What do the Breakdown Experts Advise When You Put the Wrong Fuel in Your Car

It’s a very simple mistake to make and the big three emergency breakdown assistance companies have all stated at some point recently, that it’s becoming more and more commonplace for motorists to fill up their diesel cars with petrol.

There are many contributing factors at work here, such as:

  • Motorists switching from petrol to diesel cars and then suffering a memory lapse at the pump
  • Borrowing someone else’s diesel car or hiring a diesel when you’re used to a petrol car
  • Ultra refined diesel engines that are as quiet as petrol engines
  • Pump nozzle colour code confusion for foreign motorists
  • General fatigue causing poor concentration
  • Life or work pressure and stress causing momentary distraction

So, if the worst happens and you accidentally fill up your diesel vehicle fuel tank with unleaded petrol, what can you expect to happen to your vehicle engine and why is it such a big problem?

The difference between diesel and petrol engine fuel systems

When diesel powered vehicles first appeared on the market it was fairly easy to tell that they were different from petrol powered vehicles because of the noise of the engine when it was running. Diesels were louder and made a distinctive rattling sound when idling or being revved. With modern diesel engines this is not always the case any more; refinements in design and engine bay sound proofing have made it very much more difficult to tell the difference just by listening to the car when started.

In a diesel engine, the fuel system pump works differently from a petrol engine fuel pump, it operates within very tight tolerances and the internal pressure whilst it’s running is much higher than that of a petrol engine fuel pump, with the latest designs running at between 350 and 1600 bar. The diesel fuel also acts as a lubricant for the fuel system components, ensuring that there is no build-up of friction between the metal internal components that come into contact with each other during its operation.

What happens when diesel is put into a petrol fuel system?

There are several problems caused by unleaded petrol being introduced to a diesel engine fuel system. Firstly, unleaded petrol does not have the same lubricating qualities as diesel fuel and this means that if you try to run a diesel fuel pump with unleaded petrol in it, then there is no lubrication where there is metal to metal contact of components inside the pump body and this leads to components grinding against one another and producing small fragments of metal which further contaminate the petrol. These small, sharp fragments of metal then work their way through then fuel system of the vehicle causing further damage to other components and potential blockages.

The next detrimental property of unleaded petrol is that it acts as a solvent and can damage fuel system seals which often cannot be replaced without replacing the entire component affected.

In most cases, if the diesel vehicle engine is started and the vehicle is driven after being filled with the wrong fuel type, then it will run for a short while before the effects described above kick in and the engine stalls. The usual cause of the stalling is that the small metal particles in the petrol will block the diesel engine injectors,this starves the engine of any fuel at all.

What do the emergency breakdown companies advise?

Don’t start the engine and what to do if you have. If you do happen to realise that you are putting in the wrong fuel whilst in the middle of the act, don’t start the engine. In fact, don’t even lock or unlock the car as some diesel vehicles use a low-pressure electric fuel pump which is primed if the ignition is turned on, or any electrical activity takes place with the vehicle such as unlocking it.

If you’re still on the fuel station forecourt then you need to inform the staff so that someone can help you to push the car to a safe place out of the way of other motorists using the station. If you have driven the vehicle and it has stalled further on down the road, make the vehicle as safe as you can (some kindly soul may stop and help if you’re lucky) switch on your hazard warning lights and put up your emergency reflective triangle if you have one. Whatever your situation, you then need to call your emergency assistance company and explain what has happened to them.

Call in an expert

If you don’t have an emergency breakdown membership or you want to speak to an expert in this kind of problem, there are many independent companies specialising in dealing with contaminated fuel problems. They need specialist training and licences for their activities which can be verified by the attending engineer for your peace of mind.

A vehicle fuel drain and system flush

Contrary to the scary stories that you may have read about what happens when the wrong fuel is put into a diesel engine, in all likelihood a simple fuel drain will solve the problem quickly, simply and most importantly, cheaply. Unless you are driving an exotic sports car or a modified vehicle then in 95% of cases the attending engineer will assess the situation with the vehicle and will then connect up the necessary equipment to drain all of the contaminated fuel out of your vehicle.

Fresh fuel of the correct type for the engine must be flushed through the vehicle fuel system under pressure first to ensure that any contaminated fuel is fully removed from the system and also to flush out any metal particles that may be present if the engine has been started and the vehicle has been run using the wrong fuel type. The tank is then filled up with the correct fuel and the engine started.

This is not a process that can be carried out by an inexperienced mechanic, vehicle fuel systems are complex and sensitive mechanical system – replacement of its components can be very expensive so it’s important that only experienced mechanics perform an operation such as a fuel system flush. Also, removal, storage and transportation of contaminated fuel can only be done by fully licensed individuals who are registered with the Environment Agency and hold an SPA passport.

Check Your Insurance Policy

As mentioned previously, in the vast majority of cases an inexpensive fuel system drain will be all that is required to get the vehicle back on the road and so, even if your insurance policy does cover you for putting in the wrong fuel, it probably wouldn’t be worth your while to make a claim as in most cases the costs don’t even come close to the policy excess.

If, however, you have been advised that further work or investigation is required on your vehicle, check the “accidental damage” cover if you have a fully comprehensive insurance policy. Most insurers will cover damage to the engine but not all.

Keep a close eye on your vehicle for a few weeks after the fuel drain

Once the fuel drain has been performed and you’re back on the road again, you will need to keep an eye and an ear on your vehicle just to make sure that nothing is amiss. If the vehicle is running rough and isn’t improving or if it isn’t behaving as normal then you may need to get that checked out. Again, it may be best to consult an expert from a wrong fuel removal company rather than the main dealer or a local mechanic. If there is a problem with a component within the vehicle fuel system, you’ll get better advice from an impartial expert who would only charge a nominal fee for the consultation.


If you are unfortunate and you find yourself in a wrong fuel predicament, following the advice above could save you money and hassle. Remember, it happens to a lot of motorists every day and is very rarely a huge concern.

Auto Novice

Auto Novices is a blog, that was set up in November 2011, which tries to help inform new & old automobile owners about various subjects from keeping their car in good working order right through to tips on buying a new & used motorbikes. GUEST POSTS: If you would like to produce a guest post for this blog then please contact us via the link in the navigation menu at the top of the page.