the fuel system and the catalytic converter. This can negatively impact the residual value of your car when it comes to selling or exchanging it for a new car.

There are many tell-tale signs and symptoms you may experience if you have put the wrong fuel in your car. The symptoms can be different depending on which fuel type your car is and which fuel you have put into the fuelling system.

What happens if you put petrol in a diesel car?

In diesel cars, the fuel itself is also used as a form of lubrication for specific parts of the engine, helping to keep the car running smoothly and preventing wear and tear. However, if petrol is mixed with diesel, it will act as a solvent and dissolve the lubricant, causing damage to some of the delicate components. Because of this, putting petrol in a diesel car is more serious than putting diesel in a petrol car.

As petrol circulates in the diesel fuelling system, critical components, including fuel injectors, pumps and the entire fuel system, can be irreversibly damaged. In worst-case scenarios, you may need to replace your fuel system.

If you’re aware that you have filled your diesel car with the wrong fuel, the best thing you can do is not start the engine. Turning on the car's ignition will circulate the mixture instantly. As petrol and diesel combust in completely different ways, this can lead to immediate engine damage as well as total engine failure. As little as 1% petrol in a diesel fuel system can be completely detrimental.

Symptoms to look out for if you have put petrol in a diesel car:

• A large amount of smoke coming from the car's exhaust.
• The car struggling to start or not turning on at all.
• Knocking noises when accelerating.
• Slow acceleration
• Illuminated engine warning light.

What happens if you put diesel in a petrol car?

Even though putting diesel in a petrol car is less serious, it can still cause major problems, again, due to the lubricating nature of diesel. As diesel makes its way through your car’s engine, it will coat key components and systems, leaving them unable to function properly and making it much more like for the engine to misfire.

Again, if you notice before starting your car, do not turn the engine on. If you have the diesel removed from the petrol fuel system, your car should be ok. It is said that if you have put under 5% diesel in your petrol fuel tank, sometimes the fuelling system can cope without being drained and just topped back up with petrol.

However, you should always seek professional assistance and, in most cases, the professionals will still advise draining the fuel system as soon as possible to be safe. If you’ve used diesel in a petrol car, you may experience:

• Engine misfiring due to the diesel coating spark plugs and the fuel system.
• Immoderate amounts of smoke coming from the car's exhaust.
• The engine failing to start/restart.
• The engine cutting out.

What happens if you put E10 fuel in a non-compatible car?

E10 fuel was introduced to pumps in the UK in 2021 to reduce CO₂ emissions , as E10 contains 10% renewable ethanol. E5 petrol only contains 5% renewable ethanol. Most petrol cars on the roads today can use E10 if they have been manufactured from 2011 and afterwards.

Older cars can be incompatible with E10 fuel. However, if you do have an older car and have fuelled it using E10 rather than E5, fill it back up with E5 as soon as possible. This will reduce the amount of E10 in the system while increasing the amount of E5 back to an appropriate ratio. If you find that your car won’t start in this scenario, you should call to be recovered by your breakdown cover provider.

What to do if you’ve put the wrong fuel in your car - a step by step guide

There are a few different steps to follow if you realise that you have put the incorrect fuel into your car.

1. Step 1. Do not start your engine

   If you realise your mistake straight away, it’s so important that you do not start the engine. If you do not start the engine, the incorrect fuel will not be flushed through the fuel system.

1. Step 2. Put your car in a safe spot

   If you have already started your engine, find somewhere safe to turn off the engine, stop and park the car, putting the vehicle in neutral. Call for breakdown cover and ask for someone to drain your fuel tank. DO NOT try to restart your engine under any circumstances.

1. Step 3. Call for assistance

   Seek professional help from someone qualified to assess the situation and carry out the relevant work. They may be able to drain the fuel tank and flush the system with the correct fuel. However, especially for diesels, it may be that your engine and fuel system have been permanently damaged. In this case, you may need to arrange transport for you and your car.

1. Step 4. Notify your insurance

   If necessary, you should notify your insurance provider that you have misfuelled your vehicle. Some insurers may cover you for this and subsequent repairs if your policy covers certain types of ‘accidental damage’. Contact your insurance provider directly to see if you are covered.

   Draining your car of the wrong fuel is carried out by connecting an extraction hose to a drain plug and extracting it. However, as many modern cars have sealed tanks, it can be more difficult to access and drain the fuel. This may mean that your car will need to be towed away to carry out repairs.

   In the worst-case scenario, as mentioned above, the whole fuel system may need replacing. This is an extremely costly situation to find yourself in and sometimes it may cost you less to sell the car rather than try and replace it. This is more common in situations where petrol has been entered into a diesel fuel system.

Need help after misfuelling?

At Arnold Clark, we have thousands of dedicated servicing experts who can help you with repairs if you have put the wrong fuel in your tank. Contact us or find your nearest local dealer for repairs today.

References

1. https://www.theaa.com/breakdown-cover/fuel-assist