The Folly: Petrol in a Diesel Car? How Worried to Be
If it hasn't been started, you can worry less than you likely are. Everyone in The Folly begins with "have I wrecked it?", and the honest answer is usually no: petrol resting in your tank of any diesel remains harmless till priming, cranking or driving draws it into your fuel system. Petrol in Diesel is who to call, with the engine left off, and so it stays that way. The rest of this page explains what petrol really does to a diesel, how to judge how far it has got, and how to make sure it does not happen to you a second time.
Inside your engine: what petrol in a diesel does
Diesel does two tasks on its way through an engine. It lubricates to begin with, and only then does it burn. Made to very fine tolerances, your injectors and the pump rely on your diesel flowing through them to stop their surfaces wearing against one another.
Of these two jobs, petrol handles the first poorly and the second never. It is thinner than diesel and removes the lubrication your fuel system rests on, so a pump running on a petrol blend is a pump wearing itself. And petrol is made to resist igniting under compression, which is the opposite of what a diesel needs from its fuel. That's where the clattering comes from.
Together, those make the shape of the issue plain. Contact does not do the harm — movement does. Petrol in a tank is a fuel issue. Whatever a worn pump sheds travels on, carried by the fuel, into the fuel injectors downstream, which is why a pump that has been working on the petrol is where a mechanical fault starts.
The Folly, petrol in a diesel: four situations, from mild to significant
Filled up, never started. The best case there is. The petrol is in the tank and only that tank, and the task is a drain and a flush where the car stands. This is the one scenario that can only really take place on a forecourt, since anywhere else the car has been driven to get there.
A moment's running, or just your ignition on. Petrol has left the tank and got to the filter and your lines, because your system has been primed. Still normally simple: drain, flush, and your filter sorted.
Some miles driven, and the pump has spent each of them labouring on weakened fuel. How hard the vehicle was driven counts as much as how far, and the work takes in your tank, your lines, the fuel filter and fresh fuel through the entire circuit. Driving the children to St John's Church of England Infant and Nursery School is one sort of trip. Getting as far as St Albans, under 5 miles away, makes a very different job — as does any distance on the M1, under 3 miles from The Folly, with the engine working hard all the way.
Driven on till it complained — clattering, smoke, a ragged idle, a soft throttle. The engine is telling you the fuel is no longer acting like diesel. Whether it is the verge on Dellfield Close, a lay-by or a side road, stop there then leave it. The fuel drain and flush still come first, and they deal with it: when a vehicle is stopped soon, permanent damage is very uncommon.
Myths The Folly drivers hear about petrol in a diesel
"Top it up the rest of the way with diesel and it'll be fine" — a myth. It won't. Topping up takes out nothing; it gives you a bigger tank of the same mixture. A diesel fuel system doesn't accept petrol by percentage in the way that advice supposes.
"Current diesels can handle a little petrol" — a myth, and closer to the opposite. Smaller parts and higher pressure are how a newer diesel runs its fuel, and that makes the lubrication count more, not less.
Myth: "If it starts and drives, there is no damage done." This is the one that catches people. Feed diesel to a petrol engine and it normally won't run, so the driver stops — and the damage stops with them. While your pump wears, a diesel fed petrol usually starts, idles and drives away as if nothing has happened. A running engine doesn't mean fine.
Myth: "Add some two-stroke oil and drive the vehicle home." It's an old workaround for a different fault. It doesn't put back what petrol has diluted, and the petrol is still inside your tank afterwards.
"Drain it yourself and you are sorted" — a myth, since after it has been running some petrol sits in your filter and lines, ready to go right back through your pump on your next start. Also, a can of petrol filled by the kerbside is truly risky.
Why petrol in a diesel happens so easily
It's seldom carelessness, and nobody does it on purpose. The forecourt makes it easy: the petrol nozzle is the slimmer of the two, and it slips into a diesel filler with almost no resistance, while a diesel nozzle generally won't go into a petrol vehicle at all. So the slip runs one way.
It happens when routine and the vehicle stop matching. Hire cars, courtesy cars, and cars that are new to you. One petrol and one diesel on the drive of the same household. A forecourt you know, only with its pumps arranged differently. A hurried fill en route to Radlett with a train to catch. The first fill-up in a diesel Volkswagen Golf after years of driving petrol is precisely the moment it happens. Your hand does what it has always done once you add rain, a phone call or a back seat full of kids.
How to prevent petrol in a diesel at your next fill-up in The Folly
- Check the petrol pump, not the colour. In the UK unleaded handles are normally green and diesel black, but not on every forecourt. The label is what counts.
- Check your filler flap. Plenty of cars state the fuel inside the flap or on the cap. Look at it on your first fill in any motor you do not know.
- A coloured fob or a sticker on the diesel's key does more than any amount of intending to remember, for a home with one of each.
- Before the nozzle comes off the petrol pump the first time you fuel a hired or new car, say "diesel" aloud. Daft, yes, yet it does the trick.
- Thinking about something else is when this becomes an easy mistake to make, so put your phone away then keep your mind on the pump.
- Add-on devices are available for vehicles without one, and some diesels come with a filler that won't take a petrol nozzle. Helpful — but a safety net at best, never a reason to stop checking the label.
- Noticing it mid-fill? Stop at once. A thinner mix follows from less petrol inside the tank, yet your next stage stays the same: do not start the car.
Already put petrol in a diesel around The Folly?
In that case, there's only a short order of things. Don't turn your ignition on, and don't crank it. Still at the forecourt, at Costco or anywhere else? Ask the staff to help you roll the vehicle clear of the pump. Then ring 0333 366 0103 and tell us what was put in, roughly how much, and if it has run.
AdBlue, not petrol? Then tell us. Corrosion and deposits, not lost protection, make it a different problem — The Folly has a page just for it.
The Folly petrol in a diesel jobs: why one costs more than another
There is no fair single price for this, since the four scenarios above are four separate jobs. Mostly, the cost shifts with which situation you are in. Two things follow that: the petrol that got in as a share of what that tank takes, then the place the vehicle is in and whether our van can reach it safely.
That is why our engineers ask three queries before pricing rather than after: what went in, how much, and whether it has run. Those replies are what turn a range into one firm number.
About The Folly and its setting
The Folly is a suburban area in Hertfordshire, situated approximately 14.9 miles from London and 4.8 miles from St Albans. The Folly sits in a relatively low-lying area at an elevation of 104 metres, surrounded by other nearby places such as Radlett and Battlers Green. The Folly is characterised by its predominantly detached housing stock, reflecting the area's residential nature.
Fuel drain cover across Hertfordshire, The Folly included
Radlett, Kemprow and Battlers Green are among the locations around The Folly that the same teams work, answering petrol in diesel calls across The Folly's WD postcode area. The car doesn't need to move to us: our technicians drain at a forecourt, in a supermarket car park, in a lay-by or on your own drive.
The Folly: still at the petrol pump?
In that case, you couldn't be in a better spot. Phone, and leave it switched off.