Petrol in diesel

“i put petrol in my diesel car” – Now what?

common myths;

I read on a forum that putting petrol in my diesel car will ruin the cdi/hdi fuel pump.

There’s allot of scare stories doing the rounds about this on forums, and it comes from armchair mechanics who sit on computers all day being “helpfull”, these are the kind of people who have post counts in the thousands, everyone of them spouting some regurgitated drivel from another keyboard warrior.

The amount of conflicting information on the subject of how much harm is done to a misfuelled car can leave someone quite worried.

Let me re-assure you that 99% of cars we attend don’t need anything other than the wrong fuel removed, and the right fuel put back in.

Some main dealerships have their snouts deeply embedded in the misfuel trough and are taking advantage of both motorists and insurance companies, so much so that insurance companies are now going to great lengths to keep misfuelled cars out of the dealers hands and passing the work directly to a fuel draining company.

 Putting petrol in your diesel car and not driving is certainly not something that will do any harm, assuming your car engine was not running WHILE you were pumping the fuel into it, you are most definitely in the clear, a simple fuel drain will be all you ever need, and as for the old rumour doing the rounds that the fuel starts to circulate as soon as the car is unlocked with the remote fob – thats utter tosh, (and I would know), fuel pumps do prime for 3 seconds when the key is turned to the auxiliary position, even this does not “circulate the fuel” it simply pressurises what is in the system – and the fuel will not circulate until the engine is running, of the hundreds of different cars I have worked on I have never come across one that primes itself when it is unlocked – that myth is constantly referred to by lazy journalists who are quoting wanna be mechanics

But I drove my car and it broke down after I had misfuelled it.

Yep, they all do that, funnily enough your diesel car doesn’t run so well on petrol and neither it should, lets talk about engines for a minute, in a diesel engine the fuel is injected into the piston which compresses, this compression of an air fuel mix is the very thing that ignites the fuel – compression ignition. now in a petrol,the petrol air mix ignites when the spark plug – sparks, = spark ignition. in early petrol engines there was a problem of “pre ignition” (knocking, pinging/pinking) which is when the petrol/fuel mix ignited on its own accord before the spark could kick in, not good. the fuel in the piston is only supposed to ignite when the spark plug wants it to ignite, and if it ignites earlier it causes havoc to the engine with 1 piston firing out of time and fighting the other pistons, to prevent this the fuel companies developed additives and came up with the octane rating system, which improved petrol so that it did not ignite under pressure, so unless someone fitted spark plugs to your diesel car (unlikely) it can not, and will not run on petrol. if your car did run after being misfulled – you would have still had some amount of diesel in it, many people underestimate how much fuel there is in a car when the “light comes on” – its often a quarter of a tank, (but I dont recommend you test the theory without carrying a spare can of fuel around!), – that explains why your car did not break down straight away.

I misfueld my car last night with 40 litres of petrol, after noticing it i added 30 litres of diesel and it drove 40 miles home, now it wont start, what damage have I done?

Happens all the time, we get one of these every other day.  a warm diesel engine can usually run on a strong mix of petrol, a cold diesel car will not start with a strong mix of petrol, when you fill your car up, assuming you did not push it to the petrol station – it would have been warm, when you turned it off for 3 minutes to pay for your fuel and fill up – it still would have been very warm, when you parked it on your drive for the night – it became cold, and will not start, this is typical, allow me to digress and use an analogy, a good toasty log fire can deal with the occasional wet log, but try and start a fire full of wet logs… it just doesn’t work,we dont  take the woodburning stove back to the stove shop for a major rebuild but instead we take out all the wet logs,  get some scrunched up newspaper, kindling  and some dry logs and away we go, and that is pretty much how a fuel drain works.

I read that the petrol strips the mechanical parts in the high pressure pump and this ruins the injectors because the diesel lubricates the precision parts of the pump.

in the past 30 minutes I have won two Iphone 6’s , been emailed by a lovely single lady who lives in my town and wants to meet up, and won a foreign holiday for being the one millionth visitor to a website, some  might think I am a lucky guy, but the cynic in me thinks that maybe not everything I read online is true, and the theory about CDI/HDI pumps being ruined by petrol works might be a feasible theory but it does fall down on one minor point, which is – A DIESEL ENGINE WILL NOT RUN ON A HIGH ENOUGH % OF PETROL TO CUASE ANY MAJOR HARM, now since there is a high chance that you are a concerned  recent misfueller and assuming you have even read this far, allow me to explain.  Diesel pumps work at very high pressures, crazy high, some up to 40.000 psi, thats the same pressure as being 14 miles underwater, or having an elephant standing on your little toe, fuel pumps that operate at these pressures are very well put together bits of kit that have been precision made, they do require lubrication and the tolerances are precise, the lubrication for the high pressure pump internally is diesel fuel, diesel is a slippy slidey oily substance and provides lubrication, petrol however is not, the “theory” is that in a metal on metal situation in the fuel pump, petrol will cause the parts to grind themselves to bits. however a diesel pump will only be running if the engine is running, and the engine can only run if it is still getting diesel. if it is running at all – there will still be enough diesel in the system to run it, to also lubricate the pump, and when there is not enough to lubricate the pump there will also not be enough to turn the engine.

What damage can I do by putting petrol in diesel?

some, but not as much as you think, if you leave the petrol in your car for a long time it will degrade the fuel tank (made from plastic) and the rubber/plastic fuel hoses, it can also harm the rubber seals on the fuel tank and fuel pump, it is an old housewives trick to clean oil stains with petrol, and it works a treat, plastic – which is what your fuel tank and fuel lines are made from – are condensed oil, and petrol will harm these, when we suck out petrol from diesel tanks that have been left for a while (few days at least)  the fuel that comes out is black, the blackness is dissolved fuel tank. and although I have never seen one completely ruined by petrol, I would not recommend leaving the petrol in, hence the reason I am not a fan of the top up method. petrol and diesel are very different substances, hoses and tanks that handle petrol are made to a very different specification that diesel ones, petrol hose is 3 times more expensive than diesel hose, reason being is that it is made differently to cope with the solvent like nature of the beast.

What about my fuel injectors

Again, petrol is not good for fuel injectors, in fact it is harmfull, a diesel injector is a fast moving valve, metal on metal the opens and closes rapidly, the end of your injector looks like a mechanical pencil tip, it’s small, it works hard and it cost allot to replace, the naysayers in the forums theorize that once the fuel pump mangles itself on petrol the shards will enter the injectors and rip them apart as well. but again, a diesel engine cannot run on pure petrol, and if there is enough diesel in the system for the engine to run at all, there will also be enough diesel in the system to lubricate the injectors to prevent damage, injector damage is very very very rare, in fact I am certain that the damaged injectors I have seen from misfuelling were most likely already gone, or about to go, we are talking about 1 in 500 cars, and this includes cars that have driven hundreds of miles on the wrong fuel.


Much harder to do, (in the UK) and can have serious effects if left unfixed.

A petrol will still run on 40% diesel, not that it will run well, but it will run, however because not all of the diesel burns in the piston, gravits cuases it to slide thru the piston rings and into the oil sump, when you check your oil dispstick, the “MAX” line really is the maximum you should ever fill it to, if you over fill the oil   then a ruined engine is not uncommon at all, and this is exactly what happens when enough diesel is added to the fuel tank. not to mention that diesel, although a lubricant is not designed to replace engine oil and it does not lubricate anywhere near as much as engine oil, a petrol car full of  diesel will end up with oil  diluted with diesel to a point of uselessness, and oil so full that it can ruin the engine. the first remedy is of course a fuel drain, followed by an oil change

I have read that if I have put in less than 5 litres then I should just top it up with the right fuel and it will be fine?

Another hocus pocus piece of advice from the armchair mechanics. originally the advice from the aa was”If you’ve added more than 5 litres of petrol – Drain the tank and refill with dieselIf you’ve added less than 5 litres of petrol – Top up with diesel and run normally unless the manufacturer has advised otherwise”, On the face of it – sounds logical but a landrover discovery tank, or audi q7 tank can hold 100 litres, a fiat 500 tank holds 30 litres, so using this flawed theory you could have a 5% wrong fuel mix in a range rover, or an 18% wrong fuel mix in a fiat 500 = two very different results.

So how much wrong fuel can I get away with.

Most cars will drive with up to 40% wrong fuel directly after misfuelling, and most cars will not start from cold with more than 25% wrong fuel, I have seen some cars manage to drive on 60% wrong fuel, and in some cases even more, there’s “getting away with it” and “taking a risk”, it only needs a tiny amount of petrol to start corroding plastic & rubber components in your fuel system, and running a car with mixed fuel does risk eventually causing other damage (rare), the performance and power of a misfuelled car will be reduced, and it will use more fuel than normal, some cars blow a telltale white smoke after being misfuelled, and often the emissions have a metallic smell to them

I only put a litre of petrol in my diesel, do I need a fuel drain?

it’s up to you really, and you should make the decision based on your circumstances, if it was me and the car was worth less than a couple of K I would let it slide, if however it was still under warranty and was serviced by a main dealer, or it was a lease car that may be tested when it comes off lease – I would definitely drain it to deny the lease company the opportunity to stick me for massive repairs later on, in anycase it will always be safe to drive a diesel car with a litre or so of petrol, but as explained above the petrol can harm the fuel system, what I sometimes advise is you can leave the fuel cap off overnight to let the petrol vent off, but be sure to leave your fuel cap on the drivers seat when you do this so there is no chance of you forgetting to put it back on before driving again, (never drive a car without a secure fuel cap – diesel can spil out and the motorcycling community takes great offence to this)

Does my car have an anti syphon device in the fuel tank?

No, anti syphon devices may have been fitted to cars many years ago but there are none fitted with it as standard that I know of, some truck fuel tanks that carry a thousand pounds worth of diesel do have these fitted and it is simply a thick mesh welded around the filler neck inside the tank.

Below is a picture of a fuel filler neck from a mitsubishi, this is a fairly standard neck, the thick pipe is the fillerpipe, the thin pipe is the breather pipe which allows aire to escape while fuel pours in.anti syphon device

The mistake many people make when attempting to drain the fuel from a car is to try and insert a garden or similar hose into the fuel tank via the neck to suck out the fuel, it doesn’t happen, and I seriously respect anyone who has ever done this on a modern car with any degree of success, the problem is a hose will kink and tangle on the bends and twists, and on the rare occasion you do get it into the fuel – the hose will most likely be pinched so tight that a syphon is not possible – thus people thinking there is an anti syphon system; what we use are these 

Fuel syphon tool


Flexible braided steel hose with a specially designed tip to go round corners, and a connection fitting for our suction pumps.

These are VERY easy to break and cost quite a bit of money, once they have a kink in them they become useless as they will start to pull air thru the kink instead of fuel from the tank. using one of these is quite a knack and takes some practice, a good fuel man will make it look easy but dont assume it was always that way.

We buy these from a specialist manufacturer.

What do we do with the fuel once we have recovered it.

The fuel is stored in specially made fuel tanks that have been designed to hold petrol (petrol being more volatile than diesel is what misfuel should be treated as), once we have a full tank which is every 10 days or so, a tanker comes and collects it, once collected it is bulked up alongside other companies stock and eventually it is run thru a fuel refinery again for a second time and made back into petrol and diesel, the market fluctuates with the price of fuel and we are paid a small amount for each litre by the company that collects it, the price we get for it just about covers the cost of having premises suitable for the purpose – which have to be environment agency registered and secure. there are a number of other uses for mixed fuel such as, engine parts washing, and using in space heaters (very dangerous and not for the faint hearted), I also know a man who has converted a central heating system to run on mixed fuel which works a treat.

I made some biodiesel and it doesn’t run very well, how do i remove it?

I am a big biodiesel fan and read “from the fryer to the fuel tank” back when it first came out, although I have never made my own biodiesel I am familier enough with it. poorly made biodiesel can be too thick for the car’s fuel pump, I have seen  cases now of home brew biodiesel gone wrong, in nearly all of them the maker had simply not followed the instructions carefully enough, I think everyone who wants to make biofuel who is in a position to do so should give it a go, I’m not an authority on the subject by a long way but I can tell you that if your biodiesel has any whiteness or creaminess, do not use it, properly made biodiesel looks like diesel just slightly darker, and it is not thick and overly goopy. if your a beginner with biodiesel I suggest you rope in someone experienced to check over your first few batches before you stick it in your car, the worst fuel drain we ever had was on a land cruiser full of a beginners batch of biodiesel,  it was so thick we could not even pump it out and resorted to taking it out of the tank with a small plastic cup using the sender unit hole, it took ages, and we had to flush out the tank 3 times with mixed fuel to break down all the solids. if you have made a ropey batch do not compound the problem by putting it in your car.

I have drained the fuel from my car and have 50 litres in my garden shed, what should I do?

Every car garage has a waste oil collection tank (or drum) and mixed fuel can be put in it, or your local council can point you in the right direction, If you are going to use your mixed fuel as an accelerant for a bonfire please be careful!, it will ignite, “woosh” and “wump” just like petrol, remember 1 litre of petrol can make 15.000 litres of explosive gas and as any veteren of bonfire night shenanigans will tell you – the result can be as terrifying and as deadly … it is funny.

What about putting mixed fuel in my lawnmower/strimmer/cement mixer?

Don’t bother, us fuel drainers also had that bright idea many years ago, and now we all buy our lawnmower fuel at the pumps, endlessly repairing, ungunging and changing sparkplugs has long since disabused us of that that idea.

 I misfuelld my diesel car with petrol, someone suggested I add engine oil to the mix to protect the fuel pump and increase the viscosity of the petrol to make it like diesel.

As my primary school teacher often reminded me after a battle in the lunch line “two wrongs don’t make a right” and filling a petrol ridden diesel car with engine oil is a double wrong. getting slightly technical here, petrol is 730 grams a litre, diesel is 840 grams a litre, and engine oil is 900 grames a litre, mixed fuel (and petrol and diesel mix like milk in coffee) has a gravity of about halfway between 730 and 840, for an accurate weight the exact mix would need to be worked out, but for arguments sake lets say its 50% petrol and 50% diesel, giving the mixed fuel a gravity of 785 grams a litre. add into this engine oil which is a thick sludge, and much heavier than mixed fuel. the result is you have coated the bottom of your tank in engine oil, which will eventually dilute but not in time to save your engine.

The theory behind adding engine oil to mixed fuel is that the whole mix will blend and right itself, and the oil would provide much needed lubrication to the pump and injectors, whats much more likely is when the oil does eventually work its way thru the system it will leave a thick film of soot and gunge on delicate sensors in your exhaust system, requiring replacement or at least cleaning.

I have read forums suggesting adding engine oil after a misfuel  and some people think this is a solution but I have drained out much more cars that have been topped up with engine oil (to no avail), than I have heard of it working as an effective soloution.






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