If I put petrol in a diesel car, do I need to have a new engine?

Petrol in a diesel car

In the UK it’s estimated about 15 people every hour pump the wrong fuel into their car. If they’re lucky, their journey will be delayed by a few hours and it’ll only cost them a few hundred pounds. For unlucky drivers, the bill could run into thousands of pounds and they’ll waste hours of their time dealing with insurers, hire car companies and garages. Here’s what you need to know.

How do you misfuel a car?

Putting the wrong fuel in is easy, particularly if you drive a diesel car. This is because the nozzle for a petrol pump is smaller than that of a diesel pump. Consequently, a petrol pump will slip easily into the neck of a diesel filler. Conversely you have to try really hard to put diesel into a petrol car.

What to do if you put the wrong fuel in

First of all, don’t start the engine. Don’t even turn the ignition on. Alert the garage and ask for assistance to help push your car to a safe place. If you’ve started driving, the moment you realise something is wrong pull over safely as soon as you can. This will probably be when the engine starts to lose power and sound strange. In both cases, call your breakdown service or a local garage.

What happens next?

Petrol in a diesel
Even owners of Overfinch Range Rovers aren’t immune to accidental misfuels

If you haven’t started the engine, fixing a misfuel is fairly straightforward. The rescue service will pump out the fuel tank, flush it and then refill it. Fingers crossed you should be swiftly on your way. If you’ve started your car, it’s a different story. The tank must be emptied and flushed and you’ll need to flush the fuel system too. You may get away with it only costing around £300 including a new fuel filter. But if you’ve run the engine you may need a new fuel pump, injectors, pipes, filters and even a new fuel tank. That’s why it’s believed to have cost former England football star Wayne Rooney £6000 when he mistakenly put petrol in wife Colleen’s £90,000 Overfinch Range Rover TDV8.

How to avoid misfuelling your car

Petrol in a diesel
Ford has come up with a clever device to prevent drivers accidentally misfuelling their cars

Don’t go by the pump colour. Check what it says on the pump’s trigger. If you’ve got a new car or you’re using a hire car, put reminders for yourself if it’s a different fuel to the one you’re used to. Try not to be distracted while you’re putting fuel in your car. If you’re stressed or in a hurry, pay particular attention. You can buy devices that go in your fuel nozzle to prevent the narrow petrol pump fitting into the wider diesel filler neck.

Why is putting petrol into a diesel so bad?

Diesel is a lubricant, petrol is a solvent. If you run petrol through a diesel engine, it will strip away all the lubricating oils. That means you end up with metal grinding against metal in the fuel pump. This generates pieces of metal called ‘swarf’. These travel through the fuel lines towards the engine. Once the fuel lines have been contaminated, the petrol will then pollute the fuel filter on its corrosive way to the injectors. Modern diesel engines have the fuel injected directly and precisely into the combustion chamber. The injector nozzles have such fine tolerances that it doesn’t take much swarf to block them. The engine will then stop working properly and gradually lose power before eventually grinding to a halt. That’s the point when you can start contemplating a hefty bill.

Read our DPF advice for problems affecting diesel cars