--- title: Mountjoy: Petrol in a Diesel Car, Explained canonical: https://www.petrolinadieselcar.co.uk/petrol-in-diesel/mountjoy-county-tyrone/ location: Mountjoy county: County Tyrone latitude: 54.550000 longitude: -6.600000 service: Petrol in a Diesel Car updated: 2026-10-19 --- # Should You Worry? Petrol in a Diesel Car in Mountjoy If it has not been started, worry less than you probably are. Everyone in Mountjoy starts with "have I wrecked it?", and the fair answer is generally no: petrol resting inside that tank of any diesel stays harmless until priming, cranking or driving pulls it into the fuel system. Keep it like that: leave your engine off and call Petrol in Diesel. Below, you will find what petrol really does inside a diesel, how far it may have got, and how to stop it happening to you a second time. ## Inside the motor: what petrol in a diesel really does On its way through a motor, diesel does two tasks. It burns, and before it ignites it lubricates. What stops the surfaces inside the high-pressure fuel pump and the fuel injectors from grinding against each other is your diesel passing through them — they are built to very fine tolerances. The second job petrol doesn't do at all, and the first it does poorly. Being less viscous than diesel, petrol strips out the protection a fuel system needs, so a pump on that mixture to wear itself. It's also made to resist igniting under compression — the opposite of what a diesel needs — which is why you get the clattering. Taken together, they make clear the shape the trouble takes. The harm is done by circulation, not by contact. A tank with petrol in it is a fuel fault. The start of a mechanical problem is a pump that has been running on that petrol — and whatever a worn pump sheds travels on with your fuel to your injectors downstream. ## How serious is petrol in a diesel? Four scenarios our technicians see around Mountjoy Fuelled up, not once started. The best case there is. Being at a forecourt is what makes this possible: elsewhere, the vehicle has been driven there to begin with. With the petrol held inside the tank alone, it is cleared and flushed right where it stands, and nothing more is needed. A moment's running, or just the ignition on. Your system has been bled, and so petrol has left that tank and reached the filter and your lines. Drain, flush, see to the fuel filter: still normally easy. Some miles driven, and your pump has spent every one of them working on weakened fuel. Tank, lines, filter, then fresh fuel put through the whole circuit: that's the work, and the way your car was driven counts as much as the distance. Short trips, like the school run to the local primary school, are one matter. Any mileage on the M77, over 50 miles from Mountjoy, means your engine has worked hard throughout, and a run to Armagh, under 15 miles away, makes a very different job as well. Driven on till it protested — clattering, smoke, a lumpy idle, a dead throttle. What you are hearing is the engine reporting fuel that no longer behaves like diesel. Stop wherever you are — the verge on Heugh Road, a lay-by, a side street — and leave the car there. A car pulled over early very seldom has lasting damage, because the fuel drain and the flush, which still go first, sort it out. ## What Mountjoy people get told about petrol in a diesel — and the myths in it "Top it up the rest of the way with diesel and it will be fine" — a myth. It will not. Nothing is removed by topping up — you're left with a fuller tank, still the same mix. Percentage is not how a diesel fuel system copes with petrol, however much that advice assumes. Myth: a newer diesel copes with a bit of petrol. If anything, the opposite. At higher pressure, through smaller components: that's how a newer diesel moves its fuel, so the protection matters for more, not less. Myth: "If it starts and drives, there is no damage done." This is the one that catches drivers. The motorist of a petrol engine fed diesel normally stops, because it will not run, and the damage stops with them. Nothing looks to have happened when a diesel fed petrol fires up, ticks over and drives away, which it usually does; meanwhile the pump wears. Running is not the same as fine. An outdated fix for a distinct fault lies behind this one. Myth: "Add some two-stroke oil and drive it home." It doesn't put back what petrol has weakened, and that petrol is still inside that tank later on. "Drain it yourself and you're sorted" — a myth, because after it has been running some petrol sits in the fuel filter and lines, waiting to go straight back through your pump at the next start. It's also really hazardous to drain petrol into a can by the kerbside. ## Petrol in a diesel: how does it take place? No one does it on purpose, and it is seldom carelessness. One way only, since of the nozzles: the narrower petrol one slips inside a diesel filler with almost no resistance, while the diesel one generally won't fit a petrol car. That's how the forecourt makes it simple. It occurs when habit and the car stop matching. Hire cars, courtesy cars, and motors that are new to you. Two cars on one drive, one petrol, one diesel. Your ordinary forecourt, its pumps now in a different order. Fuelling up in a rush on the way to the nearest station, with a train to catch. Years of petrol cars behind you, a diesel Volkswagen Golf in front of you at the forecourt for its first tank — that's when. With rain, a call arriving, or children to deal with, your hand just does what it has always done. ## Next time you fill up in Mountjoy: avoiding petrol in a diesel - Not every forecourt in the UK follows green for unleaded and black for diesel, though most usually do. Read the petrol pump's label; the colour is only a hint. - Check the filler flap. Plenty of motors state the fuel type inside the flap or on the cap. Look at it on the first fill in any vehicle you don't know. - Mark the key. In a household with one of each, a coloured fob or a sticker on your diesel's key does more than any amount of meaning to remember. - It seems silly and it works: on your first fill in a new or hired car, say "diesel" aloud before you lift the nozzle off the petrol pump. - While you're thinking about something else, this is an easy mistake to make — so tuck the phone away. - Up to a point, a misfuel inhibitor helps: some diesels come with a filler built to reject a petrol nozzle, and add-on devices are available for the rest. Count it as a backstop, then keep checking the pump. - As soon as you notice, stop: less petrol in that tank makes a weaker mixture. What to do next does not change, though — still do not crank it. ## Petrol already in a diesel in Mountjoy? Begin here Then, there is only a short order of things. Never start the car, and don't switch the ignition on. If you are still on the forecourt — the nearest petrol station or anywhere else — ask the attendants to help you push the vehicle clear of the pump. Then call 0333 366 0103 and tell us what went in, roughly how much, and if it has run. Corrosion and deposits form the problem with AdBlue, whereas petrol means lost lubrication. So if it was AdBlue, say so — and read its own Mountjoy page. ## What makes one Mountjoy petrol in a diesel task cost more than the next Four different jobs sit behind the four scenarios above — which is why there's no honest single price. Which of them you are in is mostly what moves the cost. The proportion matters next, meaning how much of the tank is petrol, and then the location of the vehicle and whether getting to it is safe for a technician. That is why our technicians ask three queries before pricing instead of after: what went in, how much, and whether it's run. Their replies are what change a range into one firm number. ## About Mountjoy Mountjoy is a small locality in County Tyrone, situated inland near the shores of Lough Neagh. Mountjoy sits at a low elevation of around 12 metres, close to the nearby settlements of Brockagh and Washing Bay. Mountjoy is characterized by its predominantly detached housing stock, reflecting its rural setting. Mountjoy is located near the city of Armagh, which is approximately 14 miles away. ## Fuel drain cover across County Tyrone, Mountjoy included Petrol in diesel jobs in Mountjoy — part of the BT postcode area — go to teams that also cover the locations around it, Brockagh, Washing Bay and Acton included. The vehicle doesn't need to come to us: our technicians drain at a forecourt, in a supermarket car park, in a lay-by or on your own drive. ## In Mountjoy and still at the petrol pump? The best position there is, then — that is where you are. Phone, and leave it switched off.