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

Putting the wrong fuel in your car is not just a silly and easily avoidable mistake, it can be a very expensive one.

Theoretically, putting the wrong fuel in your car is right up there with pouring something marked "Poison" into your body. You really shouldn't do either and, unless you can't actually read,  putting petrol in a diesel engine or diesel in a petrol engine should happen about as often as an adult pours milk into their eyes.

And yet, figures from the Automobile Association in the UK suggest that people in that country make this mistake every three and a half minutes, and thus the AA spends quite a lot of time helping people with a wrong fuel in car crisis.

The number seems implausible at first, and prompts thoughts of cruel jokes about British intelligence being a contradiction in terms, but if it's happening that much in the UK, it must happen every three or four seconds in the US.

A simple phone call to us for advice could help save thousands of dollars in unnecessary repair bills.

It's also fair to assume it's a common problem in Australia, so much so that a specific business has now been created to deal with the problem - Wrong Fuel Rescue

Company director Yan Van De Velde tells us his rescuers help “thousands” of stranded motorists each year, but says the exact number is commercially sensitive.

He did tell us that his clients include not just private motorists but Government and private fleets, roadside-assistant providers and car dealerships.

“Our mobilisation rate is 99.9 per cent for all affected vehicles, regardless if the vehicle has been driven or not, with an average repair time of just one hour,” Van De Velde says.

“Motorists can still benefit from us even if we don't service their area.  A simple phone call to us for advice could help save thousands of dollars in unnecessary repair bills.” 

Putting in the wrong fuel is a problem that has been exacerbated in recent years by the fact that diesel nozzles now share the same pump space with unleaded ones, rather than being stuck in the far corner of the forecourt, so here's what to do if it happens to you (aside from going and having your eyes tested immediately).

What happens if you put petrol in a diesel car?

This is both the more common mistake and, sadly, the more disastrous one when it comes to the wrong fuel in car cost equation.

Misfueling a diesel car with petrol is easier to do (particularly if you approach the fuel pumps with your eyes shut), because the petrol nozzle will actually fit inside the filler for a modern diesel.

If you do put unleaded petrol in a diesel engine, you're at risk of doing serious and possibly terminal damage to your car. Yes, it's that bad.

Once you mix petrol with the diesel that's already in your tank you're basically creating a solvent, which will start playing hell with your car's fuel system immediately, and can lead to you needing a new engine, filters, fuel pump, injectors, and a whole new fuel tank. 

Modern common-rail diesels are particularly susceptible to this kind of damage, which is cruel, because these more clever engines sound less like rattly old ones and might confuse you into forgetting you own a diesel car.

Putting a gun to your car's head and pulling the trigger is a reasonable analogy for putting petrol in your diesel engine. 

What should you do?

The good news is that there are wrong-fuel solutions, and that the most important thing to do is to recognise your mistake before you get back in the car.

As long as you don't turn the key, or press the start button, the damage can be kept to a minimum.

You're going to need to put your car in neutral, release the hand brake and get someone who feels sorry for your idiocy to help you push it away from the pumps to a safe parking spot.

The sooner you stop, the more chance you'll have of being able to rectify the problem.

Now you'll need to call for professional help, because you're going to need a fuel drain, to get that errant petrol the hell out of there.

This may call for a tilt tray to take your car to a garage, or your roadside-assistance provider, or Wrong Fuel Rescue, should be able to help.

This is the best-case scenario, and hopefully your blushes can be saved with minimal expense.

If you do drive away from the servo with the wrong fuel on board, you'll notice the problem pretty quickly as your engine will make some truly horrible sounds, and if you keep going you'll probably grind to a halt not far down the road.

Again, quick action is the key. The sooner you stop, the more chance you'll have of being able to rectify the problem. Call for help, and pray to whatever gods you believe in that you haven't killed your car.

Diesel in petrol engine

It really does take a stupendous effort of stupidity to get diesel into your petrol engine, yet somehow people still manage it (the AA says one third of its misfueling calls involve this kind of stuff-up).

What makes it tricky is that the diesel nozzle simply won't fit into the filler gap of a petrol-engined vehicle. If that's not enough of a clue that you're attempting to do the wrong thing then we'd advise you to stay away from operating any kind of machinery. And away from other human beings as well, frankly.

If you do somehow manage to shoehorn diesel into a petrol car, don't panic, because this is less catastrophic than the other option discussed above, but it's still vital not to start the engine. Diesel will cause serious damage to a petrol engine if you allow it to circulate, particularly if your car is of the direct-injection variety.

Fuel injectors that are designed to use unleaded will not take kindly to diesel.

Once again, you should make a shamefaced call for help and arrange for your tank to be completely drained.

Prevention is better than the cure

Obviously the most likely time to make a mistake like this is when you're driving a car you're not used to; either a hire car or one you've borrowed from a friend. Driving in a foreign country can up the level of confusion as well.

But it should still be a difficult mistake to make. Don't just look at the colours on the pumps, read the words on them, and make sure you're aware of what kind of car you're driving before you go to fill up.

When in doubt, there's almost always a little warning sign on the inside of the filler cap that says "Diesel only", "Unleaded only" or even "Premium unleaded only". 

If you're still confused, at all, don't just guess. Ask the service-station attendant for help.

Putting the wrong fuel in your car is not just a silly and easily avoidable mistake, it can be a very expensive one.

AdBlue to the confusion

Yan Van De Velde from Wrong Fuel Rescue says a new and growing concern for motorists is the amount of “misfuels” involving AdBlue, a mistake he says can be even worse than the usual petrol or diesel mixups.

AdBlue is a fairly new substance you might not have heard of, which some diesel manufacturers now use to help their vehicles hit emission guidelines.

Made from urea and de-ionised water, it is a light, almost colourless solution that’s now available at many service stations and is used in a modern Selective Catalytic Reduction system to transform Nitrogen Oxide fumes into harmless water vapour.

If you’re driving a vehicle that needs AdBlue, your dealer will have explained this to you. If you’re not, don’t just try it, because the results could be disastrous.

Editor's Note: This post was originally published in November 2016 and has now been updated.

Have you ever put the wrong fuel in your car? Tell us your experiences in the comments below.

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