Drivers Fact Sheets


A Guide To Misfuelling

What to do if you’ve put the wrong fuel in your car. First of all you are not alone. It’s estimated that at least 150,000 drivers (one every three and a half minutes) put the wrong fuel in their car each year. This is actually one of the more common mistakes drivers can make, through inattention, or if they’re using a car they haven’t driven very often before.

Thanks to the fact that the larger diesel pump nozzle will not fit through the restricted aperture of a car designed for unleaded, practically all misfuelling involves the narrow petrol pump nozzle being inserted into the wider diesel filler aperture.

The problem has been getting worse over the last few years with the growth in diesel car sales. Modern diesels are so quiet and it is easy to forget you’re driving one, particularly if it’s a second family car or hire car.

Why Is It A Problem?

  1. Petrol is very damaging to a diesel engine. The metal parts of a diesel engine need to be lubricated to allow them to glide alongside each other easily.
  2. Modern diesel engines are fined tuned and manufactured to work perfectly in very exact conditions, and this lubricant is essential.
  3. Petrol in diesel acts as a solvent, it will remove this lubricant and cause the parts of the engine to grate against each other and wear away. If a car is driven for any length of time, it will end up damaging the fuel pump and fuel-injection systems, and will most likely result in the need for a costly new engine.
  4. In the rare case a petrol engine is accidently filled with diesel (usually diesel nozzles are too big to fit in) it can cause permanent damage to several parts of the engine such as the catalytic converter.
  1. Some fuel system seals can be affected by the compounds in petrol too
  2. The further the contaminated petrol goes in the system the more expensive the repair – it can be cheaper in more extreme cases to fit a new engine
  3. The common rail (or HDi) diesel engines are particularly vulnerable – if fuel contaminated by pump wear debris gets as far as the common rail system you may have to replace the low and high-pressure fuel pumps, injectors, fuel rail, line filters and the fuel tank too
  4. Many systems use a low-pressure electric pump in the tank or sender unit and this begins to work as soon as the ignition is switched on, circulating the contaminated fuel through the pump and rail – so it’s important not to turn the ignition on.

As described above, the extent of the damage caused by putting the wrong fuel in your car varies depending on how much of it enters the system. To minimise damage, the engine should not be started; draining the fuel tank will then help to restore the vehicle.

If you’ve put the wrong fuel in your car, don’t turn on the ignition or start the engine as this will circulate the contaminated fuel and increase the risk of damage. Modern Diesel engines are not equipped to deal with the more volatile and significantly thinner petrol.

The results of driving away in a diesel car filled with petrol can be catastrophic for your car’s engine. The costs can be enormous. Do not try to try to flush the tank yourself, the Automobile Association do offer an excellent ‘Fuel Assist’ service removing the offending petrol from your tank, cleaning the fuel pipes and putting in some fresh Diesel fuel in to get you running again, before any damage has happened.

To re-emphasise, the most important thing to do once you’ve realized you have put petrol in a diesel car is to NOT TO START THE ENGINE.  Even if you fill the tank to the brim with petrol, as long as you don’t attempt to start the engine, the wrong fuel won’t reach the engine’s vital components and no damage will be done.

Some commentators say, as a guide, if you’ve added more than 10% (5 litres in a 50 litre tank) petrol, drain the tank and refill with diesel. If you have added less than 10% petrol (5 litres in a 50 litre tank), top up with diesel and run normally unless the manufacturer has advised otherwise. However, we would always recommend that at the time you seek professional assistance and advice from a professional organization such as the Automobile Association

How Costly Will It Be?

Short answer: It can be very expensive. The cost of driving a diesel car filled with petrol can range to a few hundred pounds for replacement parts to an entirely new engine costing thousands.

The cheapest it will be is between £150 to £300 for a tank drain if you’ve realised your error quickly and not driven the car very far.

Am I Covered?

If the car’s still under warranty you should check with the franchised dealer regarding their advice and correct remedial action – running with even a small amount of petrol in the tank might invalidate the warranty while some manufacturers advise that seals and filters should be renewed even if the engine has not been run. Be cautious, here also, as there is a view that manufacturers can overreact with their advice and take a too pessimistic view of the potential damage.

Stories abound of BMW or Mini Diesel drivers being tempted to call their local BMW dealer and ask them for assistance. The result is to have it recovered to the dealer where they’ll replace every single component in the fuel system at a cost of between £4000 and £9000. We have no verification of such incidents, however, and they may be apocryphal.

Research shows that less than a quarter of UK insurers specifically make reference to misfuelling in their policies. All of these state that they will not cover damage arising from it, whilst the remaining three quarters do not mention it at all.

This really is a grey area as in a some cases, insurance companies regard misfuelling as a genuine accident and cover it as ‘accidental damage’, although it is not specified as such within the wording of the policy. A policyholder should be entitled to complain to it if their insurer refused a claim without specifying an exclusion within the policy in the first place.

Depending on how much fuel has been drawn into the engine, you could end up with a £5,000 repair bill, which could come out of your own pocket if your insurer refuses to accept your claim.

If your car breaks down as a result of putting in the wrong fuel, you should, however, still be entitled to usual services offered by breakdown cover (either being taken to a local repairer or back to your home address).

How To Prevent It

  1. It is vital to try to prevent it in the first place. Unfortunately, the colour system on the pump handle originally used to denote petrol and diesel pumps has been replaced with green pumps as petrol stations jump on the green bandwagon.
  2. The simplest way to avoid it is to always ensure you are aware of the type of fuel your car needs. This is important if other people drive the car, or if you’re using a company car or hire car.
  3. You could also take the steps hire car companies have taken and stick a label above the fuel tank stating which type of fuel it uses. This should prevent any accidental misfuelling and help a car with multiple drivers avoid any mishaps.
  4. Alternatively, You could buy the latest Ford. The manufacturer has introduced a new Easyfuel system, which actually prevents drivers filling up diesel cars with damaging petrol. The petrol pump nozzle is physically stopped from entering the engine in models such as the Ford Mondeo saloon and estate, making it an ideal car for people prone to misfuelling.
  5. Always check your receipt after paying for the fuel, it should tell you what you have just put into the car.
  6. Generally, petrol cars are fine because diesel nozzles will not usually fit into a petrol tank.


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