Misfuelling: What to do if you put petrol in a diesel car

Person counting cash at a petrol station with the fuel pump filling the car

In the UK around 15 people every hour pump the wrong fuel into their car. It’s commonly known as misfuelling and 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.

BookMyGarage infographic showing petrol pump and stats around misfuelling cars

How do you misfuel a car?

Putting the wrong fuel in is alarmingly easy, particularly if you drive a diesel car. This is because the nozzle for petrol pumps is smaller than diesel pumps. 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 filling station staff and ask for assistance to help push your car to a safe place. If you’ve started driving, the moment you realise something is awry pull over safely as soon as you can. This will probably be when the car starts to misfire or lose power. In both cases, call your breakdown service or a local garage.

What happens next?

If you haven’t started the engine, fixing a misfuelling mistake is a fairly straightforward procedure. The fuel tank will be pumped out, flushed and then refilled. Fingers crossed you should be on your way fairly swiftly. 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 new fuel pumps, injectors, pipes, filters and even a new fuel tank. That’s why it’s believed to have cost England footballer Wayne Rooney £6000 when he mistakenly put petrol in wife Colleen’s £90,000 Overfinch Range Rover TDV8.

How to avoid misfuelling your car

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 fragments 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 misfire and gradually lose power before eventually grinding to a halt. That’s the point when you can start contemplating a really hefty bill.

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Mandy Weston

Mandy Weston

Mandy is an ex-mechanic, with 22 years’ experience in the motor industry. As an in-house motoring expert, Mandy is the go-to woman for any relevant questions that our customers have; both garages and drivers. From specific problems with your car to general maintenance, Mandy is a reliable source of information and advice. Her passion for motoring is a huge factor to her success and the huge wealth of knowledge that she has. She now uses her remarkable grasp of the industry to write regular content for our readers to help drivers understand their car better, avoid being ripped off by garages and save money on their motoring requirements.