St Andrews: Petrol in a Diesel Car? How Worried to Be
Likely less than you already are — as long as nobody has started the car. "Have I wrecked it?" is the first question anyone around St Andrews asks, and the truthful answer is usually no, because petrol left in a diesel tank harms nothing till it's drawn through the system — by ignition priming, by the starter, or by driving. Leave the motor off then ring Petrol in Diesel — that keeps it like that. Below, you will find what petrol actually does inside a diesel, how far it may have got, and how to stop it happening to you again.
What petrol in a diesel does to the engine
On its way through a motor, diesel does two jobs. It protects to begin with, and after that does it burn. Designed to very fine clearances, the fuel injectors and your pump depend on your diesel moving through them to keep their surfaces from wearing against one another.
Of these two tasks, petrol handles the first poorly and the second never. 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 runnier than diesel, it strips out the protection the system hinges on, and any pump running on such a blend is wearing itself.
Together, those leave the shape of the trouble plain. The damage is done by circulation, not by contact. All a tank with petrol in it amounts to is a fuel issue. The start of a mechanical one is a pump that has been running on that petrol — and whatever a worn pump sheds travels on with the fuel to the injectors beyond it.
Petrol in a diesel in St Andrews: how serious is it? Four situations
Fuelled, and never started: nothing beats it. That petrol is inside the tank and just your tank, and the job is a drain and a flush where the car sits. This is the one scenario that can only really occur at a forecourt, as anywhere else the car has been driven to get there.
Started for a second, or only the ignition on. With the fuel system primed, the lines and filter now contain petrol that came from that tank. Drain, flush, see to the fuel filter: still usually straightforward.
Your pump now worked a couple of miles on weakened fuel — that is what driving the car does. Tank, lines, filter, then fresh fuel put through the entire system: that is the work, and the way your car was driven counts as much as the miles. Short trips, such as the school run to the local primary school, are one matter. A run as far as Dundee, under 15 miles away, is a very different task. So is any mileage on the M90, under 25 miles from St Andrews, where your engine has been working hard throughout.
Driven until it complained. Knocking, a lumpy idle, smoke, a throttle that has gone soft. The fuel is not behaving like diesel any more, and your engine is telling you. Whether it's the verge on South Street, a lay-by or a side road, stop there then leave it. The drain and flush still go first, and they sort it out: when a car is stopped quickly, permanent damage is very rare.
Myths St Andrews drivers hear about petrol in a diesel
Myth: topping up to full with diesel makes it fine. It does not. Topping up takes out nothing; it gives you a fuller tank of the same mixture. The idea behind that guidance — that diesel fuel systems accept petrol by percentage — is wrong.
If anything, the opposite is true. Myth: "A modern diesel can cope with a bit of petrol." Lubrication counts more on modern diesels, not less, since their fuel passes through smaller components at higher pressure.
The myth that catches people off guard: if it starts and drives, it must be unharmed. Diesel in a petrol motor normally leaves a motor that won't run: the driver stops, and so does the damage. Nothing looks to have happened when a diesel fed petrol fires up, ticks over and drives away, as it usually will; meanwhile the pump wears. Running is not the same as fine.
"A drop of two-stroke oil and drive it home" — a myth, taken from a fix from years ago, for a separate problem. What petrol has thinned remains thinned, and that petrol remains inside the tank regardless.
The next start draws petrol back round your pump out of the lines and the filter, where some of it sits after the engine has run. That's why draining your tank yourself doesn't sort it — a myth. And, a can of petrol drained at the roadside is truly dangerous.
Petrol in a diesel: how does it happen?
On purpose? Never. Through carelessness? Seldom, if ever. The forecourt makes it simple: the petrol nozzle is the narrower 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 occurs when habit and the car stop matching. Any motor that's new to you — a hire car, on loan as a courtesy car, or brand new. Two cars on one drive, one petrol, one diesel. Your ordinary forecourt, its pumps suddenly in a different order. A hurried fill on the way to Leuchars with a train to catch. Precisely the moment it occurs: after years of driving petrol, someone fills a diesel Volkswagen Golf for the first time. Throw in a back seat full of kids, a phone call or rain, and the hand takes over.
Preventing petrol in a diesel next time you fill up around St Andrews
- A green handle for unleaded and black for diesel is normal in the UK — but not on every forecourt. Go by the label on the pump, not the colour.
- 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 a motor you do not know.
- One of each in the house? Label your key to the diesel — a coloured fob, a sticker — because meaning to remember is not enough.
- It seems silly and it does the trick: on the first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the petrol pump.
- Thinking about something else is when this becomes an easy mistake to make, so put your phone away and keep your mind on the petrol pump.
- Add-on devices exist for motors without one, and some diesels come with a filler that won't take a petrol nozzle. Helpful — but a safety net only, never a reason to stop checking the label.
- Noticing it mid-fill? Stop there and then. A weaker mixture results from less petrol inside that tank, yet the next step remains the same: don't crank it.
Already put petrol in a diesel around St Andrews?
Short and simple, then, is the order of things. Don't switch the ignition on, and do not start it. If you're still at the forecourt — Shell or anywhere else — ask the attendants to help you roll the vehicle clear of the petrol pump. What went in, about how much, and if it has run: tell us those when you call 0333 366 0103.
If it was AdBlue instead of petrol, say so. That is a different issue — corrosion and deposits rather than lost lubrication — and it has its own page for St Andrews.
What makes one St Andrews petrol in a diesel task cost more than the next
There is no fair single quote for this, because the four scenarios above are four separate tasks. What moves your cost is chiefly which of them you are in. Two things follow that: the petrol that got in as a proportion of what that tank takes, then the spot the vehicle is in and whether our van can reach it safely.
What was put in, how much, and if it has run — three questions our technicians ask before quoting, not after, for exactly that reason. With those replies, you get a straight figure instead of a range.
About St Andrews, briefly
St Andrews is a coastal town situated in the county of Fife, Scotland. It is located near the eastern shore, roughly midway between Dundee and Edinburgh. The town is known for its historic significance and picturesque setting, making it a notable destination. St Andrews boasts a range of distinctive features that set it apart from other towns in the region.
Across Fife and in St Andrews: fuel drain cover
Mount Melville, Prior Muir and Kincaple are among the spots near St Andrews that the same teams work, answering petrol in diesel call-outs across St Andrews's KY postcode area. Our technicians drain where your vehicle is — your own driveway, a lay-by, a supermarket car park, a forecourt — so it never needs to come to us.
Still at the pump around St Andrews?
Then, you could not be in a better place. Keep the car switched off, then ring.