It may sound ridiculous, but every year around 150,000 of us do it…pull up to the pump at a filling station and put in the wrong fuel; diesel instead of petrol or indeed, petrol instead of diesel.

It’s a simple mistake to make, especially if you’ve recently changed your car to one with a different fuel type, or if another driver, more used to the opposite fuel type has borrowed it or if you’re simply tired, stressed and not thinking straight.

Whatever the reason, the consequences can be very costly.

By far the worst of the two scenarios is putting petrol into a diesel car. Diesel partly acts as its own lubricant and components such as the fuel pump need that slight oiliness to coat and protect the surface of its moving parts. The trouble is petrol will cut through that protective layer leading to metal-on-metal contact which will very quickly cause irreparable damage.

Diesel and petrol nozzles on forecourts are differently sized to try to avoid this mistake and diesels are usually bigger than the opening in a petrol tank so they shouldn’t fit in. That’s on most cars though, but certainly not all of them, and it still means that a petrol-into-diesel scenario is all too easy. Some car brands, Ford for example, have a sort of `safe-fuel’ system where you can’t fit the wrong one in, but the majority of cars on our roads aren’t so well protected.

So, what do you do if you spot your mistake?

red car at petrol station

Petrol into a diesel engine

The golden rule is under no circumstances should you turn on the ignition or start the engine. If you leave it alone the wrong fuel will just stay in the tank but the moment you turn the key it will make its way through the pump and into the injection system where it will wreak havoc. It isn’t just the fuel pump, but the whole network from the tank through the filters, feed pipes, injection system and into the cylinders. You really don’t want to be faced with a repair bill for that lot!

Turn the ignition key only as far as needed to unlock the steering and the filling station staff or some friendly motorist should be able to help you push the car to a safe spot. You can then call a roadside repair firm for help. It is worth noting that some, the AA for example, will send out a technician to drain the tank even for non-members. If you’re not a member of any such organisation (and you really should be) the staff may be able to give you a number to call.

There really isn’t anything else you can do except leave the ignition off and wait for an expert to come and drain the tank. You may want to alert your insurer and start thinking about the small print in their policy to see what it covers…

Once that has been done and the right fuel put in your next step is to call your local service centre, explain what’s happened and book the car in for a thorough inspection.

Diesel into a petrol engine

This is less likely because as we said before, the nozzles on a diesel pump are generally bigger than the opening in the car’s tank. It shouldn’t be possible and yet breakdown recovery firms still report cases of it, possibly if the tank has run dry and is refilled from one of those small plastic emergency containers or a jerrycan but filled with the wrong fuel.

The good news is that putting diesel into a petrol engine will cause less damage than vice versa, but it still won’t do the car any favours and the engine management system will definitely be rather confused. If you’ve noticed your mistake in time and only pumped in a small amount the engine may still run albeit not in full tune, and you are still best advised to book the car into a service garage asap and have them drain and flush the entire system.

In a petrol car the air/petrol mix is ignited by a spark from the spark plug whereas a diesel engine just uses compression, squeezing the fuel until it combusts so diesel will likely clog up the fuel injection system and plugs.

more fueling

How do I stop a misfuelling mistake?

Other than taking a few seconds to check you are not putting a green (petrol) pump into a diesel car, you can buy aftermarket kits which make it impossible to put in the wrong nozzle, but unless you’re a qualified mechanic you may not want to start messing with the fuel cap and in fact you’re probably safer not doing so. Fuel leaking out in a corner or when you’re accelerating or braking is really not a good idea….

Misfuelling sounds a really silly mistake, but as 150,00 drivers a year will tell you, it’s a much easier mistake to make than you think.

AdBlue in the wrong tank

There is a third scenario which is putting in something called AdBlue, an additive to cut pollution from diesels, into a car not designed for it. This is perhaps a little unlikely because normally you have to buy a bottle rather than get it from a pump, but if by some mischance you put it into the wrong tank and start the engine, it can damage the fuel pump, injection system and what is called the catalytic reduction system.



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