Wrong Fuel?Petrol in Diesel FAQ’s 368

Here is a list of the wrong fuel questions we get asked – on or before nearly every job!

If you have any questions, or need a fuel drain now, please contact us.

Q; I drove my diesel car for 2 miles on petrol after I filled it up, then it just died on me. Will it need to be repaired ?

Answer; you didnt drive it for 2 miles on petrol whats more likely is you drove it for 2 miles with petrol in the tank.Once the petrol made its way through the low pressure sender pump, thru all the fuel lines, up to and through the filter (holds about a pint of fuel),  through the hoses to the high pressure pump, thru the pump, into the common rail resovoir, and finally into the high pressure lines into the combustion chamber, that’s normally the exact point that the engine stops turning, and in fact you will find you maybe got about 20 yards on petrol, which is the time it took for the engine to stall. The good news is, this isn’t as catastrophic as you might think and there is a simple, effective solution in a mobile fuel drain – a service we provide all over the country and we attend most call outs with 50 minutes, 24 hours a day. there are many different things that can happen with misfuelling, I have covered most of them below.

Q; What does petrol do to a diesel car's fuel system?

Answer: This  picture (below)  is the suction  line I use to remove wrong fuel . this was taken when doing a job that had 50 litres of petrol in a diesel mini, this was being taken from the fuel tank as the car had not been started. this hose is full of fuel, about 70% petrol, 30% diesel.

Is wrong fuel harmfull wrong fuel in hose

what does wrong fuel do to a car

This is a picture of the same hose, the car was a Diesel VW Polo, it has been fill’d with 90% petrol, and 10% diesel, the car had been left overnight after loosing power and being towed back to the customers house, the suction line was attached to the high pressure pump inlet, as you can see the fuel has gone black, this is becuase the petrol has desolved  rubber in the fuel lines,  this car sat for a day with a very strong mix of petrol, it had also been driven until failure, the dark colour is a mix of rubber and plastic that has dissolved in petrol (fuel hoses are made type specific to reduce costs for car manufacturers,i,e diesel for diesel hoses only and petrol in petrol hoses only), I would not expect this to have done the car any real harm over an 18 hour period, just a microscopic layer of fuel line has been stripped off from the inside, these lines are fairly thick so a short bit of contamination once is fine, if you do want to make a habit of misfuelling regularly and leaving the car sitting for days with the wrong fuel in it you will eventually need new fuel lines. and plastic fuel components

Q; What happens to the fuel once we take it out

Answer; at the end of the day (or during a very busy day), I unload all of my mixed fuel into 220 litres drums, at our garage, (a licence is required to store mixed fuel from the enviroment agency), the mix is collected every 7 to 10 days by a garage services company, these are the same guys who take away used engine oil, brake fluid etc.. from garages everywhere. the mixed fuel is batched seperetly and in-between me and all the other companys that offer this service, I would guess that there is between 50 to 200 thousand litres a week of mixed fuel “made” in the UK per week. – once it is batched up into viable quantities usually 28.000 litres (tanker load) it is refined in one of the main refinery’s using a distillation process – identical to the one used for making crude oil, the petrol diesel mix seperates at 60 – celsius, i,e the petrol fumes off, it is then condensed and seperated, from what I have heard this is done twice to get a proper result, I would assume that the mix would then go back into the supply chain and be used as it was originally intended!

Q; I put the wrong fuel in - have I damaged my car?
Answer; it is very unlikely that you have, pretty much 1 for 1 call outs I attend are back up and running again when drained, if you have not started your car, you will not have damaged it. if you have driven your car and it stalled, or would not re-start then the chances of any lasting damage are very slim, there’s a bit of a safety mechanism that works by defualt and it goes like “petrol will ruin your diesel car but your diesel car will not run on petrol long enough to get ruined, therefore, PETROL WILL NOT RUIN YOUR  DIESEL CAR”

Q; My car conked out, have I wrecked it?

AnswerNo, as above “conking out” is your engines way of saying “I am not moving untill you get the wrong fuel out of me and the right fuel in”, I attend many “conk outs” “died” “made horrible noises” etc.. once they have been drained down , 20 litres of correct fuel put in -and the system fully purged upto the high pressure pump fuel inlet –  then , they all ran fine afterwards

Q; My car has a common rail pump, are you sure its going to be fine?

Answer; YES!, the vast majority of new diesel cars have sophisticated common rail pumps, we attend them day in, day out what I have written above applies specifically to common rails.

Q; Whats worse, diesel in petrol, or petrol in diesel?

Answer; the wrong fuel in varying quantities does different things, petrol in diesel will stall it, or depending on the mixture it might just still run, albeit sounding like a bag of spanners, – with reduced power  Diesel in a petrol car will make it lose power, and run flat. The worst case scenarios (and these are the WORST) are;

1) petrol in a diesel pump will eventually damage the pump (If you are unlucky enough to get a mix that just runs) , if you have not noticed it, and are completely oblivious to the car’s behaviour, you would also have to do a fair ammount of driving with a heavy foot, this would require a new common rail pump, piezo injectors and fuel lines,  the high pressure fuel pump which operates at 28 oddd thousand PSI of pressure and is a highley enginerd unit relies on diesel going thru it to provide lubrication, petrol does not lubricate, in fact it has “anti lubricating” properties, hence why petrol is good for cleaning out oil stains. what happens is the inner working of the high pressure pump start chaffing metal grinds on metal, this destroys the pump, then the metal chafe gets drawn thru to the piezo injectors and ruins them as well, now  before you go and sell a kidney to pay for a new diesel pump and injectors, please read the read the rest of these FAQ’s because full scale damage like this is very very rare.

2) diesel in petrol worst case scenario, the diesel will enter the cylinder and not burn, it will slide past the piston rings, into the oil sump and therefore the cars oil system, this will increase the oil level, to a point that could cause total engine failure, i.e mangled rods, bent pistons, complete block failure, or the thining of the oil can reduce lubrication to the engine and it could suffer from a full seizure/big end failure/ruined propshaft. Again this would not be an easy thing to do, the car would be very low on power and not running right, you would have to do a good bit of driving in it to achieve the above.

Q; Land rover/mercedes/bmw/audi etc all insist I bring it into the garage for a full drain and it will cost ££££££££££ what should I do?

Answer: main dealers and “tooth sucking garages” will sometimes take a misfuel case for all its worth. I have heard of people having misfueled cars collected by main dealers (these cars had not been started and were collected from the petrol station forecourt) and then the insurance company picks up a tab for 5 or 6 thousand pounds for parts that absoloutley dont need replaced, (and I would suspect never get replaced), this is all very well untill you lose your no claims bonus and when you trade your car in eventually you have to explain why it had so much major work when it was brand new.

Q; But the maindealer says I must bring it in otherwise the engine and pump will be ruined

Answer; I’ve got a fair bit to say about this, probably the most high profile example of manufacturers being totally inflexible  was the Icelandic volcano eruption, and the disruption to flights, it went like this, there was a small amount of ash detected in the air space, as a precuation the authorities asked Boeing & Rolls Royce what was a “safe” and what was an “unsafe” amount of ash for jet engines to fly in, the responce they got was “zero”, therefore all flying was banned. because none of the big engine makers would go out on a limb and come up with a “safe” level of ash everything came to a stop – (until the airlines started flying empty planes round in circles to prove that it was actually safe) Coming back to the automotive industry, the car manufacturers have a similar one policy fits all approach to wrong fuel in cars, , and that is certain things must get replaced after a fuel drain regardless of how much fuel was put in, for how long, or whether the car was driven. They win both ways, firstly they sell lots of parts and millions of hours of labour that wouldn’t otherwise be sold – and secondly they never have to deal with someone claiming warranty if there is any possibility of wrong fuel being used , they know that petrol in small temporary quantities won’t destroy a diesel engine, but they could never say it, because it they are wrong on just one car once… its would be one too many, that’s the manufacturers approach, and the main dealer network quickly caught up with the idea of turning it into a lucrative money spinner and starting milking customers and  insurance companies for thousands of pounds.

Q; I've read everything here but the stuff I read on the net and in car forums says otherwise, I still don't know what to do?

Answer; I don’t blame you, I have seen the various discussions in forums on the net – it makes for scary reading if you have put petrol in a diesel car, you will find most of them are repeating threads from older posts, who have re-hashed threads from even older posts, that were written by armchair mechanics , the amount of conflicting information is also a clue as to the validity of the content,  there’s a lot of people talking a lot about something they know very little about, I do fuel drains, its my livelyhood, I do dozens every week, I have attended thousands of call outs, I have seen customers who I did fuel drains on after they had driven 10.0o0 20.000 and even 50.000 miles in their cars after putting the wrong fuel in and being drained, none of them had any damage that developed after the misfuel.

Q; I'm in a hire car, should I call them and have them arrange something

Answer. (I dont mind getting a legal letter on this one becuase I know it to be true) I received a call from a man in Canada on Tuesday, last week he was in the UK and misfuelld in a HERTZ rental car,  the forecourt attendant gave him my card, before he called me he had a quick look at the rental agreement, it  was quite clear that he was obliged to tell Hertz what had happened, so he did, and they sent the AA.  his card was then debited for £500 odd pounds, now the AA charge between £100 if they are quiet, to £230 if they are busy for a misfuel (no it is not covered by AA membership), so how did he end up paying £500?, Hertz called the AA and I assume got a whacking corporate discount and had the job done for 50 or 60 quid, they then put the boot in and charged the customer £500, he didnt have a choice in the matter because they had his card details, so if you are in a hire car you could either, call an independent mobile fuel drain company, or if that makes you uneasy – call the AA yourself , and pay for it yourself, you will save a small fortune.

Q; I have read all the above, now I think I will top up with the right fuel and chance my luck will that be ok?

Answer; hmm… I would always recommend a full fuel drain, not because I want your business but because petrol does not do any good in diesel engines, it can corrode rubber hoses and plastic seals that are fuel type specific (petrol and diesel behave differently on different materials, , so the car manufacturers use different plastics and rubber in the fuel system) , as much as I believe wrong fuel will not wreck a car, that is based upon the wrong fuel being taken out as soon as the mistake has been spotted, petrol in a diesel engine will eventually cause problems if not dealt with, but if you are driving an old knacker that will be scrapped when it fails its MOT that is due in a month then you might as well just risk it, but otherwise DRAIN DRAIN DRAIN.

Q; Can I keep the wrong fuel after you drain it, I want to use it in my lawnmower

Answer. there are easier ways of getting out of mowing the lawn other than ruining your lawn mower with a petrol diesel mix, maybe you should feign injury or concrete over the garden. Really misfuel is not worth trying to use, even if you think you have just put 80 quid of petrol in your range rover sport and assume you have a 99% petrol mix, allow me to elaborate.

when you drive a car your fuel sloshes around the tank, (even with baffles)  when it gets low and sloshes too much it draws air into the fuel system, this is what running out of fuel feels like, however the tank is not totally empty at this point, it merely cannot supply a constant flow of fuel without interruption,  but there is still around 5 to 10 litres in the tank, this changes the sums when you are calculating the “purity” of the mixed fuel, and you will actually have a fair bit more diesel in the mix than you thought you had, allot of people tell me that the “gauge said 3 miles left” and when Í’ve finished the drain and they fill back up they find they got 20 quid more in it that they ever had, and are genuinely surprises how big the tank actually is,  that said, if you really feel attached to it and have suitable containers and somewhere safe to store it, then I might just let you keep it, I will expect coffee, biscuits and perhaps a sandwich or two for the privilege.

Q; Are there a lot of idiots/wallies/Muppets who misfuel like me?

Answer; I have met some of the nicest people doing fuel drains, in fact it has restored my faith in humanity! You most likely are not a Muppet, fuel filling pumps are very unstandard and there is no uniform colour coding of pumps, the AA released a snippet a few years ago that 300.000 people per year misfuel, I  think the true number is many times that, and the number of people who only misfuel by a pound or so is astonishing, people who lead busy lives or drive more than one car are more likely to misfuel, some older customers joke that maybe they have gone senile – and it doesn’t even occur to them that they have driven a petrol car for 40 years and just got a diesel fiesta because there son insisted they get a more economical car – (and were embarrassed about the 1985 Cortina that mum and dad  were running around in), don’t beat yourself up, you most likely live a busy life and have responsibilities that take up a lot of your attention, misfuelling is not the beginning of dementia, nor is it indicative of a low IQ, anyone who gives you a hard time or carries the joke on too long, is probably the sort of person who has a go at everyone about everything, so don’t take it personally, one day it will happen to them too. The “at risk groups” are mothers with young children, (try filling up with  toddlers),  anyone in a rush, fleet drivers who run a petrol home car and diesel work car, and of course Muppets  joke;-)

Q; Does the petrol seperate and float on top of diesel or does the diesel float on top of petrol

Answer; neither, they blend instantly in the tank and it becomes a fairly even mixture, although they are different compounds, (technically speaking the carbon chain on diesel is longer) , they both come from crude oil,  petrol and diesel spend millions of years in the ground as one and the same as part of crude, only refining seperates them, and once recombined they will mix together and stay together  untill refined.

Q; I have heard of petrol being used as an additive in diesel in colder climates, why?

Answer, yes, it can be used as an ad-hoc winter fuel additive, in remote freezing areas that do not have a well established supply chain of fuel, (think Arctic weather stations), in the UK however we have a very good supply chain of fuel and winter additive is added by the refiners as early as September, the engines that can tolerate a bit of petrol are lower technology diesels, that do not have the high pressure pumps that are found in the cars of today, I would not recommend using Petrol as an additive to diesel, I heard from a  skip hire company in edinburgh that used to add petrol into diesel – but they stopped doing it in the 90’s.

Q; My engine management light has come on - what does that mean?

Answer: Engine management lights come on for a number of different reasons, on Volkswagons for example when they have been driven on the wrong fuel and drained, they often take a while to restart sometime 3 bursts of 30 seconds cranking, it is not un-common for the low oil light to come on when it first fires up, it goes off after a minute or 2,  engine management computers are not very informative, they either say – “no problem” or “problem”, petrol in a diesel car will confuse sophisticated engine management computers, they are designed to regulate air and fuel flow, air pressure, exhaust pressure, exhausttemperature, ignition timing, all to give optimum performance, economy and emissions, when you put petrol in the tank it throws all the calculations off, some very intelligent engine management units cars will even try to “adjust” to the contaminated fuel, and when they fail as they always do, they will sometimes read a fault, the vast majority of fault codes generated by a wrong fuel scenario are temporary, which means once a fuel drain has been performed – the engine will detect that the conditions that caused the problem are no longer present – thus assumes the problem is solved and the EM light will go out, It is very rare that a light stays on after a fuel drain and restart, this is one of the reasons why I would always suggest a professional drain on a new car, a professional drain will remove as much fuel as is possible, which mitigates possible damage

Q; I put the wrong fuel in, do I need a new fuel filter?

Answer: No you not need a new filter. This idea came about from the “logical” assumption that the filter would be contaminated with condensed amounts of wrong fuel and thus continue to weep wrong fuel into the system, a filter does not hold much fuel – and petrol and diesel certainly do not “condense” 300 to 400 ml of fluid is what a filter holds, the filter is simply a housing, with a cardboard, paper type filament to catch solid debris, a diesel filter has 2 purposes, to prevent water from getting into the engine via a water trap – which needs emptied on servicing or by dashboard warning light sensor,  and to stop solid debris from the tank from getting into the engine, diesel and petrol are neither, and petrol does not “sit” in a fuel filter, likewise when diesel is put in a petrol car it does not sit in the system  when a fuel drain is performed of a car that has been driven to failure on wrong fuel a flush is done of the filter, this involves pumping the correct fuel through the filter, which purges the filter completely, after a proper fuel drain a filter will be clear of the wrong fuel, I think that this is sometimes used as an excuse by some garages to inflate the price of a fuel drain, the truth is that most fuel filters cost a whopping 4 pounds, and take a lenghty 6 to 8 minutes to change, I have seen garages charge over 150 pounds for a filter change, and If I was morally bankrupt I would be getting in on it to, but the truth is that there no reason to change a filter unless it is due per the service book, that said some people still want filters changed during a fuel drain for peace of mind, which I have done occasionally if the filter is supplied, for the princely sum of a cup of tea and a heap of biscuits, (chocolate).

Further reading

A very good article on this subject was written by auto express, here is the link here


This was re-hashed by the daily mail here


Q.  Who owns Fuelfixer

A.  Wrong Fuel company, Fuelfixer Ltd, is owned by the managers of the company, Stuart Guy and Daniel Garside. It is headquartered in East Grinstead, West Sussex. The business was started by Murdo Guy in 2010. Murdo now operates Tubby’s Tyres, based in Crawley.

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368 thoughts on “Wrong Fuel?Petrol in Diesel FAQ’s

  • karen

    I put 20 quid of petrol in my diesel corsa. I drove 10 miles home when I went to start car next day I realised what I had done straight away and turned car off. my mechanic friends drained the petrol out and we refilled tank with diesel. all seemed ok for 2 days then car started to lose power when I slowed down at a junction or a round out when I try to get my speed back up it loses power I pull over switch car off then on again an all is fine has done this 4-5 times in 2 days any advise

    • TheFuelMan Post author

      Hi Karen,
      Sorry for taking so long to get back to you. The first thing to find out is if your friend managed to get all the contaminated fuel out of the vehicle (including clearing out the lines and filter). It is most likely the case that there is still a fair amount of it left causing the problem with a lack of power.
      Let me know.
      The Fuel Man

  • John Mansfield

    Thankyou very much FuelMan for your quick reply, I feel reassured by yourself that there shouldn’t be any concern, I am going to put an additive in as you suggested, Thankyou once again, John.

  • JP

    Superb info and am all the better for reading it. Diesel Peugeot started playing up recently:engine cuts out sometimes when slowing down or when idling(though next journey may be fine and so on – i.e an erratic fault). Always super-careful when fuelling but wanted to eliminate this as poss cause. Had originally topped up with 1/2 tank fuel,drove approx 90 miles before it then started playing up following day. Cousin takes the car for 3 days – no probs. Car back+been playing up since! Has probably been driven for approx 100 miles since. Neither garage nor auto electric folk who have taken a look have mentioned anything re wrong fuel as a poss theory and I’m 99% certain it is not that. Your website I thinks backs this up. Thanks

  • John Mansfield

    Hi FuelMan, Please could you help, i drive a Range Rover Evoque 2.2 diesel 2013,.Today i accidently put 48 pence worth or 0.45% of a litre of unleaded petrol instead of diesel, i realised and stopped straight away i then continued to fill the tank which took 45 litres of diesel to fill.The tank itself holds 60 litres of diesel, i am concerned i may have damaged the engine, should i consider having the tank drained, or is percentage of petrol against diesel to low to be of concern, what do you recommend. Thanks John

    • TheFuelMan Post author

      Hi John,
      I’ve never heard of that amount of wrong fuel causing any issues at all (I hope your case isn’t the first). If it were my car it’s not something I would be overly concerned about.
      Obviously for legal reasons I have to remind you that every case is individual and that any amount of contaminated fuel can cause issues. But in all honesty I wouldn’t just be surprised if it damaged anything – I would be shocked.
      If you want to mitigate any possible chances you should get a lubricating additive into the fuel.
      Hope this helps.
      The Fuel Man

      • Mike Cryne

        Hi Fuelman,
        I have stumbled on you site by accident while trying to resolve my gas (petrol) in diesel dilemma in the USA. There are not any services like yours in the US that I can find, only dealers and local mechanics / garages, with VW TDI experts in sparse supply. I am greatly relieved to hear of the results normally achieved by a complete drain and filter change and in some cases (very small contaminations) simply diluting with diesel and adding a diesel conditioner w/lubricant. That said I fear that I am the unlucky one who drove our 2006 golf TDI (BEW PD engine in states) about 25 miles before it died on the highway. I did not figure out it was petrol contamination for about two weeks, I looked at the last receipt for about 6 US gallons and it said diesel ( apparently the stations diesel tank was contaminated by a miss filling). I have now opened the tank and cleaned it as well as drained the lift pump, fuel lines and changed the filter. I primed the filter with diesel purge, put 5 gals fresh diesel in the tank and added 5 oz of Kleenex diesel additive with lubricant, cycled the lift pump a dozen times and tried to start with no luck. The battery is fully charged and the engine cranking over nicely it has tried to catch a few times but seems to not have fuel. I fear I may have gotten a mix of petrol to diesel that allowed the engine to run just long enough to ruin the injectors and more. The lift pump seems to work fine but there might be air somewhere in the system yet. Here in the states very few people have specific expertise in the fuel mistake/recovery area due to the lack of available diesel personal vehicles and as a result you only hear horror stories about catastrophic failure and multi thousand dollar repair bills, almost no one including TDI forums will tell you that a complete tank drain, line flush and filter change with fresh fuel will solve many problems, in fact 90% or better of what you hear is catastrophe failure and $1000 – $3000 for 2006 and earlier TDI’s and $3,000 -$8,000 for 2009 and up (no ’07 or ’08 TDI’s in US). I would have gladly paid for a service to drain my vehicle and dispose of the 15 gallons of @20% petrol diesel that I have to deal with. I can’t help wondering if a “fuel repair” specialty company would do well in major metropolitan areas in the US, i.e. NYC, DC, LA, Philadelphia , Chacigo. Mike

        • TheFuelMan Post author

          Hi Mike,
          Sorry to hear about your situation. When you turn the ignition, is there fuel? Disconnect the fuel line and check if fuel spurts out when you crank it. If not it could be a problem with the lift pump. If you are getting fuel then it should eventually start on cranking (I find that spraying a tiny bit of break-cleaner into the air intake helps give it a little push).
          Let me know if that works out for you.
          As regards setting up shop in the US – I’ve actually looked into this before. Unfortunately last time I checked there aren’t enough diesel cars per square/km in the States to make it a profitable venture. We have tonnes of diesel cars here – and the country is tiny – and therefore easy to get around.
          The Fuel Man

  • James B

    Hi FuelMan,
    I own a 2001 Saab 9-3 TID.
    Today I went to fill up the car and realized that I was adding Super Unleaded not Diesel into the tank. I stopped but not before I added a half a tank to the car. I drove it (at most) 300 meters to a self service garage. I drained the fuel from the tank using a pump. Added 2 gallons of DIESEL fuel and now the vehicle will not start.

    Any suggestions on what I can do to make this thing start?

    Thanks in advance.

    • TheFuelMan Post author

      Hi James,
      It sounds likely that you still have unleaded in the fuel system. Draining the tank on a misfuelled vehicle that has been run is very often not enough to get it going again. It will also require draining and flushing out the system (lines, filters, etc.). This needs to be done carefully to ensure that there are no air-locks which would also prevent the vehicle from starting.
      I very highly doubt that driving the vehicle 300 metres would have damaged anything so I would suggest that the drain just needs to be completed.
      Hope this helps.
      The Fuel Man

  • Colin

    Hi FuelMan …Thanks for reply,

    So far so good ive basically mirrored what Bruce done and have kept it brimmed with high grade and added another double dose of diesel additive/lubricant and fingers crossed 200miles since misfuel. It starts fine and fires up in a second or two and a lil bit of white smoke ot first, sounds a little tappynoisy under acceleration but seems to be getting quieter after each top up.
    My one question if you could answer is, When I add diesel will the petrol rest on the top? Or would it mix with the diesel ? Only I dont think Bruce could confirm that in his post.
    Thanks Again.

    • TheFuelMan Post author

      Hi Colin,
      Good to hear your luck is holding.
      As to petrol resting on top of diesel – no. They mix like milk and water. If you were to leave it standing for a loooong time you would find a higher concentration of one at the top – but they would still be pretty well mixed. Bear in mind that they come out of the ground mixed, and it takes further refining to re-separate them. Also remember that hot fuel is constantly being pumped back into the fuel tank when the engine is running, causing any newly added fuel to be almost instantly mixed in as well.
      The Fuel Man

  • Colin

    I have a 2005 1.8 tdci Transit connect,
    My low fuel light came on and i pulled into fill up but made the error and pit 10 ltrs worth in without realising, drove off and engine management light came on under acceleration in 5th gear it was then the penny dropped and i noticed my error… pulled into next station [10 miles down road} and brimmed to full on higher grade diesel and added diesel additive the management light did not re-apear and i completed my journey with no issue { so far} it also started this morning ok and seems to be running fine, do you think ive just been lucky or could i have problems ahead.
    I will continue to brim with higher grade diesel and add another dosage of additive when i get to 3rd of tank so just hoping, any advice or guidance on whether i should get system drained or would i have experienced the symptoms by now?
    Thanks for any help you can offer in Advance
    (Just a quick add, the journey from brimming with high grade was roughly 70 miles.)


    • TheFuelMan Post author

      Hi Colin,
      Sorry for the delay in getting back to you.
      Have a look at this comment from Bruce. He was in a similar situation and he describes what he did and the results of it.
      Best of luck,
      The Fuel Man

    • TheFuelMan Post author

      Hi Andy,
      This is a question we get asked quite a lot as it is very much a grey area for most people.
      Have a look at this comment from Jules, and the subsequent replies from me and from the motorist. It might help ease your mind.
      Best of luck,
      The Fuel Man

  • Bruce

    Last weekend put 3.4lt of unleaded into my 64l diesel tank – it had 20lt in, panicked and took a risk and brimmed it with 40lt – so about 5% contamination. Drove 1000 miles over next four days, kept topping up (5 times) so tank never below 2/3rds full with BP Ultimate rather than ordinary diesel. Kept engine revs to under 2,000 by keeping within speed limits! By my calcs, assuming petrol mixed evenly with diesel contamination should now be around 0.8%. Have put in 101 litres since first brimming. Car drives absolutely fine, returning 45mpg over long journey. My calcs suggests another 100 litres (say 5 top ups of 20lt) will bring contamination down to 0.1%

    So have I been lucky or can I expect future problems – and in a years time when the car goes in for service (after a further 8000 miles) will VW dealer be able to detect what I’ve done – car is still under warranty. Is it still worth getting a full drain?

    • TheFuelMan Post author

      Hi Bruce,
      With only 3.4Ltres in a full 64Ltre tank (filled with BP Ultimate) I do not expect you will have any trouble. Your contamination level is sufficiently low – and you are using diesel with added lubrication (BP Ultimate) – that any negative effects of the petrol in the system should be almost entirely nullified.
      I doubt very much that a fuel drain will make any difference to the vehicle now. And after another 8,000 miles I very highly doubt anyone will know what’s happened (I don’t think anyone could even tell now bearing in mind that some of the fuel you can buy at some supermarket petrol stations is a bit dodgey and, under analysis, probably looks a like Ultimate diesel with a bit of unleaded mixed in).
      Best of luck,
      The Fuel Man

      • Bruce

        Thought I would leave an update here so readers have so real data on outcomes. Now driven 1,750 miles since the 3.4 litre misfuel – and have put in a total of 223 litres of diesel since – 155 litres were BP Ultimate or Shell V Power. Haven’t noticed anything at all out of the ordinary from the engine, noise wise or performance wise, and have got 39.9mpg overall since the misfuel and on some journey’s up to 49mpg. Prior to misfuel average was 37.3mpg. Have filled up 10 times never letting fuel get less than 2/3rd full apart from today when tank got down to a bit above half full – 30 litres needed. Have kept revs to under 2000 as much as possible but on occasion had to make the engine work hard up long 16% hills.
        Had a nervous moment when car went in this week to dealer for a warranty recall to have a ‘regulating flap’ fitted, but dealer didn’t make any comment about the car.
        Have tried to calculate how much petrol is left mixed in with the diesel. Making the assumption that every fill mixes entirely evenly with what was there already (not necessarily the case), I’ve calculated that with my fill pattern the tank has gone from a % petrol of 5.1% at misfuel (after brimming) in the diesel down to 0.12% now. It’ll get down to 0.01% after another 120 litres (2 more tankful’s, 6 fills).
        I’ll provide an update at the 2,500 miles since misfuel or earlier if there’s a problem. I’ll be continuing to fill up every 20 litres or so with Shell V-Power. Good job diesel has got cheaper!

        • TheFuelMan Post author

          Hi Bruce,
          Thank you very much for this information – very useful for our readers!
          Glad that it has all worked out for you.
          The Fuel Man

          • Bruce

            Thought I’d give a final update – have now gone 2800 miles with no problems at all. Have continued to refuel to brim every 25litres or so. My calcs show that petrol should now be just 0.01% of a tank full compared with 5.5% at start.
            Looking back on the experience I would now go for a tank drain – too much worry otherwise! At the time I was filling up and immediately going on a 1,000 mile weekend round trip and short of time. Delaying starting for 2 hours whilst tank was drained would have made for a much more relaxing time. However by doing the 1000 miles (almost all dual/motorway at 70mpg) in 3 days I got the petrol heavily diluted quickly and reassured myself quickly …. and avoided having to tell the wife! Still not sure if there might be long term damage which hasn’t shown up yet, but car is still under warranty ( a year to go). Will be keeping putting in V-power/Ultimate to minimize any long term effects.

  • Cheryl

    Thank you so much for your very informative reply.

    In addition to the advice I had received, on SaturdayI did also contact two garages that I have used in the past
    and BOTH of them told me that I shouldn’t have a problem at all; in fact, one of them told me he
    would not actually see it as necessary to drain the system at the % I had put in. BUT BUT BUT,
    I was still worried – still am. Knowing it is an older car – and thus less likely to have a problem – means,
    to me, that I should take the best care of it as I really do not want to have problems looming over
    the horizon. It would be nice to think I shall replace the car soon – but, to be honest – I don;t see it happening.
    So I do plan to arrange to have the system drained before I have much more driving to do. I have driven
    probably 40 miles max.over the weekend (since the problem Friday evening) – would this mean that
    there is no point in draining?
    Again, thank you for your help so far.

    • TheFuelMan Post author

      Hi Cheryl,
      First off I would say that I can’t find any fault with the advice you have been given on this matter. In the majority of cases I’ve seen like this there have been no long term effects of the misfuelling. However there is always a chance that something could go wrong, specifically on an older car that already has substantial wear and tear on the fuel system. If you are really worried about it – get it drained. This will give you the peace of mind you are looking for as it will ensure no further damage can be done to the vehicle. But I don’t know that I would bother too much with it if it were my vehicle. I would probably just go for getting a lubricating additive and have the fuel filter changed.
      Give us a call and we’ll be happy to further assist you.
      The Fuel Man

  • Cheryl

    Hi – fpr the first time in 8 years, I put the wrong fuel into my much loved but very old (R reg) Passat 1.9 TDi.
    I noticed at about five and a half litres and stopped, thinking I was going to pass out from the stress. The tank was just under
    half full prior to putting the petrol in and I was about to top up to full for a saving of 4p on a litre. Ha ha.
    I called Fuel Doctor who after checking car, amount I had put in etc., told me to fill the tank and all would be fine; under 10%
    of the tank capacity will be OK.
    This was also backed up by the attendants in the petrol station.
    Drove home perfectly fine, no issue whatsoever. This car has been very good and always starts first time and runs well,
    although is obviously old and does sound like a taxi.
    Now have read this entire comment thread and am worried that other. more insidious problems will arise (plastic pipes “melting”,
    filters clogging and even injectors being ruined….. etc.) and I am now a wee bit worried – – to say the least ;o(
    I don’t know if you will see this or have chance to respond – and I am not even sure if your company covers my area as
    I don’t see it on your list (South Yorkshire) – but I would be very very grateful for your advice based on my situation.

    • TheFuelMan Post author

      Hi Cheryl,
      I’m sorry to hear about your situation. Rest assured that we do cover your area – in fact we are fully nationwide.
      About your car: I would tend to side with the advice that you have already been given. If I’m not mistaken the 1997 Passat has a 70litre tank and therefore your contamination level is about 7.5% (assuming it was topped up afterwards).
      Realise that the “under 10%” advice that you were given is simply a guideline based on experience and is not a hard and fast rule. I’ve seen people get away with higher contamination levels, and also seen problems develop with lower levels.
      It is my opinion that putting this much petrol through a common-rail diesel engine is equivalent to running a very crude and abrasive cleaner through the system. Most engines will have no particular problem with it – but some will. Unfortunately there is no way to tell which way the pendulum will swing without trying it.
      If the future health of the vehicle and peace of mind are likely to be very important to you, you might want to look at getting it fully drained out and cleaned.
      If you are willing to take the risks (and I honestly don’t believe that they are huge) then I would keep topping it up. I would also find a fuel system additive and get that into the tank (look for any diesel additive that says it helps to re-lubricate the system – this is important). Additionally you should get the fuel filters changed at your soonest convenience due to the fact that the petrol will almost certainly loosen fuel line gunk and deposits which will make their way into the filter and could help to clog it up.
      I hope this has been helpful. If you have any further questions don’t hesitate to contact us.
      The Fuel Man

  • Jeff Evans

    Very good information, thanks for telling me in time to save me from selling my body parts to pay for a main dealer fix; ps the wife says my body parts are too knackered to sell anyway, regards jeff.

  • ijroute

    Absolutely brilliant write up from someone who definitely knows what there talking about, reading this as put my mind a little easier as I have had to be towed back today after picking up a new (to me) car and was totally unaware that I had put petrol in instead of diesel thought the blue smoke and stalling was due to the car wanting a good “blow out” and that’s what I intended doing today until I got a few miles down the motorway and it just cut out without warning!

  • gerard


    Fuelled seat 1.2 ltr diesel car with £20 worth of petrol and then drove it for 20 miles before it broke down. At this point a large amount of white smoke was coming from car.

    It has been into seat garage and they have drained fuel and cleaned tanks however not fixed as yet. They have then completed a further clean of filters and still no fix.

    Today they have informed me when engine starts that a large amount of water and diesel is coming out from exhaust and indicating that a new engine maybe required.

    Have you heard of this before?


    • TheFuelMan Post author

      Hello Gerard,
      I’ll be honest – the water is a surprise to me as it’s not something that is normally associated with misfueling. A small amount of water vapour (and steam when the engine is still cold) is completely normal. However a “large amount” is not. This could indicate a clogged muffler or, more seriously, a blown head gasket. As regards the diesel coming out of the exhaust after a misfuel, this could most likely be caused by a faulty injector.
      If I were you I would get the muffler checked and the injectors tested. This should tell you what you need to know and the extent of the damage.
      The Fuel Man

  • Michelle

    Hi. I put 3litres of unleaded in my Nissan Diesel Juke today realised – and filled it up with Diesel . It’s been to Nissan for a service today that was already booked. Now I’ve collected it the mpg details keep changing every few seconds on the dash board! Flashes 99mpg then 28mpg etc. Is that something the garage has done wrong or because I’ve put unleaded in? Thank you.

    • TheFuelMan Post author

      Hi Michelle,
      When does this mpg change happen – while driving? It could be something as simple as the display was changed from showing Average Fuel Consumption to showing Current Fuel Consumption. If this is the case then resetting it to show average mpg will solve the problem. Take a look at this:
      Nissan Juke Multi-Function Display
      If this is not the case let me know and I’ll look into it further.
      The Fuel Man

  • Aaron

    Dear Sir,
    Greetings from Panama! I have a 1.9L TDI Diesel Skoda Octavia (Pumpe Düse unit injector engine, apparently). I stupidly put in a 90% tank of unleaded and drove 25km. It misfired a couple of times then but I wasnt sure because the road home is very bumpy. Then When I stopped and lowered the revs it knocked very hard. I turned it off and tried restarting which it would not do so towed it the rest of the way home. I obviously will drain the tank, but should I drain the rest of the system too? Do you think I have ruined the engine? What a fool I have been!
    Thank you for any advice!

    • TheFuelMan Post author

      Hi Aaron,
      Yes, get the system drained as well – it can hold quite a lot of contaminated fuel. Then fill it up again and re-prime the system. Should be fine from there. I’d also get an additive into the system to lubricate the pumps as it has been driven quite a while.
      The Fuel Man

  • vrs

    so it would be best to get it drained when i change fuel filter? even though she is running perfect? and there is now 68 euro worth of diesel gone into it? and only 20 petrol

  • vrs

    I put 14 litres of petrol (20 Euro) in my 2012 octavia vrs. I was told to top it up with diesel as the tank was empty to start with. so i filled it up to the brim. (50 Euro) i then drove for about 100kms and put another 10 Euro diesel in and drove again for another 100k and topped up with 10 Euro more diesel. my mechanic is changing fuel filter in the morning and said that the car will be ok as i diluted it so much. What do you think. should i still get it drained. it seems to be running ok and starting ok

    • TheFuelMan Post author

      On an older car I would say just keep topping it up and use a fuel additive to help relubricate the pumps. However, on a newer vehicle – especially one under warranty – I would err on the side of caution and get it drained. If an old car packs up it’s not as big a deal as a new one as regards cost to get it fixed or replaced.
      The Fuel Man

  • Mark

    Added 1.5l litre of petrol to Diesel Kia Sportage (2013). Fully topped up with diesel and drove 15 miles. Should I get it drained, or should additive be safe enough?
    Thanks in advance

    • TheFuelMan Post author

      Hi Mark,
      Most likely brimming it and using an additive will be ok. Obviously the only sure-fire solution is to have it removed but you will probably be ok like this.
      The Fuel Man

      • Mark

        Thanks for the prompt reply,
        Sorry to be pain, but can you advise what brimming it means (Just keep topping up with Diesel?). Is there a particular kind of additive I should use