Petrol in Diesel FAQs

Here is a roundup of the most frequently asked questions regarding petrol in a diesel car.

If you have put petrol in your diesel car and cannot find the question or answer that you are looking for then please email us and ask.

What is a “misfuel”?

A misfuel is where petrol has been put in a diesel vehicle, or diesel into a petrol vehicle. The term also loosely covers any sort of fuel contamination, whether that be water, sub-standard fuel or any other contaminants in the vehicle’s fuel supply.

What is a “Fuel Drain”?

Simply put, a fuel drain is the process of removing contaminated or unwanted fuel from a vehicle and disposing of it. For a vehicle that has not been run on the contaminated fuel this could be as simple as draining the contaminated fuel directly out of the vehicle’s fuel tank via the filler neck. Complexities are added into the situation with regard to vehicle anti-syphon systems, fuel tank inaccessibility via the filler neck, more than one fuel tank, how to safely drain and handle the fuel, etc. The list is as long as there are different types of vehicles.
Additional complexities are added by having the vehicle run on the contaminated fuel. In these cases the entire fuel system (fuel tank, fuel lines, filter, pumps and injectors) will need to be flushed out to purge the contaminated fuel from them and replace it with clean fuel. With run petrol vehicles it is also often necessary to clean the spark plugs to remove the diesel fuel oil that builds up on them. (Our technicians are trained in these repairs)

How long does a fuel drain take?

We aim to have the customer on his way again within an hour of arriving on scene.
Some unrun vehicles can be fully drained and ready to go in a matter of 20 minutes – others drain more slowly and could take almost an hour to empty out. Run vehicles always take longer than unrun ones due to the extra process of flushing the fuel system.
Despite all the above we aim to have the customer on his way again within an hour of arriving on scene.

Does the vehicle need to be recovered?

The only reason that the vehicle would need to be recovered is if it is parked in a location that would expose the engineer or other traffic to harm or injury while they are working on it. An example of this is at a busy junction or on a busy motorway. The best course of action here is to consult with the engineer beforehand. We will be able to advise if you should call your breakdown company for recovery to a safer location, or maybe we would be able to tow the car to a safer location.

Can I drive the vehicle after the fuel drain or does it need to go to a garage?

The purpose of the fuel drain is to remove contaminated fuel so that the vehicle can be used immediately.
There should be no need for anything further to be done with the vehicle once the fuel drain is completed. Obviously, contaminated fuel has the possibility of damaging sensitive engine parts, which is why it needs to be drained in the first place. There is always a risk that some part of the engine has become damaged as a result of the contaminated fuel, and in these cases it would need to be taken to a service centre for repair.

NOTE: Of the more than 1,000 misfuels attended by NW Fuel Drain in 2018, 2 had sustained damage requiring them to be taken to a mechanic for repairs.

If something has been damaged because of the misfueling, can the engineer fix it?

Our engineers are all fully trained mechanics. If it something that can be repaired on scene then they will endeavour to rectify the fault. We are however limited on repairs as we only carry a select amount of tools and equipment.

Have I wrecked my engine?

It’s unlikely that you’ve wrecked anything. If you have put the wrong fuel in you shouldn’t drive it, and you should get it drained as quickly as you can. Driving with the wrong fuel can damage some engine parts, and the longer it’s driven the worse the damage could be. Of course you may not even be able to start the engine if there is a lot of wrong fuel in – but we don’t advise that you try.

How much wrong fuel can I get away with, if I fill up the tank with the right fuel?

Motoring organisations and vehicle dealerships advise that any contaminated fuel can cause problems, and should be drained rather than being topped up. They have a “zero-tolerance” policy. Diesel has lubricating qualities for parts such as fuel pumps, whereas petrol removes oils which these parts require. Diesel does not burn off in a petrol car and will work its way into the vehicle’s oil supply, thereby diluting it and reducing its effectiveness, which could lead to complete engine seizure and failure. Engine management systems, fuel pipes, injectors, cylinders and catalytic converters can also be damaged by wrong fuel. Taking this advice into consideration the answer would be: How much “wrong” fuel can I get away with – None. The risk to the vehicle generally outweighs the cost involved in correcting it.

I’ve put the wrong fuel in a hire car: what should I do?

Rental agreements usually require that you inform the hire company in the event of any mechanical problem with the car. They will usually then arrange for a fuel drain through their own contracts with a fuel drain company. Unfortunately there are many stories around of motorists being hit with large bills for the work, well in excess of the fees of independent fuel drain companies. Since the hire company has your credit card and your authorisation to charge it, there is little you can do about such excessive fees after the fact.
We can and do attend and fix hire cars directly, and treat them exactly the same as any other vehicle, and for the usual (not inflated) fee.

Is a fuel drain covered on my insurance?

Some insurers will cover the cost of a fuel drain, others will not. Some motorists however elect to pay for a fuel drain directly rather than face increased premiums even if the drain could be claimed for. Additionally, a fuel drain from an independent company such as NW Fuel Drain is likely to cost less than the insurance excess and therefore you would probably have to pay for it anyway. We advise motorists to contact their insurer directly if unsure. In any case, we provide detailed VAT invoices for all work carried out.

My car broke down after misfueling. Do I need to have it towed to a garage?

If misfueling caused the breakdown it will usually be fine after draining it and restarting with the correct fuel. Damage to engine parts is a risk if the vehicle is driven with contaminated fuel, but in our experience it is rare provided the wrong fuel is drained before too long.

I’ve been told my fuel tank can’t be properly drained because of the design of the car. Can you still help?

Modern car designs certainly make it harder to drain the fuel tank, and it often requires specialist equipment and techniques. However, our technicians are equipped to deal with all vehicles.

What’s worse, diesel in petrol, or petrol in diesel?

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 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 amount of driving with a heavy foot, this would require a new fuel pump, injectors and fuel lines. The high pressure fuel pump which operates at around 28thousand PSI of pressure and is a highly engineered unit relies on diesel going through it to provide lubrication, petrol does not lubricate, in fact it has “anti-lubricating” properties, and 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 and this destroys the pump. Then the metal chafe gets drawn through to the injectors and can ruin them as well.
2) Diesel in petrol can let the diesel enter the cylinder and not burn. It will slide past the piston rings, into the oil sump and therefore into the car’s oil system. This will increase the oil level to a point that could cause total engine failure, i.e. damaged rods, bent pistons, complete block failure, or the thinning of the oil can reduce lubrication to the engine and it could suffer from a full seizure/big end failure/ruined prop shaft. Additionally, the unburnt diesel can pass through to the exhaust system, coating everything in a film of diesel oil and causing damage to the catalysts and sensors. 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 to achieve the above.

But the main dealer says I must bring it in otherwise the engine and pump will be ruined.

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 in 2010, and the disruption to flights. It went like this: there was a small amount of ash detected in the air space. As a precaution 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 response 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 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 if they are wrong on just one car once… it 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.

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?

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 older vehicle 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.
NOTE: It is especially important to drain a vehicle that is still under warranty. The main reason for this is that the next time you take it in for an annual service, if the dealership finds that there is any contaminated fuel in the system, even traces of it, they will immediately know that it has been misfueled and will tell you that to keep the warranty valid a full drain will need to be done, with the possibility of needing to replace parts or all of the fuel system. Obviously, as the misfueling was the customer’s own fault, you will most likely have to foot the bill for this. Prices here range from £500 to £6,000.

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

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, mixed fuel 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 I’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 surprised 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 no problem.

Does the petrol separate and float on top of diesel or does the diesel float on top of petrol?

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 separates them, and once recombined they will mix together and stay together until refined.

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

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. 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.

My engine management light has come on – what does that mean?

Engine management lights come on for a number of different reasons, on Volkswagens 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, exhaust temperature, 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. If once the vehicle is running correctly the eml stays illuminated our technicians have onboard diagnostics to effectively clear any error codes caused by the wrong fuel fill up.

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

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 lengthy 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 is 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.

If you’re still questioning what you’ve read then by all means give us a call 01513742421 or send us an email