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

Misfuelling

Putting the wrong type of fuel into a car is easy to do. Known as misfuelling, it tends to happen when drivers are in a hurry or distracted. You’re not in your usual motor, maybe it’s your partner’s, a colleague’s, or a hire or courtesy vehicle. Your mind might be on other things and out of habit you lift the petrol pump from its holder and start filling your diesel vehicle. Hopefully you’ll realise your mistake before you drive away…

The good new is, it’s virtually impossible to put diesel in a petrol-engined car. The neck of the petrol filler is tighter than a diesel pump nozzle. The bad news is the wide neck of a diesel fuel filler easily takes a petrol pump nozzle. The even worse news is that putting petrol into a diesel does far more harm than the other way around. Read on to find out all about misfuelling and how to prevent it.

Why is misfuelling so harmful?

Lubrication is vital for engines, and in diesels the fuel helps to lubricate the fuel pump. Petrol on the other hand is a solvent. It strips away lubrication. This means in your fuel pump you have metal grinding against metal, which you never want anywhere in an engine.

How petrol harms a diesel engine

Once the petrol has stripped the lubricant out of the fuel pump, tiny fragments of metal from the parts rubbing together will enter the fuel system. Many of these will be caught by the fuel filter. However, some will make it all the way to the injectors and that’s when the real damage is done.

Misfuelling

You don’t want to be putting the green pump into a diesel car…

Modern diesel injectors work to very fine tolerances, releasing a fine spray of fuel into the cylinders. Even the tiniest foreign body will block the jets, throwing everything out of kilter. It could cause a misfire or the engine might even seize.

What’s the result?

It can be a costly mistake. If you fill a diesel car with petrol and start the engine, you will need to have the petrol pumped out of the fuel tank and the engine flushed through at the very least. It’s probably safest to have a new fuel filter put in. But you could be looking at a new fuel pump, injectors, pipework, and fuel tank. If the damage is extensive, it could be cheaper to have a whole new engine.

What to do if you misfuel

First of all don’t even think about starting your car and driving it. In some cars just turning on the ignition primes the fuel pump meaning the damage begins before the engine is running. Go into the service station and ask if someone will help you to push your car to one side so it’s not in the way. Whatever you do, don’t drive your car.

Then call your breakdown company. Any professional misfuelling service should be able to pump the petrol out of your car, flush it through and get you on your way, hopefully with little in the way of lasting damage.

How to prevent it

A simple and cheap way is to get a sticker set from a craft shop and make a ‘diesel’ sticker that you put on your fuel cap and inside the fuel filler flap. Alternatively, there are various devices that are designed to prevent a petrol nozzle going into a diesel tank, while still enabling you to fill up from the black pump. Perhaps the most famous is the Fuel Angel, after its appearance on TV’s Dragon’s Den. The SoloDiesel is also worth looking at.

2 comments on “Misfuelling: what to do if you accidentally put petrol in a diesel car

  1. Eric Hayman February 5, 2018 8:55 am

    Forget different size nozzles – why not differently shaped nozzles and tank fillers? Say triangular and square? Years ago I put diesel into my petrol Ford Escort. It took many miles to use the diesel/petrol mixture (my first journey after the misfuelling was about 80 miles on the M4), and the exhaust was quite smokey. Yet there were no obvious ill-effects to the engine.

  2. Carrosserie February 6, 2018 12:33 pm

    Such a helpful post. Thank you! 🙂

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