I’ve put petrol into my diesel tank – what do I do?

Have you accidentally used the wrong fuel at the pump? It’s surprisingly common, so don’t beat yourself up too much. The important thing is to recognise the mistake as quickly as possible and act fast to rectify it. This month, we look at what you should do if you have used the wrong fuel and why making that mistake could be damaging for your car.

Don’t start the engine

Many who make this mistake realise it at the pump, which is a good thing. The important thing is that you do not start the engine. Remove the keys from the ignition, put the car into neutral and push it out of the way before calling your mechanic for a tow.

If you have already set off and realised that something is wrong, pull over and turn the engine off as soon as possible to avoid doing any further damage.

Why is it so damaging?

There are two possible problems with using the wrong fuel:

  1. You have added diesel to your petrol tank – which is less of a problem
  2. You used petrol in your diesel car – a much more damaging mistake to make if the engine is started

Let’s take these one at a time. Diesel in a petrol engine is less likely to result in any permanent damage, but you still want to avoid circulating fuel around the engine if possible. The reason it is less damaging is because petrol engines use spark plugs to ignite the fuel; when diesel is used instead the most likely outcome is that it clogs the spark plugs and doesn’t start as diesel is ignited through compression. If the engine does start then you are likely to begin experiencing misfiring, the engine will cut out, and you could see smoke from your exhaust pipe.

Using petrol in a diesel engine is a much more serious problem and could result in expensive and potentially permanent damage to your engine. Petrol will increase the friction between the various parts of your engine – unlike diesel which is a more lubricating fuel – and this is what will cause damage. The longer a diesel engine runs on petrol, the greater the damage which is why it is important to avoid turning the engine on in the first place if you can.

Why is it such a common mistake?

The most common mistake is adding petrol to a diesel engine, which as we have seen is the more damaging of the two problems. It could be that you are driving a rental car, a company or friend’s car, or have just made the switch from a petrol to a diesel car and have not yet got into the habit of picking up the diesel pump.

The diesel nozzle is larger than most petrol tank necks, making it easier to spot the mistake at the pump. On the other hand, the petrol nozzle will fit most diesel filter necks making it easier to fill up before realising the mistake.

How to fix it

The way to fix both mistakes is by having the fuel tank drained and flushing your fuel system to remove all traces of the wrong fuel. If you have already driven off and the damage is more widespread you may need to replace parts such as the fuel pumps, filters, injectors, or the fuel tank. If the damage is severe you may even need a new engine.

For help and advice if you have used the wrong fuel in your car get in touch with us at Bromley Vehicle Test Centre straightaway by calling us on 020 8460 6666. We will make sure you are safely back on the road in no time.