--- title: Explained: Petrol in a Diesel Car Williamstown canonical: https://www.petrolinadieselcar.co.uk/petrol-in-diesel/williamstown-rhondda-cynon-taf/ location: Williamstown county: Rhondda Cynon Taf latitude: 51.607790 longitude: -3.442860 service: Petrol in a Diesel Car updated: 2026-10-24 --- # Should You Worry? Petrol in a Diesel Car in Williamstown Probably less than you are now — provided nobody has started it. Petrol sitting in a diesel tank does nothing to your motor till something pulls it through your fuel system — your ignition priming your pump, your starter, or the drive on its own. So the truthful answer to the first query everyone around Williamstown asks, "have I ruined it?", is generally no. Leave the motor off and ring Petrol in Diesel — that keeps it that way. Below, 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. ## Petrol in a diesel: what does it do to the engine? Passing through an engine, diesel is doing two things at the same time. Oiling comes before burning; it does both. What stops the surfaces inside the high-pressure pump and the fuel injectors from grinding against each other is the diesel passing through them — they are designed to very fine clearances. The second task petrol does not do at all, and the first it does badly. It's thinner than diesel and strips away the protection the system depends on, and 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 reverse of what a diesel needs from its fuel. That is where the clattering comes from. The shape of the problem is plain once you put those together. The harm is done by movement, not by contact. Petrol in the tank is a fuel fault. The start of a mechanical one is a pump that has been working on that petrol — and whatever a worn pump sheds travels on with your fuel to your injectors beyond it. ## Four scenarios our technicians see around Williamstown: how serious is petrol in a diesel? The best case there is: filled up, not once started. On a forecourt, and really only there, that petrol is still limited to that tank — because everywhere else your car was driven to reach it. The job is emptying and purging on the spot. Ignition on, or started for a moment. The system has been bled, and so petrol has left the tank and got to your filter and the lines. Usually nothing more than draining, rinsing and sorting the filter, so still easy. Driven a couple of miles. Now the pump has spent those miles running on diluted fuel. Tank, lines, filter, then fresh fuel run through the entire circuit: that's this work, and the way the car was driven matters as much as the distance. Driving the kids to the local primary school is one kind of trip. Driving as far as Cardiff, under 15 miles away, makes a very different job — as does any miles on the M4, under 10 miles from Williamstown, with your engine working hard all the way. It complained, and only then stopped: smoke, knocking, a throttle gone soft, an idle gone lumpy. The fuel isn't acting like diesel any more, and your motor is telling you. Pull up on the spot — on the verge on Brook Street, at the kerb, down a side road — and do not move it. The fuel drain and flush still come first, and they sort it out: if a vehicle is stopped quickly, permanent damage is very uncommon. ## Stories Williamstown motorists hear about petrol in a diesel It will not be fine. The myth: "Fill it the rest of the way with diesel and it'll be." Topping up takes out nothing; it gives you a fuller tank of the same mix. Percentage is not how a diesel fuel system copes with petrol, whatever that guidance supposes. Myth: "A newer diesel can cope with a bit of petrol." If anything, the opposite. At higher pressure, through smaller components: that is how a modern diesel handles its fuel, and so the lubrication counts for more, not less. "It starts, it drives, so there is no harm" — the myth that trips up most drivers than any other. The driver of a petrol engine fed diesel usually has no choice but to stop, since it will not run, and the harm stops there. Nothing looks to have happened when a diesel fed petrol fires up, ticks over and drives away, which it usually does; meanwhile the pump grinds. Running isn't the same as fine. Myth: "Add some two-stroke oil and drive the vehicle home." It is an old workaround for a separate fault. None of what petrol diluted is put back by it — and afterwards, that tank still contains that petrol. If your engine has been run, part of the petrol is in the fuel filter and the lines, not that tank — and the next start sends it straight back through the pump. So the myth "drain that tank yourself and you are sorted" does not stand up. Besides, petrol cleared into a can by the road is truly dangerous. ## How it happens: petrol in a diesel Carelessness is seldom the cause, and no one means to do it. One way only, since of the nozzles: the smaller petrol one slides inside a diesel filler with almost no resistance, whereas the diesel one generally won't fit a petrol motor. That is how the forecourt makes it easy. It happens when habit and the car stop matching. Courtesy car, hire car, or a motor that's brand new. A household with one petrol and one diesel on the driveway. A forecourt you know, but with its pumps arranged differently. A train to catch, and a hurried fill on the way to Dinas Rhondda. Years of petrol cars behind you, a diesel Renault Megane in front of you at the forecourt for its first tank — that is when. With rain, a call coming in, or children to deal with, the hand just does what it has always done. ## Next time you fill up in Williamstown: avoiding 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. - The first fill in an unfamiliar vehicle is the time to check your filler flap — inside it, or on the cap, plenty of motors state the fuel they take. - One of each in the house? Label the key to the diesel — a coloured fob, a sticker — because intending to remember is not enough. - Before the nozzle comes off the pump the first time you fill a hired or new vehicle, say "diesel" aloud. Silly, perhaps, yet it works. - Tuck your phone away. This is an easy mistake to make whilst you're thinking about something else. - A misfuel inhibitor helps, up to a point. Some diesels have a filler built to reject a petrol nozzle, and add-on devices exist for vehicles without one. They are a safety net, not a reason to stop checking the pump. - Noticing it mid-fill? Stop at once. A weaker mix follows from less petrol in that tank, but the next stage remains the same: do not fire it up. ## Put petrol in a diesel in Williamstown already? Then, there's only a short order of things. Don't crank it, and do not switch your ignition on. Ask the staff to help you roll it clear of the pump if you're still on the forecourt, whether that's Shell or anywhere else. Next, 0333 366 0103. We'll ask what got in, roughly how much, and if it has been run. Corrosion and deposits form the trouble with AdBlue, where petrol means lost protection. So if it was AdBlue, say so — and see its own Williamstown page. ## One petrol in a diesel job in Williamstown costs more than another. Why? The four scenarios above are four different jobs, so no single quote for this would be honest. Which of them you're in is mostly what moves your cost. The share matters next, meaning how much of your tank is petrol, followed by the location of your car and whether reaching it is safe for our van. Before any quote, not after, we ask three queries: the fuel that went in, its amount, and whether it has run. That is why. Given the replies, you get a straight figure rather than a range. ## About Williamstown, for a moment Williamstown is a suburban area in Rhondda Cynon Taf, situated on the riverside at an elevation of 183 metres. Williamstown lies near the communities of Penrhiwfer, Edmondstown, and Penygraig, and is approximately 14 miles from Cardiff. Williamstown has a population of around 3775 people, predominantly living in terraced housing. Williamstown is also relatively distant from London, at a distance of 142.6 miles. ## Fuel drain cover around Williamstown and across Rhondda Cynon Taf The vans answering petrol in diesel call-outs around Williamstown, in the CF postcode area, equally work the locations near it, Penrhiwfer, Edmondstown and Penygraig included. Our technicians drain where your vehicle is — your own driveway, a lay-by, a supermarket car park, a forecourt — so it does not have to come to us. ## Williamstown: still at the petrol pump? Then you are in the best place there can be. Keep the vehicle turned off, and ring.