How Worried Should You Be? Petrol in a Diesel Car in Pen-y-fan
If it hasn't been started, worry less than you likely are. "Have I ruined it?" is the first thing anyone around Pen-y-fan asks, and the truthful answer is normally no, because petrol resting in a diesel tank harms nothing till it's drawn through the fuel system — by ignition priming, by your starter, or by being driven. Keep it like that: leave your engine off then call Petrol in Diesel. The rest of this page covers 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.
Your engine and petrol in a diesel: what happens
Two jobs fall to diesel on its route round a motor. It ignites, and before it ignites it protects. What keeps the surfaces inside the high-pressure pump and your injectors from grinding against each other is the diesel flowing through them — they're designed to very fine clearances.
Of these two jobs, petrol manages the first badly and the second not at all. A pump working on a petrol mix is wearing itself out, because petrol is thinner than diesel and strips out the lubrication the fuel system relies on. Petrol is also made to resist igniting under compression — the opposite of what a diesel needs from its fuel — and that is where any clattering starts.
Put those together and the shape of the trouble is clear. It's circulation that does the harm, not contact. All a tank with petrol in it amounts to is a fuel fault. The beginning of a mechanical problem is a pump that has been labouring on that petrol — and whatever a worn pump sheds moves on with the fuel to the injectors beyond it.
Four scenarios our engineers see in Pen-y-fan: how bad is petrol in a diesel?
Filled but never started — the best it gets. Being on a forecourt is what makes this the case: elsewhere, the car has been driven there to begin with. With that petrol held in that tank alone, it is emptied and flushed right where it stands, and nothing more is needed.
Started for a moment, or just your ignition on. The fuel system has been bled, and so petrol has left the tank and reached the fuel filter and the lines. Still usually simple: drain, flush, and your filter dealt with.
Driven a couple of miles. Now the pump has spent those miles running on thinned fuel. Mileage is only half of that; how hard it was driven counts equally. Either way the work runs to that tank, the fuel lines, the fuel filter and fresh fuel through the entire system. Short trips, such as the school run to the local primary school, are one matter. A run as far as Swansea, under 10 miles away, is a very different job. So is any mileage on the M4, under 5 miles from Pen-y-fan, where your engine has been labouring hard the whole time.
It protested, and only then stopped: smoke, rattling, a throttle gone soft, an idle gone uneven. That is the motor telling you its fuel no longer is acting like diesel. Stop where you are — on the verge on Llwynwhylwyg Terrace, at the kerb, down a side road — and do not move it. The drain and flush still go first, and they sort it out: if a vehicle is pulled over soon, permanent damage is very uncommon.
Pen-y-fan people and the 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." Nothing is removed by topping up — you are left with a bigger tank, still the same mixture. The assumption behind that guidance — that diesel fuel systems tolerate petrol by percentage — is wrong.
Myth: "A current diesel can cope with a bit of petrol." If anything, the opposite. More precise components and higher pressure are the way a newer diesel runs its fuel, and that makes the lubrication count more, not less.
This is the one that fools drivers. Myth: "If it starts and drives, there is no damage done." Feed diesel to a petrol motor and it normally won't run, so the driver stops — and the harm stops with them. Nothing looks to have happened when a diesel fed petrol fires up, ticks over and drives away, as it usually does; meanwhile your pump grinds. Fine and running aren't the same thing.
Myth: "Add some two-stroke oil and drive the car home." It's an outdated fix for a distinct fault. What petrol has weakened stays weakened, and the petrol stays in the tank regardless.
"Drain it yourself and you are sorted" — a myth, because after it's been running some petrol lies in your filter and lines, ready to go right back through the pump at the next start. It is also really risky to drain petrol into a can at the roadside.
How does petrol in a diesel take place?
No one does it on purpose, and it is rarely carelessness. A diesel nozzle generally will not go into a petrol car, yet the narrower petrol nozzle goes into the filler of a diesel almost without resistance — so forecourts make this one mistake easy, and just in that direction.
When routine and the car stop matching, it occurs. A motor that is new to you — a hire car, on loan as a courtesy car, or just bought. One petrol and one diesel on the drive of the same household. A well-known forecourt with the pumps in a different order. Fuelling up in a rush en route to Llanelli, with a train to catch. Exactly the moment it occurs: after years of driving petrol, a driver fills a diesel Volkswagen Passat for the first time. Throw in a back seat full of kids, a phone call or rain, and habit wins.
Avoiding petrol in a diesel next time you fill up in Pen-y-fan
- Not every forecourt in the UK follows green for unleaded and black for diesel, even if most usually do. Read the petrol pump's label; the colour is only a clue.
- The first fill in a new vehicle is the time to check your filler flap — on it, or on the cap, many cars state which fuel to use.
- A coloured fob or a sticker on the diesel's key achieves more than any amount of meaning to remember, in a house with one of each.
- "Diesel", aloud, before you pick up the nozzle — each time a new or hired car needs its first tank. It seems daft. It works.
- Thinking about something else is when this becomes an easy mistake to make, so tuck the phone away then keep your mind on the petrol pump.
- Add-on devices are available for motors without one, and some diesels are built with a filler that will not take a petrol nozzle. Useful — but a safety net at best, never a reason to stop checking the label.
- Less petrol makes a thinner mix, so stop as soon as you realise — while remembering that the next step is unchanged. It is still not to fire it up.
Pen-y-fan: already put petrol in a diesel?
Then, there is just a short order of things. Starting it is out, and so is switching on the ignition. Yet at the forecourt, at Tesco or anywhere else? Ask the attendants to help you roll the vehicle clear of the petrol pump. Next, 0333 366 0103. We will ask what went in, about how much, and if it's been run.
If it was AdBlue and not petrol, say so. That is a different problem — corrosion and deposits and not lost protection — and it has its own page for Pen-y-fan.
Pen-y-fan petrol in a diesel jobs: why one costs more than another
Four separate jobs sit behind the four scenarios above — which is why there is no honest single quote. Which of them you are in is mostly what shifts your cost. The ratio counts next, meaning how much of your tank is petrol, and then the location of the vehicle and whether getting to it is safe for a van.
What got in, how much, and if it's run — three queries our engineers ask before costing, not after, for precisely that reason. A range turns into a straight figure when the replies are in.
About Pen-y-fan and where it is
Pen-y-fan is a small suburban area in Carmarthenshire, situated near the coast and close to the cities of Swansea and Llanelli. Pen-y-fan has a relatively low elevation of around 6 metres, and its proximity to the sea has influenced its development. Pen-y-fan is characterised by its semi-detached housing stock, which is typical of many suburban areas in Wales. Pen-y-fan is also near several other small communities, including Trostre and Coed Cae.
Fuel drain cover across Carmarthenshire, Pen-y-fan included
Trostre, Coed Cae and Bigyn are among the locations near Pen-y-fan that the same vans work, answering petrol in diesel jobs across Pen-y-fan's SA postcode area. No need to bring your car to us. Forecourts, supermarket car parks, lay-bys and your own driveway are all spots where we work.
Pen-y-fan: still at the petrol pump?
The best position there is, then — that is where you are. Phone, and keep it switched off.