Car Advice

Misfuelling explained – What to do if you put the wrong fuel in your car

Each year thousands of people misfuel their car causing thousands of dollars worth of damage. Here’s what to do if you put the wrong fuel in your car.

Note: This article was first published in 2016 and while it’s becoming harder to misfuel a vehicle because the nozzles are different sizes, etc, but for those with older cars it’s still a possibility. 

I CAN STILL remember the phone call. It was mid afternoon and I was just gathering my bits and pieces together to head out the door and collect a Range Rover TDV8. The call was from Land Rover’s then (but no longer) PR boss, and it went a little something like this…

‘…Ah, Isaac. Um, I’m sorry to say this but you won’t be able to collect the Range Rover for testing today.’ Me: ‘Why?’ Land Rover: ‘Because… (long pause)… I’ve just filled the thing with petrol, driven off down the road and stuffed the engine.’ Me: ‘Bugger’.

Now, you might never have grabbed the wrong bowser and stuck petrol into a diesel vehicle, or diesel into a petrol car but each year in Australia, according to the motoring clubs, around 10,000 people do just that. But don’t feel too bad, though, the number of people who pump the wrong fuel into their car in the UK is around 150,000 each year and it’s fast becoming one of the most common reasons for roadside assistance call outs in that country.

Not so long ago misfuelling your vehicle was almost unheard of, and that’s because, in Australia at least, the majority of passenger vehicles ran on petrol. Back then, it was usually only farmers and truck drivers who drove diesel-drinking vehicles and they usually had their own pump at the farm (the farmers, I mean) and, if you did a diesel vehicle in town then you used to have to, first, find a petrol station that sold diesel it was usually a petrol station on the highway, or just out of town, and usually tucked away in a distant corner of the forecourt where trucks would fill up on the oily stuff. And, so, this meant that if you had accidentally put petrol into your diesel vehicle, then you probably didn’t deserve to have a driver’s licence in the first place.

Then, as the popularity of diesel vehicles in this country began to increase, petrol stations started to offer at least one diesel bowser in amongst the petrol ones – people who’d never owned a diesel vehicle began buying them. And the potential for misfuelling began to grow until we arrived at the situation we have now where most service stations offer a diesel pump on the same bowser as petrol, and with more and more people buying diesel vehicles the opportunity for misfuelling continues to grow… especially if you’ve never owned a diesel before which means the chance for an accidental tankful of petrol is greater, with some claiming that this usually occurs on the fourth or fifth tank. See, some people claim it’s because the vehicle is no longer ‘new’ making it easy to forget you own a diesel and fall back into old habits (meaning, filling the thing with petrol).

Mostly the problem occurs with petrol being pumped into a diesel vehicle as the neck on a diesel tank is generally wider than that of a petrol tank. It’s also worth mentioning that misfuelling isn’t covered by your vehicle’s warranty or by your vehicle insurance policy, meaning that any damage caused will be paid for out of your pocket.

What to do if you’ve filled up with the wrong fuel

Okay, so you’ve somehow managed to put the wrong fuel into your car, don’t worry, these things happen. But, whether you’ve realised your mistake 30 seconds in or not until you’ve brimmed the tank it makes no difference, the result is the same and the entire fuel tank will need to be drained.

It’s important that, no matter what else you do, you don’t start your car. See, by not starting your car you can keep the ‘wrong’ fuel in the fuel tank, thus making the fix a little simpler and less expensive. But, should you start your car before realising you’ve filled it with the wrong fuel, well, then you’re in a whole world of trouble as the ‘wrong’ fuel will mix and begin to circulate around your vehicle, more on this shortly.

So, if you’ve realised your mistake and haven’t started the car, return the pump to the bowser and head inside and tell the fuel station attendant what you’ve done that. Hopefully they’ll have seen this before and be sympathetic. And, hopefully, if you’re on your own, they’ll help you to push your car to one of the car parks at the petrol station. From there you can call your roadside assistance provider and tell them what you’ve done, and they should be able to provide help in draining the tank, or direction to someone who can.

But, if you’ve started the vehicle and driven a short distance before realising your mistake, well, you’ll need to have your vehicle towed to a mechanic for a thorough check over. And this is where the costs can mount, as it’s likely you’ll be up for a new fuel tank, new fuel lines, fuel filters and fuel pump(s) and injectors – although this really does depend on the vehicle and the circumstances. And, if your vehicle is still under warranty when this has occurred you’ll need to let the dealer where you purchased the vehicle know what’s happened as misfuelling, and subsequent damage caused by it, isn’t covered by either your warranty or insurance. They might even request that the vehicle be towed to them for the work to be carried out.

Why is it a problem if I put petrol into my diesel vehicle, and vice versa?

Firstly, and in case you didn’t already know, diesel and petrol are different. Yes, they’re derived from the same base petroleum product (although diesel can also be produced from vegetable oil – bio-diesel), but the chemical structures of the end products are different. As are the engines and how they convert the chemical energy from the fuel into mechanical energy.

diesel and petrol engines

In basic terms, a petrol engine uses a spark plug to ignite the petrol in the cylinder, this small explosion causes the piston to begin moving. In a diesel engine there is no spark plug and so they operate with a higher compression ratio than a petrol engine; the air inside the cylinder is compressed and heated (at more than 540-degrees C) then diesel is squirted in causing combustion and the cylinder moves.

So, the engines work differently and that’s because the fuels behave differently. See, petrol is essentially a solvent and, so, when it’s tipped into a diesel engine strips away the oily coating that diesel provides to a diesel engine (diesel acts as a lubricant). It means, that with this protective coating washed away the seals will dry out and crack, and because of the metal-on-metal friction the engine will likely overheat, seize up and fail. Thus putting petrol into a diesel engine can become an expensive mistake.

If you do it the other way around and put diesel into a petrol engine then the main problem you get is that the oiliness from the diesel will coat everything inside the engine and because diesel doesn’t burn very well and burns slowly at that (it requires a high compression ratio to create the necessary ignition), a spark plug will be fairly ineffective at igniting a puddle of diesel in the cylinder and so you’ll end up with a coughing, smoking mess.

So, putting diesel into a petrol engine and then starting it up will be a problem and require a lot of stuff to be replaced, but running petrol in a diesel engine will cause a more dramatic failure of the engine. Some technicians report it takes twice as long to sort out a diesel engine that’s had petrol run through it than diesel in a petrol engine.

There’ll be some out there running bio-diesel, but the amount of people doing this is likely to be small and given you can’t find it at your local petrol station we’ll ignore it as a potential issue and stick with petrol and diesel (that can be purchased from a petrol station) only.

So, I haven’t run the vehicle and the tank has been drained

Fingers crossed you didn’t start your vehicle and had the fuel tank drained without any of the ‘wrong’ fuel getting into the system. How long it will take to drain the tank and how much it will cost and even where it can be done will depend on where you live, so I won’t try and estimate the costs. And it will be impossible for you to try and drain the tank yourself, so don’t even consider it.

But, with the tank drained it’s likely you’ll want to fit a fresh fuel filter and will likely need to fit another new one after the first few tankfuls of fuel. Once this is done, you’re right to start up your car and get back on with your day. That said, one mechanic we spoke with said it could be possible to get away with simply cleaning the fuel filter… it all depends on the vehicle and the circumstances.

If it was a case of diesel into a petrol tank it’s likely that once the tank has been drained you’ll only need a flush through before restarting the vehicle. You can expect to hear a bit of a cough and see plenty of smoke as any diesel residue is burnt off, but this is normal. There are additives you can get that will help to get rid of diesel residue.

If you put petrol into your diesel then the process is the same, the tank will be drained and flushed through and you’ll be able to start the vehicle.

Some newer automatic cars will require you to put the key into the ignition and start it before they’ll let you move out of P for Park and into N for Neutral. Unless you’ve filled your car from empty, it’s likely you’ll have enough of the ‘correct’ fuel left in the lines and near the pick-up in the tank that being run for a few moments won’t be too big a deal. Just remember to tell roadside assist, or your mechanic that you’ve had to start the vehicle.

How do I prevent misfuelling in the future?

As mentioned earlier, most fuel filler flaps will have a sticker stating what fuel the vehicle should be filed with. So, pausing for a moment to have a look, especially if you’ve borrowed a friend’s car or are driving a rental vehicle, is a good idea.

If you want to be a little more proactive then there are items that will physically prevent you from inserting the wrong fuel nozzle into the tank. One such product, called SoloDiesel has five safety latches that are designed to block a petrol pump from being inserted into a diesel vehicle, but allow a diesel nozzle to be used. The SoloDiesel can be retrofitted and takes the place of the vehicle’s regular fuel tank cap. Click here for the full list of supported vehicles.  There are other products, like Diesel Key which also replaces the regular fuel tank cap and only the correct nozzle (diesel) will ‘unlock’ it. Click here.

Extra reading:

Petrol Vs Diesel – Which should I choose?


Isaac Bober

Isaac Bober

Isaac Bober was born in the shadow of Mount Panorama in Bathurst and, so, it was inevitable he’d fall into work as a motoring writer. He began his motoring career in 2000 reviewing commercial vehicles, before becoming editor of Caravan & Motorhome magazine. He then moved to MOTOR Magazine before going freelance and contributing to Overlander 4WD, 4×4 Australia, TopGear Australia, Men’s Health, Men’s Fitness, The Australian, CARSguide, and many more.