The Dunks: Petrol in a Diesel Car? How Worried to Be
Not started? Then less worried than you likely are. "Have I wrecked it?" is the first question anyone around The Dunks asks, and the honest answer is generally no, since petrol resting in a diesel tank hurts nothing till it's drawn through the system — by ignition priming, by your starter, or by being driven. Keep it like that: leave your engine off and ring Petrol in Diesel. The rest of this page explains what petrol really does to a diesel, how to tell how far it has got, and how to make sure it doesn't happen to you again.
Inside the motor: what petrol in a diesel does
Diesel has two tasks on its way through an engine. It ignites, and before it burns it protects. Very fine gaps go into the injectors and the high-pressure pump, and just your diesel flowing through stops those surfaces grinding together.
Petrol does the first job badly and the second not at all. The knocking comes from petrol being made not to ignite when compressed, which is precisely the opposite of what a diesel needs. On top of that it is thinner than diesel, it strips out the protection your system depends on, and any pump running on such a blend is wearing itself.
Taken together, they make clear the shape the trouble has. The harm is done by circulation, not by contact. All a tank with petrol in it amounts to is a fuel problem. Once your pump has been labouring on that petrol, a mechanical problem has begun: anything shed by a pump that is grinding is carried onward to the fuel injectors with the fuel.
How serious is petrol in a diesel? Four scenarios our engineers see around The Dunks
Filled but never started — as good as it gets. Just your tank has petrol in it, and so the task is draining the tank and cleaning your system where your car stands. This scenario can only really occur on a forecourt: anywhere else, the car got there by being driven.
Ignition on, or started for a moment. The fuel system has been bled, and so petrol has left that tank and reached your filter and the lines. That's generally still an easy job — a drain, a flush, and the filter dealt with.
A few miles driven, and your pump has spent every one of them labouring on diluted fuel. The work is that tank, your lines, the fuel filter and fresh fuel through the whole system, and how hard the car was driven counts as much as how far. On the school run to Farndon Primary School, the miles is short. A run as far as Wrexham, under a mile away, is a very different task. So is any miles on the M53, under 15 miles from The Dunks, where the engine has been labouring hard throughout.
It protested, and only then stopped: smoke, knocking, a throttle gone soft, an idle gone uneven. The engine is telling you your fuel is no longer acting like diesel. Stop where you are — on the verge on Saint David's Crescent, at the kerb, down a side road — and do not move it again. Permanent damage is very unusual if a vehicle is pulled over soon, and the drain and flush that still come first sort it out.
The Dunks people and that petrol in a diesel stories they hear
It will not be fine. The myth: "Fill it the rest of the way with diesel and it will be." A bigger tank of the same mixture is all you get, because topping up takes out nothing. That advice assumes a diesel fuel system tolerates petrol by percentage. It does not.
Not so: a newer diesel copes with a bit of petrol. Rather, the reverse. At higher pressure, through finer components: that's how a modern diesel moves its fuel, and so the lubrication counts more, not less.
"It starts, it drives, and so there is no harm" — the myth that trips up most motorists than any other. Feed diesel to a petrol motor and it usually will not run, and so the motorist stops — and the harm stops with them. Nothing seems to have happened when a diesel fed petrol fires up, ticks over and drives away, as it usually does; meanwhile your pump grinds. A running engine does not mean fine.
Not so: a two-stroke additive lets you to drive home. It was an old workaround, meant for another fault. That petrol is still inside the tank afterwards, and whatever it has weakened is not put back.
The next start sends petrol back round your pump from the lines and the fuel filter, where some of it sits once your engine has been run. That's why a DIY tank drain doesn't leave you sorted — a myth. Clearing petrol into a can at the kerbside is also genuinely risky.
How does petrol in a diesel occur?
No one does it on purpose, and it's seldom carelessness. A diesel nozzle generally won't go into a petrol car, yet the smaller petrol nozzle goes into the filler of a diesel nearly without resistance — so forecourts make this one mistake easy, and just in one direction.
The moment routine stops matching your car is the moment it happens. Courtesy car, hire car, or a motor that's brand new. Two cars on one drive, one petrol, one diesel. The pumps in a different order on a well-known forecourt. Fuelling up in a rush en route to Wrexham Central, with a train to catch. Precisely the moment it takes place: after years of driving petrol, someone fills a diesel Volkswagen Golf for the first time. Throw in rain, a phone call or a back seat full of kids, and the hand does what it has always done.
Next time you fill up in The Dunks: preventing petrol in a diesel
- Green for unleaded and black for diesel is usual in the UK — but not on each forecourt. Go by the label on the petrol pump, not the colour.
- In a motor you do not know, look inside the flap over your filler or at the cap before your first fill: plenty of motors state the fuel there.
- Good intentions do less than a sticker or a fob in a bright colour on the diesel's key when a household has one of each. So mark it.
- It sounds silly and it works: on the first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the petrol pump.
- Put the phone away. This is an easy mistake to make while you're thinking about something else.
- A misfuel inhibitor helps, up to a point. Some diesels have a filler designed to refuse a petrol nozzle, and add-on devices exist for cars without one. They are a safety net, not a reason to stop checking the pump.
- Stop as soon as you realise. Less petrol in your tank makes a weaker mix. It does not change what to do next, which is still not to crank it.
Petrol already in a diesel around The Dunks? Begin here
The order of things is short, then. Leave it unstarted, with the ignition off as well. Ask the staff to help you roll it away from the petrol pump if you're still at the forecourt, whether that's Tesco or somewhere else. Ring 0333 366 0103 next, with three facts — the fuel, the rough volume, and if it's run.
Say so if it was AdBlue rather than petrol: it is a separate problem, with its own page for The Dunks, since AdBlue brings corrosion and deposits instead of lost lubrication.
The Dunks petrol in a diesel jobs: why one costs more than another
The four situations above are four separate jobs, so no single price for this would be fair. What shifts your cost is chiefly which of them you're in. Two things follow that: the petrol that got in as a proportion of what the tank holds, then the spot the vehicle is in and whether a technician can reach it safely.
Three queries come first, before quoting, instead of after — what was put in, how much, has it run — and that is the reason. The replies turn a range into a straight figure.
About The Dunks, for a moment
The Dunks is a locality situated on the riverside in Wrexham, with an elevation of around 75 metres. The Dunks lies close to the city centre of Wrexham, approximately 0.7 miles away, and is near other local places such as Rhosnesni and Maes-y-Dre. The Dunks has a population of around 2934 people, predominantly living in semi-detached housing. The Dunks is located in a region with easy access to major cities, being around 160.8 miles from London.
The Dunks and Wrexham: where our fuel drain cover runs
Rhosnesni, Maes-y-Dre and Hightown are among the locations around The Dunks that the same vans work, answering petrol in diesel jobs across The Dunks's LL postcode area. No need to move the vehicle to us. Forecourts, supermarket car parks, lay-bys and your own driveway are all locations where we work.
At the petrol pump in The Dunks, still?
Nothing beats it: that's where you now are. Keep it off. Call.