What should I do if I put the wrong fuel in my car?

Friday, 28 December 2018

The recent mix up that saw a South Island petrol station with diesel in its petrol tanks and vice versa is a undoubtedly a screw-up of fairly spectacular proportions. And, to be honest, one that is hard to fathom how it actually managed to happen - it would have required several mistakes to be made and quite a few people to not discover it - but mistakes happen. Even quite big ones like that.

And there is one thing you will know for sure if you have ever worked at petrol station - people regularly put the wrong fuel in their vehicles, even if it is in the right tanks to begin with. As you can see from the video above, it even happens to the president of the United States

So quite apart from how the wrong fuel gets into the wrong place to begin with; what should you actually do if it happens?

Even if the right fuel is in the right pump, people can still get the wrong stuff in their vehicles. What should you do if that happens?

Let's get the worst one out of the way first - petrol into diesel.

**READ MORE:

The different grades of fuel have different coloured handles to help prevent mistakes. The diesel one even usually has a locking flap with a big warning on it.

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Petrol (left) and diesel nozzles are different sizes to try and stop you putting the wrong fuel in. It doesn't always work though.

* Silly Car Question #10: How do I refuel my car?**

This is potentially very bad indeed - not only does petrol have none of the lubricating qualities of diesel (that diesel engines rely on), it also acts as a solvent, reducing lubrication of the diesel in the tank and causing damage to the pump, injectors and other parts of the fuel system.

Bad things can happen in here if you put the wrong fuel in your vehicle, so the best advice is to not start the engine if you do get it wrong.

Added to this, petrol will also depress the flash point of the diesel, potentially leading to premature ignition that could damage the engine as well. Due to the high pressures of injection systems in modern common rail diesel engines literally any contaminants (including petrol) come with a high risk of destroying the engine - with even just a small amount of petrol in the system.

If you have put petrol in your diesel don't - under any circumstances - start the engine. Starting it will only pump the disastrous mix of petrol and diesel though the entire fuel system, potentially doing a lot of damage and ultimately costing you a vast sum of money.

Talk to the staff at the petrol station immediately, as they will almost certainly have the contact details of a company that can drain the fuel from your tank. If they don't - or you are at an unmanned station - then pull out that little internet connected computer you carry in you pocket and find someone (but do I really need to tell you that? Really?).

But is just draining it enough? Well, there is debate over that - if it is an old diesel, just draining the tank, filling it back up with diesel and taking your chances that it will dilute the residual petrol could be worth the risk - but remember, it is still a risk. Doing that with a modern common rail system, however, would be a way bigger risk, and it could well need more drastic measures that will involve changing fuel filters and thoroughly cleaning the tank, which will take time and cost lots.

If you haven't started the car and no fuel has gone through the system, this may seem like a rort from a greedy mechanic looking to take advantage of your mistake (and both experts and commenters here on Stuff disagree on that subject), but considering that just 1 per cent of petrol contamination will lower the diesel flash point by 18 degrees C, we would suggest you ask yourself this; is it worth taking the risk? We wouldn't.

Now on to the less potentially damaging error of putting diesel in a petrol vehicle.

Again, once you realise you should rather obviously stop the pump and notify staff. Again, you shouldn't start the vehicle, but it won't be as disastrous as petrol in diesel if you accidentally do.

Do you need to drain the tank? That depends on how much diesel you have put in and how much room there is left in the tank. If you have brimmed it with diesel from empty, then there is little alternative. Just draining it will be fine though - no need to replace anything.

If you have just pumped a small amount into a half full tank of petrol, then just filling it right up with petrol should be fine - but once again, there are risks.

One of the biggest problems with contaminating your petrol with diesel is the reduction in octane. The octane rating is essentially the measurement of petrol's ability to ignite at the right time and not too early. Petrol with a reduced octane rating thanks to the addition of diesel will ignite too quickly once it is injected into the chamber that will, at best, produce a knocking sound and, at worst, damage the pistons and rods.

Mixing 2 per cent diesel into petrol will lower the overall octane rating by 1 point, meaning that 10 per cent diesel contamination lowers octane by 5 points, which is enough to create problems in most engines - usually just a bit of knocking and some smoke though.

You will notice that there are a lot of 'probablys' and 'shoulds' through this part, because while there are risks, they aren't as huge as the petrol in diesel equation. A lot depends on your car and, of course, your willingness to roll the dice and see what happens, but the odds are far more in you favour here.

Then there is the wrong petrol in a petrol vehicle error - what happens if you put 91 in a car that demands 95 or higher? Not much. It may be a bit grumpy and underpowered, but it will get over it when you put some of the higher octane stuff back in it again. Yes, there are risks, but really only if you continuously try to run on a lower octane rating than your car requires.

Higher octane fuel in a car that only requires 91? Nothing bad there whatsoever - in fact you may even notice you car running a bit better on the higher octane fuel. Some mistakes are definitely worse than others.