What happens if you put petrol in a diesel car

Aug 28, 2023

If you've accidentally put the wrong fuel in your car, you're not alone.

According to the AA, the absent-minded error affects around 45 motorists every day. Whether it's mistakenly putting petrol in a diesel car or diesel in a petrol car, misfuelling can be disastrous for your engine as well as your bank balance.

Here's what to do.

Putting petrol in a diesel car is far more serious than filling a petrol car with diesel.

Diesel cars use fuel as a lubricant, but petrol is corrosive so it can cause extensive damage.

Some of the main components that could be adversely affected by putting petrol in a diesel car include the fuel pump, fuel lines, fuel filter and fuel injectors.

The most common signs of a misfuelled diesel are that the engine will make a loud knocking sound whilst accelerating, sluggish acceleration, excessive exhaust smoke, idling will be rough, the engine warning light will illuminate, and your car might stall or struggle to restart.

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The good news is that if you wrong-fuel your petrol car, it's less serious than the other way round and your action is less likely to cause any permanent damage to your fuel system or engine.

Mistakenly putting diesel into a petrol car is also less common, partly because diesel nozzles are larger than the filler neck on most petrol cars.

Common symptoms of misfuelling a petrol vehicle include a misfiring engine, excessive smoke from your exhaust and your engine cutting out and/or failing to restart.

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If you're lucky enough to realise you’ve put the wrong fuel in your car before starting your engine, do the following:

Leave the engine switched off and don't even put your key in the ignition

Inform the staff at the service station.

You'll be advised to leave your car where it is, or it will be pushed to a safe place

Call your breakdown provider. Hopefully, they will be able drain and flush your vehicle's fuel system and get you on your way, unless there's more serious damage.

If you don't have breakdown cover, there are garages and companies that deal specifically with misfuelling that can be called out.

If it is more serious, call your insurance provider as soon as possible to check if misfuelling is covered.

However, if you realise you've wrong-fuelled after you started up your car, both types of fuel will start circulating, damaging your engine. Here's what to do:

If you’ve already started your engine, turn off the ignition immediately.

If you're already moving, pull over to a safe spot, then switch off.

Again, call your breakdown provider and inform your insurance company.

Stop and think when you arrive at a service station to refuel and don't allow yourself to get distracted.

Read the label on the pump and nozzle and double-check the information on the inside of your car's fuel flap. A circular ‘E10’ or ‘E5’ label denotes petrol, while a square ‘B7’ sign means it's a diesel dispenser. ‘E’ stands for ethanol and ‘B’ for biodiesel.

Check the colour of the fuel nozzle (black for diesel, green for petrol), but don't rely 100% on this colour coding.

If you’re driving a new or unfamiliar vehicle, check and double check the labelling on the inside of the car's fuel flap.

If you're a diesel car driver, fit an adaptor which replaces the existing filler cap. This prevents you from accidentally putting a petrol nozzle into a diesel filler neck

If you’re still worried, the best way to avoid misfuelling is to put a sticker on the outside of the fuel flap reminding you which fuel is required.

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