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

Thursday, 19 September 2019

Can you save money by filling your car with cheaper, lower octane fuel than it is supposed to use?

Never the damage petrol prices can do to your bank account: a recent Motor Disputes Tribunal case has highlighted the engine damage that can be caused by putting the wrong grade of fuel in your car.

A buyer who purchased a 2010 Volkswagen Golf returned it to the dealer after a year when it developed engine problems. The Tribunal ruled that the damage was caused by the owner consistently running the vehicle on 91-octane fuel when it fact it required 95.

It also ruled that it's the owner's responsibility to know what type of fuel a vehicle requires.

If 91 is used in cars requiring higher octane fuel, they run the risk of pre-igniting or 'knocking'. Knocking occurs when the petrol combusts early and pushes down against a piston while it is still moving upwards during the compression stroke.

* Invercargill Z station fuel mistake

It's your responsibility to know what octane rating your car requires: look on the fuel flap or in the handbook.

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

Recent Motor Disputes Tribunal case focused on a 2010 VW Golf that was run on 91 - but required 95.

In this case, that pre-detonation eventually cracked one of the piston rings - a major and expensive repair.

Filling with 91 instead of 95

Nervous yet? The first takeaway from this is that you should really know what kind of fuel your car requires. The minimum octane rating required is usually printed on the inside of the fuel flap, especially if it's a model that needs the more potent stuff.

But accidents happen. Mistakes can be made. If you do accidentally fill up with 91 when your car requires 95 or higher, is it time to panic and get the tank drained to prevent damage?

The answer is no. While long-term use of lower-grade fuel can have catastrophic consequences, a one-off or even occasional top-up won't do any damage.

It's easy to get mixed up between petrol grades. And the odd mistake won't hurt.

Although you may notice some degradation in performance and an increase in fuel consumption, especially in a modern car that will have sensors and computer control that automatically adjusts the timing to compensate for your little filling mistake.

The other way around is fine of course. There's no problem filling your 91 car with 95 or 98.

In fact many people choose to do that, as the more potent fuel theoretically burns more efficiently and may improve consumption generally, giving you more range out of a full tank.

There's a bit of subject mathematics around that of course, because the improved efficiency has to be offset by the extra cost per litre. But feel free to give it a try.

Filling with petrol instead of diesel

Getting the wrong grade of petrol is easy to do but unless you're a repeat (and repeat and repeat) offender, it's usually not a serious problem.

Getting confused between petrol diesel is much more difficult but also a much more immediate problem.

As soon as petrol goes into the tank of your diesel car, you need to act. It's not major if you haven't started the car. So don't.

Diesel in a petrol tank is all about relative volume.

Petrol lacks the lubricating qualities required by a diesel engine and it can also act as a solvent, potentially causing damage to the pump, injectors and other parts of the fuel system.

Because diesels run at very high compression, even a small amount of petrol in the system could cause huge damage.

So once you've topped up with the wrong stuff, talk to the service station staff immediately. They'll be able to put you in touch with a mobile service that can drain your tank.

Whether more is required depends on the car and whether you're feeling brave. Given that a tiny amount of petrol can cause major damage, it pays to seek advice from a mechanic or technician about whether it's wise to get the tank more thoroughly cleaned and change the filters.

That sounds like a lot of work and expense and there's debate over how much risk there is when the tank has been thoroughly drained. But it's fuel for thought, especially if you have a newer vehicle that will be more susceptible to contaminants.

Or diesel instead of petrol

Putting diesel in your petrol tank is not as serious as the other way around, but you still shouldn't start the car.

If it was just a few squirts before you realised the error, then you could potentially just fill with petrol and take the risk. If you're topped up all the way, then of course you'll need to get the tank drained. But no further work should be required.

Like we said, it's your risk. The big danger from diesel in a petrol car is a lowered octane rating. Ten per cent diesel in a petrol tank will lower the octane rating by five points, which is pretty serious.

That's like the difference between 95 and 91 petrol. Which takes us right back to where we started.