Put the wrong fuel in a car? Don’t panic! Help is at hand

Put the wrong fuel in a car? Don’t panic! There are often things you can do to prevent expensive vehicle recovery and garage repair bills.


Accidentally putting diesel into a petrol (gas) tank

This scenario is definitely the lesser of the two evils. Petrol engines rely on an explosion of fuel to run, and as diesel is much more difficult to ignite you’re not going to get far before the car brakes down. It’s unlikely to do any damage to any of the engine components other than lubricate them (although it may damage the catalytic converter if fitted).

Just like above, if you’ve only put a little diesel in the tank just fill it up with petrol to dilute it and you should be on your way.


Putting petrol into a diesel car accidentally

The reality is that modern diesels are finely tuned to use the fuel as efficiently as possible, and putting a large amount of petrol into a diesel and then running it won’t do the car much good. For example, the fuel pump on a diesel relies on the lubricating effect of the diesel to keep it lubricated and stop it from seizing.

If you’ve put a small amount of petrol in your diesel car don’t panic. Most people realise within the first gallon that they’ve made a mistake and there is a quick fix that will get you on your way again without causing any damage (and with no expensive repair bills.

Fill the tank up with diesel.

The average fuel tank of a car is around 65 litres. So if you’ve only got 5 litres of petrol in it, adding filling the tank full will dilute the diesel significantly enough to make it safe to drive and have you on your way. You’ll probably notice absolutely no difference.

In fact, in some countries people deliberately add petrol to a diesel tank to stop the diesel solidifying in the tank and lines (which can make a car difficult to start). My friend does this in the cold winters in Canada.

If you’ve put the wrong fuel in a car you definitely don’t want to run a diesel car on petrol only, but diluting 10% to 90% is very unlikely to do any damage at all and will get you on the way.


I’ve filled the tank completely with the wrong fuel – what should I do?

This is a much bigger problem that just adding a little fuel, and you’re going to have to seek the advice of a professional.

The only way you’re going to remove the fuel is to drain the tank. As most modern cars have an anti-syphoning device fitted to prevent people stealing your fuel you’re very unlikely to be able to get any sort of device into the tank to do this yourself. In addition, almost no fuel tanks have a drain point and almost all have tank sender units at the top (under the rear seats), not the bottom.

Some points to remember:

  • Petroleum (gas) is a highly flammable liquid. Never do anything which might risk exposure.
  • Petrol vapour is also highly volatile and should only be removed in controlled conditions – away from sources of ignition (including mobile phones)
  • Petrol burns skin – never attempt to syphon it using your mouth if you’ve put the wrong fuel in a car. The fumes are also very dangerous to the lungs
  • Although diesel has a higher flash point than petrol it’s still a chemical and should always be treated as such

I’m afraid that at this point it’s time to call your garage or breakdown company.


I’ve put the wrong fuel in a car and the tank needs emptying. How much should it cost?

The cost of having the wrong fuel removed will depend on a number of factors, including:

  • How easy it is to access the tank
  • The labour rate that your repair garage charges
  • Callout charges (if you can’t get the vehicle to them)
  • The cost of disposing of contaminated fuel

Some breakdown services will offer a free service to people who have put the wrong fuel in a car, so check with them if you have cover.


My garage is telling me I need to have the entire fuel system replaced. Is this correct?

In general, no.

Sadly, however, there are some garages who will tell you that you must have the entire system replaced. This is usually to cover their back (in case of future problems) or because your vehicle is covered by warranty.

In reality it’s very unlikely that every part is going to need to be replaced. The most common problems will be:

  • a damaged fuel pump
  • damaged injectors
  • a damaged catalytic converter

If your garage if telling you that you need the entire lot replaced, ask them to replace the injectors and fuel pump. Once replaced, they can run an emissions check on the catalytic converter. If the catalyst is OK then there’s really no point in replacing it and it will save you a small fortune.


How to prevent putting the wrong fuel in a car

There are some ways that you can avoid putting the wrong fuel in a car. These include:

  • add a sticker next to the fuel cap to remind you of the type of fuel, such as this diesel only or unleaded only sticker
  • a diesel nozzle on a fuel pump is usually bigger than the petrol to prevent you adding the wrong fuel – if you’re forcing the nozzle in this is a warning sign
  • colour the fuel cap red for diesel, green for petrol as a subtle reminder

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