Tongwynlais: Petrol in a Diesel Car? How Worried to Be
Not started? Then less worried than you likely feel. Everyone in Tongwynlais begins with "have I ruined it?", and the fair answer is usually no: petrol left inside the tank of a diesel stays harmless till priming, cranking or driving draws it into your fuel system. Keep it that way: leave your engine off and ring Petrol in Diesel. The rest of this page sets out what petrol actually does to a diesel, how to tell how far it has got, and how to make sure it doesn't happen to you again.
Petrol in a diesel: what does it do to the engine?
Diesel does two tasks on its way through an engine. Before it burns, it lubricates — then it ignites. What keeps the surfaces inside the high-pressure pump and the fuel injectors from wearing against each other is your diesel passing through them — they are built to extremely fine tolerances.
Petrol does the first job poorly and the second not at all. Being less viscous than diesel, petrol strips out the protection a fuel system needs, leaving a pump running on the blend wearing itself. It is equally made to resist igniting under compression — the reverse of what a diesel needs — which is why you hear the clattering.
The shape of the issue is plain when you put those together. The damage is done by circulation, not by contact. Where that petrol is only in your tank, it's a problem of fuel, nothing more. 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 the fuel to the fuel injectors downstream.
How bad is petrol in a diesel? Four scenarios our technicians see around Tongwynlais
Filled up, not once started. The best case there is. The petrol is inside your tank and only that tank, and the job is a drain and a flush where the vehicle sits. This is the one situation that can only truly take place at a forecourt, since anywhere else the vehicle has been driven to get there.
A moment's running, or merely the ignition on. Once primed, the fuel system has pulled petrol from your tank up to your filter and the lines. Normally little more than draining, flushing and sorting your filter, so still straightforward.
Your pump now worked a couple of miles on diluted fuel — that's what driving it does. How hard the car was driven counts as much as how far, and the work takes in the tank, the fuel lines, your filter and clean fuel through the entire system. Short trips, such as the school run to the local primary school, are one thing. Getting as far as Cardiff, under 5 miles away, makes a very different task — as does any mileage on the M4, under a mile from Tongwynlais, with your engine running hard the whole time.
Driven till it protested. Rattling, a ragged idle, smoke, a throttle that has gone soft. The engine is telling you the fuel is no longer acting like diesel. Pull up on the spot — on the verge on Merthyr Road, at the kerb, down a side road — and don't move it. A car pulled over quickly very rarely has permanent damage, since the fuel drain and the flush, which still come first, sort it out.
What Tongwynlais motorists get told about petrol in a diesel — and the stories in it
Myth: "Fill it the rest of the way with diesel and it'll be fine." It will not. Nothing is removed when you top up — the tank simply holds more of the same blend. The idea behind that advice — that diesel fuel systems tolerate petrol by percentage — is wrong.
Myth: a modern diesel copes with a bit of petrol. If anything, the reverse. A modern diesel runs its fuel through more precise parts at higher pressure, which makes the lubrication count more, not less.
One myth that catches motorists off guard: if it starts and drives, it must be unharmed. The driver of a petrol motor fed diesel usually stops, because it will not run, 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 wears. A running engine doesn't mean fine.
Myth: "Add some two-stroke oil and drive it home." It's an outdated fix for a different issue. It doesn't put back what petrol has diluted, and that petrol is still inside that tank later on.
Myth: "Drain that tank yourself and you are sorted." Once your engine has run, some of the petrol is no longer inside that tank: it's in your filter and the fuel lines, and it goes straight back through the pump on the next start. Draining petrol into a can at the roadside is also truly dangerous.
How petrol ends up: petrol in a diesel
No one does it on purpose, and it's seldom carelessness. One way only, because of the nozzles: the slimmer petrol one slides inside a diesel filler with almost no resistance, whereas the diesel one generally will not fit a petrol vehicle. That's how the forecourt makes it simple.
It occurs when habit and the car stop matching. Any motor that's new to you — hired, on loan as a courtesy car, or just bought. A household with one petrol and one diesel on the drive. The pumps in a different order on a well-known forecourt. Filling up in a rush en route to Taffs Well, with a train to catch. Years of petrol motors behind you, a diesel Volkswagen Golf in front of you at the forecourt for its first tank — that is when. Throw in rain, a phone call or a back seat full of kids, and the hand does what it has always done.
How to avoid petrol in a diesel at your next fill-up around Tongwynlais
- A green handle for unleaded and black for diesel is normal in the UK — yet not on each forecourt. Trust the label on the petrol pump, not the colour.
- In a car you don't know, look inside the flap over the filler or at the cap before your first fill: plenty of motors state the fuel type there.
- A coloured fob or a sticker on the diesel's key achieves more than any amount of intending to remember, for a household with one of each.
- It seems daft and it works: on the first fill in a new or hired car, say "diesel" out loud before you take the nozzle off the petrol pump.
- Put your phone away. This is an easy mistake to make while you are thinking about something else.
- A misfuel inhibitor helps, up to a point. Some diesels have a filler built to refuse a petrol nozzle, and add-on devices are available for cars without one. They are a backstop, not a reason to stop reading the petrol pump.
- Spotting it halfway? Stop at once. A weaker mix results from less petrol in your tank, but the next step stays the same: do not start the vehicle.
Tongwynlais: already put petrol in a diesel?
Then, there's just a short order of things. Starting it is out, and so is turning on the ignition. The staff on the forecourt — whether at Asda itself or wherever you filled up — will usually help roll it clear of the pump if you ask. What got in, about how much, and whether it's run: tell us that when you phone 0333 366 0103.
Corrosion and deposits are the issue with AdBlue, where petrol brings lost protection. So if it was AdBlue, say so — and see its own Tongwynlais page.
Tongwynlais petrol in a diesel jobs: why one costs more than another
An honest single price does not exist here: four scenarios, four separate tasks. What shifts your cost is mostly which of them you are in. Where the vehicle is counts too, and whether a van can safely get to it, after the ratio of petrol to what that tank holds.
Ahead of any quote, not after, come three questions: your fuel that got in, the volume, and if it has run. That's why. Those answers are what change a range into one firm number.
About Tongwynlais, briefly
Tongwynlais is a village situated on the banks of the River Taff in Cardiff, with a mix of terraced housing and a population of around 1273 residents. Tongwynlais sits at an elevation of 43 metres, close to the nearby communities of Morganstown, Forest Farm, and Pantmawr. Tongwynlais is located near the Welsh capital, Cardiff, which is approximately 4.8 miles away. Tongwynlais has a relatively low elevation, typical of riverside villages in the region.
Tongwynlais and Cardiff: where our fuel drain cover reaches
The vans answering petrol in diesel calls in Tongwynlais, in the CF postcode area, equally work the places around it, Morganstown, Forest Farm and Pantmawr included. The vehicle does not need to move to us: our technicians drain at a forecourt, in a supermarket car park, in a lay-by or on your own driveway.
At the petrol pump in Tongwynlais, right now?
Then, you could not be in a better position. Keep it turned off, then ring.