Wrong fuel: a guide on what to do if you put petrol in a diesel car

Misfuelling your car can be a costly mistake if you try to drive it, but here's what to do if you put petrol in your diesel car

petrol pump Citroen C4 HDi

Extraordinary as it may sound, motoring organisations say that every three to four minutes here in the UK, another unfortunate motorist will be pumping the wrong fuel into their car’s petrol or diesel tank.

If it’s never happened to you be grateful, but don’t pour scorn on those unhappy drivers who’ve put petrol in a diesel car by mistake. A moment’s distraction, tiredness, stress, or even plain old forgetfulness could see any one of us joining the statistics at our next trip to the filling station.

Mixing petrol in diesel fuel isn’t the end of the world as long as you realise your mistake at the pump. Yes you’ll be faced with the hassle and cost of getting your fuel tank pumped, but the real damage can occur when you drive away. If you’re unlucky, you could end up with a bill costing literally thousands of pounds.

Most misfuelling incidents – around 95 per cent – occur when petrol is pumped by mistake into a diesel car. It’s harder to misfuel the other way around, as diesel pumps have wider nozzles that don’t fit into modern petrol car filler necks.

Spot your mistake quickly as you’re filling up, and there’s a chance you’ll get away scot-free. That’s because it’s commonly agreed you can mix up to 5 per cent petrol into diesel fuel without disastrous consequences.  If there’s no room in the tank to top it up with diesel at the required 95 per cent ratio, you’ll need the services of somebody who can drain the tank.

What happens when you put petrol in a diesel car?

As long as you don’t turn the key in the ignition, there’s not too much to worry about other than having to confess your error to the garage attendant. That, and the £200 or so it usually costs for a breakdown van or similar to come and drain your tank.

Petrol station

Turning the key is the biggest mistake you can make, as even if you don’t actually start the engine, illuminating the dashboard lights could mean your fuel pump whirrs into life. It’s supposed to prime the engine with diesel before it starts, but instead could be sucking petrol up the fuel lines. This means they’ll need draining and flushing as well as the fuel tank. If you get as far as starting the engine, there’s a whole lot more that can go wrong besides.

Petrol in diesel: the worst-case scenario

Modern diesel engines employ lots of technology to eke out their impressive mpg figures and minimise emissions, including expensive high-pressure fuel pumps, and common rail injectors with very fine tolerances.

Those fuel pumps are lubricated by diesel fuel itself, as it passes through. Replace that diesel with petrol, and its solvent properties mean the lubrication effect is eliminated too. A fuel pump running without lubrication will soon begin to create internal friction as its metal surfaces grind together. It will then begin to disintegrate, and that in turn creates swarf – potentially microscopic particles of metal that can do even more damage to your car's engine further down the line.

It’s the diesel engine’s high-pressure injectors that are next in the firing line. They force fuel into the engine cylinders through very fine holes, and with a carefully engineered spray pattern. If swarf gets as far as the injection system it will simply block some or all of the holes, and a replacement common rail injector system will potentially cost thousands of pounds.

Further misfuelling problems can be caused by petrol’s corrosive qualities, which may degrade seals in a diesel system. At the very least, the entire system will need to be inspected and assessed for damage, as well as flushed through with a cleaning agent by your garage.

If you have driven with petrol in your diesel, don’t panic!

It’s important to note at this point that driving your car a very short distance on the wrong fuel is not guaranteed to damage vital components in the ways described above. In fact, misfuelling recovery specialists such as Fuel Doctor reckon many motorists have driven off forecourts without realising the dangers, only calling for help when their engines stutter and stall a few miles down the road – without long-term adverse effects.

However, driving a diesel car with petrol in the tank will cause serious and expensive damage in fairly short order, and the potential cost of extensive repair is not worth the gamble.

Even if you get away with running the engine or driving without damage, the incident will still be more expensive. That’s because draining and flushing the fuel lines and engine is a more laborious process than simply draining and flushing the tank.

Petrol in your diesel car: our 5 top tips

  1. Stop fuelling immediately: If you’ve only put in a splash of petrol, some experts say there’s a chance you can get away with it – as long as there’s not more than 5 per cent petrol in your diesel. If you do want to risk driving on, stop frequently to top up the diesel and thus reduce the percentage of petrol in your tank as quickly as possible.
  2. Inform the filling station attendants: They will take it in their stride, as it happens to hapless motorists all the time. If they want you to move your car, ask them to help you push it – remembering to turn the key in the ignition just far enough to unlock the steering. Don’t illuminate the dashboard lights or start the engine!
  3. Call your breakdown service or a specialist misfuelling service: The filling station will likely have contact details for a suitable company if your breakdown service can't assist. Don't be harassed into using an operator you're not comfortable with though, as you can find well-known nationwide operators using google and a smartphone.
  4. Wait with the car (as long as it’s parked safely): The tank can usually be drained and flushed in situ in around 30-40 minutes. Afterwards you’ll be able to fill it up again – hopefully with the right fuel this time - and drive away.
  5. Lock the car and leave it if you can't wait: Don't worry about locking the doors with central locking if you don’t want to wait with the car. It won’t affect the fuel system. 

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