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Help, I've Put Petrol In A Diesel Engine - FAQ

Last Edited: 01/10/2015
In this guide:

    Noticed your error at the petrol station?

    If the answer is yes then DO NOT start the engine, DO NOT put your keys in the ignition, and DO NOT unlock the car. If you do turn the key in the ignition then you could prime the fuel pump and contaminate engine components. This can happen even if you began turning the key and stopped yourself at about halfway.

    Some modern cars also prime the fuel pump when you simply unlock the doors. So before doing anything at all with your key, call a breakdown assistance service and wait for them to come and pump all of the fuel out of your tank. You can then fill up properly and be on your way.

    Theoretically if you were near empty and you only put in a small amount of wrong fuel, you could then completely fill the tank with the correct fuel at a nearby station and be on your way.

    Some engines can cope with up to 10 per cent contamination, so if you only added one litre of the wrong fuel and still have space for 50 litres of the correct fuel you could risk it. However, even just a little contamination is enough to severely damage modern common-rail diesel (CRD) engines. We always recommend calling the manufacturer to get their advice before you think about starting your engine.

    Disclaimer: We take no responsibility for your actions. We are not trained mechanics. These tips are provided as a guide, bringing together advice from around internet and various forums. You should always seek the advice of a trained vehicle mechanic or breakdown service when dealing with car maintenance issues such as this.

    Have you noticed your error after you've left the forecourt?

    We'll assume your reading this paragraph having noticed your mistake but also having already left the forecourt and have since pulled over on the side of the road. In that case, make sure your vehicle is stopped in a safe location. It's better to risk your engine than it is to put your life, or the lives of others in danger.

    If you're on a motorway, then you should ideally park next to one of the emergency phones which are located at one mile intervals. You can read more about what's best to do when you need to stop on the motorway for an emergency in this guide.

    Once pulled over in a safe location, you should put your hazard lights on. If it's dark, leave your sidelights on too. It is wise to stand on the verge so that you are well out of the way of fast moving traffic. From here you can call a breakdown service, but remember to always face the traffic when you are using the phone.

    Please note, if your car is under warranty, you could invalidate this warranty if you do not allow the maker of your car to fix the problem. Bear this in mind before letting a local breakdown company do any work on the car, since it could cost you more in the long run. It's best to get it towed first, and give yourself some time to decide your next move.

    Have you called the breakdown company and been shocked by the price of rescue?

    It's probably best you pay them and avoid wrecking your engine, since this could prove far more costly in comparison. The list of replacement parts needed to fix a misfuelled engine is long, and could run into thousands of pounds.

    Are you unsure if you have misfuelled?

    You should be easily able to confirm if you've misfuelled by checking your receipt. Alternatively, here are some symptoms you can look out for:

    • The vehicle is difficult to start, especially when the engine is warm
    • The engine gets very noisy under load
    • Your exhaust is smoky when driving
    • The engine won't stop once started
    • Sudden loss of power
    • Complete loss of power

    Worried about what your wife/husband/friends/boss will say when they found out your mistake?!

    If it's any consolation, past research has indicate that misfuelling mishaps occur frequently. As many as 400 times a day in fact. We don't know how many of these people go on to drive the car, but if you have personally then the subsequent repair charges can range from a few hundred to several thousand pounds. On the flipside, if you've noticed your error while still at the petrol station, it will likely only cost a couple of hundred to get someone out to pump the tank, plus the cost of refuelling of course!

    A report in 2014 showed that the british police force spend around £300,000 a year as a result of officers not paying attention at the pumps, which is an average of 4 police cars damaged per day!

    A freedom of information request in 2015 revealed that police officers from Thames Valley Police alone have misfuelled cars in each of the past seven years, resulting in a massive £17,352 bill.

    How to prevent misfuelling

    There are a range of simple yet vital questions you can ask yourself before making a trip to a fuel station:

    • Have you checked if the car you're driving is a petrol or diesel model? This is particularly relevant if you're using a hire car or company car.
    • Is everyone who drives your car aware what type of fuel it uses?
    • Have you (if necessary) labelled your fuel tank with a 'Diesel Only' sticker?
    • Do you double-check fuel pumps at petrol stations? In the UK you'll almost always find that Green pumps are unleaded, and Black pumps are Diesel. Remarkably, they're the opposite way around in some countries so be careful if you're driving abroad!
    • Think you might have filled up with unleaded? Your receipt should tell you.
    • If you are attempting to dilute unleaded petrol with diesel ensure your car is not a modern common-rail diesel. If it is then do not even consider diluting.
    • It should only be possible to dilute 10 per cent of unleaded or under. Make sure you know the capacity of your fuel tank and make a judgement about whether it's worth the risk.
    • You may be able to drain a fuel tank yourself but if in doubt seek expert help. There are specialist companies that will come to your house and drain your fuel tank.

    How does the wrong fuel type affect engines?

    Breakdown organisations put the estimate figure of petrol misfuelling incidents at around 150,000 cars a year, with repair bills totalling hundreds of millions of pounds. Petrol strips a diesel engine of the lubricant it needs to keep it ticking over, as well as damaging seals. It will almost always lead to the engine seizing, by which point the whole engine block and many vital components will likely be destroyed beyond repair.

    To make matters worse, pre-ignition systems start fuel pumps in new diesel models when car doors are unlocked. This reduces the time it takes for new diesel engines to warm up, but means that diesel engines can be ruined simply by unlocking the doors. Modern technology doesn't always help when it comes to human error.

    How much will this mistake cost?

    Brace yourself for some bad news. If you've avoided turning on the engine or stopped quickly after turning on the engine, a fuel-tank drain may be sufficient. This can cost anywhere from £130 or more. It really depends on the area you've misfuelled in, and therefore which breakdown companies are within range. Don't be surprised if you're charged more if you're somewhere remote, since you're paying for the worker's hourly rate which includes the time taken to reach you. That's on top of the charge given for the safe disposal of the unwelcome fuel stored in your vehicle's tank.

    If you've driven the car after misfuelling, it is going to be costly. Vital components will be damaged, and unless you're driving a go-kart the bill for repair will likely stretch well into the thousands.

    Am I covered on my breakdown cover?

    Misfuelling is something that will not be covered by your manufacturer, though they may help tow your car to a dealership for you.

    Am I covered on my insurance?

    Misfuelling is generally excluded from all car insurance policies, including those which offer comprehensive protection.

    What if I lease my car?

    You will have to meet the cost of any repairs yourself if your car's on PCP or contract hire - or any other form of leasing contract. Misfuelling and then firing up your engine really is about the worst thing you can do to your car, so expect bills of at least several thousand pounds for a replacement engine and fuel system.

    Will my warranty be invalidated?

    That probably depends on the amount of damage. Your manufacturer may threaten to invalidate your warranty on your fuel system and related parts unless you let them do the repair work, but there may be room for negotiation.

    Can I dilute the petrol with diesel?

    Diluting contaminated fuel may be possible on older cars that lack the fine, high-pressure diesel injection systems of modern diesels. However, this is a high-risk move which could result in the destruction of your engine.

    It may be possible to dilute a proportion of petrol to diesel under 10% in older cars, though there's no guarantee this will work. Your car may run roughly for a while and you may risk long-term damage that is not immediately apparent.

    We don't recommend you try this but if you do risk it you need to be completely sure about your car's fuel tank and engine system - try this on a common-rail diesel and you'll simply wreck your engine.

    How does this mistake even happen?

    Modern diesel engines can be hard to differentiate for drivers with an untrained eye, or ear. Where traditionally diesel engines were clattery, dirty and slow to warm up, modern diesels drive and sound very similar to petrol engines. People simply forget they are driving a diesel as the associated audio and visual clues are simply not there.

    Ford has been the first manufacturer to develop technology designed to prevent a misfuelling mishap, with its Easyfuel system. This prevents drivers filling up their diesel models with a petrol pump.

    An unleaded petrol pump nozzle cannot enter a diesel tank on Mondeo and Focus models equipped with the system. Typically diesel nozzles are too wide to fit into any petrol tank, so the problem rarely crops up.

    To make matters more confusing, the increasing uniformity of petrol pump colours does not provide motorists with a visual cue as to the fuel type.

    Whereas diesel pumps were traditionally black, unleaded petrol green and four-star leaded petrol red, modern-day diesel pumps are occasionally green to enhance fuel suppliers' eco credentials, with BP being the most notable culprit.

    With the massive rise in popularity of diesels, misfuelling is an inevitable problem which only in recent years has been addressed properly by manufacturers.

    Diesel in a petrol engine

    Theoretically it should be hard to fill a petrol tank with diesel, as diesel nozzles are too wide to fit into the tank. That doesn't stop there being cases where this in fact does occur. Using diesel in a petrol engine may destroy the engine's catalytic converter, necessitating a costly replacement.

    How much Biodiesel/Ethanol can I safely put in my car?

    If you've accidentally filled your tank with biodiesel or ethanol the likelihood is you'll need to drain your tank, unless your car is correctly modified to receive these fuels.

    What more can I do to prevent this mistake?

    Aside from buying a Ford model with the Easyfuel protection system, or a similar feature from another manufacturer, the best thing you can do is to clearly label your vehicle with a sticker above the petrol tank, as car-hire companies generally do. This is particularly important if your car is used by a number of drivers.

    Some new cars nowadays will already have some kind of reminder on the fuel cap anyway, like a sticker for instance.

    Alternatively, you can also buy little gadgets which you stick inside your fuel flap that are light activated, and shout out "Diesel, Diesel...." when you open the fuel cap. Gadgets like these can be found at various car maintenance stores.

    Until manufacturers work with fuel suppliers to address the issue, it's simply a case of having your wits about you and bearing the astronomical repair costs in mind. Or you could always buy an electric car and never have to worry about this problem ever again.


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