--- title: Explained: Petrol in a Diesel Car Felinfoel canonical: https://www.petrolinadieselcar.co.uk/petrol-in-diesel/felinfoel-carmarthenshire/ location: Felinfoel county: Carmarthenshire latitude: 51.697870 longitude: -4.143980 service: Petrol in a Diesel Car updated: 2026-10-09 --- # How Worried Should You Be? Petrol in a Diesel Car in Felinfoel Likely less than you already are — as long as nobody has started the vehicle. Till the ignition primes your pump, your starter turns, or the vehicle is driven, petrol in a diesel tank simply sits there and does nothing. Which makes the truthful reply to that first Felinfoel question — "have I wrecked it?" — normally no. To leave the car that way, phone Petrol in Diesel and leave your engine switched off. Further down, this page sets out what petrol really does inside a diesel, how far it might have gone, and how to stop it happening to you again. ## What petrol in a diesel does to your engine On its way through a motor, diesel has two jobs. Before it ignites, it lubricates — and then it combusts. Your high-pressure pump and the fuel injectors are built to extremely fine tolerances, and the diesel passing through them is what stops those surfaces from grinding against one another. Poorly at the first, not at all at the second: that is petrol. It is runnier than diesel and takes away the lubrication the fuel system hinges on, so a pump running on a petrol mixture is a pump wearing itself. And petrol is designed to resist igniting under compression, which is the opposite of what a diesel needs from its fuel. That is where the rattling comes from. Put those together and the shape of the problem is clear. The damage is done by movement, not by contact. Any tank with petrol in it is a fuel issue. After your pump has been working on that petrol, a mechanical problem has begun: whatever is shed by a pump that's wearing down is carried onward to the injectors with the fuel. ## How bad is petrol in a diesel? Four scenarios our technicians see in Felinfoel Fuelled, and not started: no case is better. Being at a forecourt is what makes this the case: elsewhere, the car has been driven there to begin with. With the petrol held in the tank alone, it's cleared and rinsed right where it stands, and nothing more is needed. Started for a second, or only the ignition on. Petrol has come out of that tank and got to the fuel filter and your lines, since the fuel system has been bled. That's normally still a straightforward task — a drain, a flush, and your filter dealt with. The pump now spent a few miles on thinned fuel — that's what driving the vehicle does. Tank, lines, filter, then clean fuel put through the whole circuit: that's the work, and the way the vehicle was driven counts as much as the distance. A school run to the local primary school is one matter. Very different once more: a trip to Swansea, under 15 miles away, or any distance on the M4, under 5 miles from Felinfoel, where your engine kept labouring hard. Driven until it protested. Clattering, a lumpy idle, smoke, a throttle that has gone soft. That's the engine telling you its fuel has stopped acting like diesel. Leave it where it stops: a lay-by, a quiet street, the verge on Park View. Permanent damage is very rare when a car is pulled over soon, and the fuel drain and flush-through that still go first sort it out. ## Felinfoel motorists and that petrol in a diesel myths they hear It will not be fine. The myth: "Fill it the rest of the way with diesel and it'll be." A bigger tank of the same mixture is all it gives, because topping up removes nothing. Percentage isn't how a diesel fuel system tolerates petrol, whatever that guidance supposes. "Modern diesels can handle a little petrol" — a myth, and closer to the opposite. At higher pressure, through finer components: that's how a current diesel handles its fuel, and so the lubrication matters more, not less. This is the one that catches motorists. Myth: "If it starts and drives, there is no damage done." Feed diesel to a petrol engine and it usually won't run, and so the driver stops — and the damage stops with them. Feed petrol to a diesel and it usually fires up, ticks over and pulls away like nothing happened — all the while its pump is wearing. Being able to run is not the same as being fine. "A drop of two-stroke oil and drive it home" — a myth, taken from a fix from years ago, for a distinct problem. None of what petrol weakened gets put back by it — and later, the tank still contains the petrol. "Drain it yourself and you're sorted" — a myth, because after it's been running some petrol lies in the filter and lines, ready to go straight back through your pump on the next start. Also, a can of petrol drained by the roadside is truly dangerous. ## How does petrol in a diesel take place? Carelessness is rarely the cause, and no one means to do it. One way only, since of the nozzles: the narrower petrol one slides inside a diesel filler with almost no resistance, whereas the diesel one mostly won't fit a petrol motor. That is how the forecourt makes it easy. Habit and the car getting out of step is how it occurs. Any car that's new to you — a hire car, on loan as a courtesy car, or brand new. Two cars on one driveway, one petrol, one diesel. A forecourt you know, but with its pumps laid out differently. A rushed fill en route to Llanelli with a train to catch. Years of petrol vehicles behind you, a diesel Volkswagen Golf in front of you on the forecourt for its first tank — that is when. Throw in rain, a phone call or a back seat full of children, and the hand repeats what it has always done. ## Next time you fill up around Felinfoel: avoiding petrol in a diesel - 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. - Inspect the filler flap. Plenty of cars state the fuel inside the flap or on the cap. Look at it on the first fill in any car you don't know. - One of each in the household? Label the key to the diesel — a coloured fob, a sticker — because intending to remember isn't enough. - It sounds silly and it works: on your first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the pump. - Put the phone away. This is an easy mistake to make whilst you're thinking about something else. - Add-on devices are available for cars without one, and some diesels come with a filler that won't accept a petrol nozzle. Useful — but a safety net at best, never a reason to stop checking the label. - As soon as you notice, stop: less petrol inside the tank makes a weaker blend. What to do next doesn't change, though — still don't crank it. ## Felinfoel: already put petrol in a diesel? Then, there is only a short order of things. Leave it unstarted, with your ignition switched off as well. Ask the attendants to help you roll it away from the pump if you are still on the forecourt, whether that's Texaco or anywhere else. Next, 0333 366 0103. We will ask what was put in, roughly how much, and if it has been run. If it was AdBlue rather than petrol, say so. That is a separate fault — corrosion and deposits and not lost protection — and it has its own page for Felinfoel. ## What makes one Felinfoel petrol in a diesel job cost more than the next There is no fair single quote for this, because the four situations above are four different tasks. Which of them you're in is chiefly what moves your cost. Next it is the proportion — how much petrol got in against how much that tank takes — and then where the car is and whether a van can get to it safely. What got in, how much, and if it's run — three queries our engineers ask before pricing, not after, for exactly that reason. Those answers change a range into a straight figure. ## About Felinfoel itself Felinfoel is a suburban area in Carmarthenshire, situated near the coast and approximately 10 miles west of Swansea. Felinfoel sits at a relatively low elevation of 32 metres, with nearby areas including Swiss Valley and Pentre-Poeth. Felinfoel has a mix of housing types, but semi-detached homes are particularly common in the area. Felinfoel is located near the larger urban centre of Swansea, but retains its own distinct character. ## Felinfoel and Carmarthenshire: where our fuel drain cover runs Our vans answering petrol in diesel calls around Felinfoel, in the SA postcode area, also work the locations near it, Swiss Valley, Pentre-Poeth and Tyrfran included. No need to move the car to us. Forecourts, supermarket car parks, lay-bys and your own driveway are all locations where we work. ## Still at the petrol pump around Felinfoel? The best spot there is, then — that's where you are. Turned off it remains; then phone.