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

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You turn into the petrol station on your way home from work, thinking about what’s for dinner or the next episode of reality TV, and halfway through filling up you realise you’ve accidentally put the wrong fuel in the car. It’s a common mistake, and one that we’re all likely to do at some point in our lives, but what should you do?

In this article, Canstar Blue will look at common fuel types that you’ll see at your local petrol station, and what to do if you’ve accidently put the wrong type in your ride.

Fuel types explained

Confused Woman

When you pull up to the bowser, there are always a few options to choose from, and we’ve all been tempted to go for a cheaper choice at some point. But what’s the difference between them, and will filling up with any type spell the end of your car?

The main difference between fuels is their octane rating, or Research Octane Number (RON). The octane rating relates to the fuel’s ability to resist pre-ignition or ‘knocking’ which is an uncontrolled explosion of fuel in the cylinder rather than a controlled burn. Essentially, the octane is an index of a fuel’s resistance to burning too quickly, which is less efficient for the engine.

  • Regular unleaded petrol is 91RON, premium unleaded is a minimum 95RON, while higher grade premium is usually 98RON

So, can you fill up with any petrol? The short answer is no, you’ll have to be careful which hose you grab to fill your car. It’s best to stick with what’s recommended for the engine, which is usually listed on the inside of the fuel flap. However, if you’re driving a car that usually takes 91RON, you can fill up with 95 or 98 no problem, with some petrol companies even claiming that it’s better for your engine.

But if you’re just filling up with a higher RON occasionally, chances are you won’t notice much of a difference, except maybe on your next bank statement. Similarly, if you drive an older car, you won’t notice much of a difference if you decide to fill up with a premium petrol, although it won’t damage your engine either.

But what happens when you fill up with 91 when your car usually takes premium 95 or 98? That’s when you might find yourself in trouble. A lower octane fuel may not sit well with your engine, causing loss of power or acceleration, as well as the loss of fuel economy and ‘knocking’, which might impact on your comfort behind the wheel.

Ethanol blends

‘But what about E10?’ E10 is a mixture of up to 10% ethanol in petrol, and is becoming a popular option for motorists. If you drive a 91RON car, you can substitute E10, but if your car isn’t suitable to E10, stay clear, as you can damage your car’s fuel lines and engine.

E85 is also becoming increasingly prevalent, bumping up the ethanol blend up to 85%, and can be used a substitute for some fuel types, although it’s best to check with the manufacturer before you fill up.

What happens when you put petrol in a diesel car?

Petrol Pumps

While mixing up petrol may only briefly impact the performance of your car, mixing up petrol and diesel could spell the end for your engine. Diesel works on a very fine tolerance at high pressures, and if petrol is added into the mixture, the lubrication properties are reduced, which can damage the fuel pump, as well as the injectors and the engine.

If you realise that you’re putting petrol in your diesel car while still at the bowser, it’s recommended that you stop and call roadside assistance, or services like Wrong Fuel Rescue. Starting the engine will cause the fuel to circulate through the system, causing further contamination, so to keep damage to a minimum, stay away from the ignition.

If you don’t realise your mistake before you drive off, your car will let you know soon enough, as the engine will start making unusual noises. Quickly find a safe place to pull over and call for assistance. To rectify the situation, more often than not you’ll only need a fuel drain, but it’s recommended that you get your car checked out by a mechanic if you’ve driven any distance.

  • It’s also recommended that you don’t siphon the fuel yourself, and to wait for the professionals to help you get back out on the road

What about putting diesel in a petrol car?

in petrol engine

While you’ll suffer a similar fate if you fill your petrol car with diesel, you may struggle fitting the nozzle into the tank, as diesel nozzles are traditionally bigger than their petrol counterpart. If you do manage to wrangle the nozzle and fill up on diesel, you may damage the fuel injectors, particularly if you’ve started the engine and driven away. To rectify the situation, the same recommendations apply, including staying away from the ignition and calling roadside assistance.

How can I avoid putting the wrong fuel in my car?

Unfortunately for those who have already suffered from filling up with the wrong fuel, there’s no trick to avoiding the same problem in the future, with paying attention when you swing into the petrol station the only way to avoid ruining your car and your day. This advice is especially relevant to those who are driving a hire car, or have borrowed mum’s car to duck down to the shops, so always take a moment and be sure before you fill up.

It should also be noted that any damage done to your car as a result of putting the wrong fuel type in isn’t covered by warranty and your insurance may not help either, meaning it can really cost your back pocket. So, like any other time you get behind the wheel, it pays to stay alert.

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