Should You Worry? Petrol in a Diesel Car in Mynydd-bach
If it hasn't been started, you can worry less than you likely are. Everyone in Mynydd-bach begins with "have I ruined it?", and the honest answer is generally no: petrol resting in that tank of any diesel remains harmless until priming, cranking or driving draws it into the fuel system. To keep it like that, phone Petrol in Diesel and leave your engine switched off. The remainder of this page covers what petrol actually does to a diesel, how to judge how far it has got, and how to make sure it does not 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. It protects to begin with, and after that does it burn. The high-pressure pump and the injectors are designed to extremely fine gaps, and the diesel passing through them is what keeps those surfaces from wearing against one another.
Poorly at the first, not at all at the second: that is petrol. It's thinner than diesel and strips away the lubrication the fuel system hinges on, and so a pump running on a petrol mix 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's where the knocking comes from.
The shape of the trouble is plain when you put those together. The harm is done by circulation, not by contact. All a tank with petrol in it amounts to is a fuel issue. A pump that has been working on that petrol is the start of a mechanical one, and whatever a worn pump sheds moves on with your fuel to the injectors further on.
Four scenarios our technicians see around Mynydd-bach: how serious is petrol in a diesel?
The best case there is: filled up, never started. Only the tank has petrol in it, so the job is clearing your tank and cleaning your system where the car sits. This scenario can only truly occur on a forecourt: anywhere else, the car got there by being driven.
A brief spell of running, or merely the ignition on. Your system has been bled, and so petrol has left your tank and got to the fuel filter and your lines. That is generally still an easy job — a drain, a flush, and your filter dealt with.
Those few miles driven meant the pump working on fuel thinned by petrol throughout. Tank, lines, filter, then clean fuel put through the whole system: that is this work, and the way the vehicle was driven matters as much as the distance. On the school run to Tutshill Church of England Primary School, the distance is short. Any distance on the M48, under 3 miles from Mynydd-bach, means the motor has worked hard throughout, and a run to Newport, under 15 miles away, makes a very different job as well.
It complained, and only then stopped: smoke, clattering, a throttle gone soft, an idle gone uneven. The motor is telling you your fuel is no longer acting like diesel. Leave it wherever it stops: a lay-by, a quiet street, the verge on Weyloed Lane. The fuel drain and flush-through still go first, and they deal with it: when a vehicle is stopped soon, permanent damage is very rare.
What Mynydd-bach drivers get told about petrol in a diesel — and the myths in it
"Top it up the rest of the way with diesel and it'll be fine" — a myth. It will not. Nothing comes out when you top up — the tank simply holds more of the same mixture. That guidance assumes a diesel fuel system handles petrol by percentage. It doesn't.
Myth: a modern diesel copes with a bit of petrol. Rather, the reverse. Lubrication matters more on newer diesels, not less, since their fuel passes through smaller components at higher pressure.
This is the one that fools people. Myth: "If it starts and drives, there is no harm done." Diesel in a petrol motor usually means a motor that won't run: the driver stops, and so does the damage. A diesel fed petrol normally starts, ticks over and drives away as though nothing has happened, while the pump wears. 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 different fault. It doesn't put back what petrol has thinned, and the petrol is still inside your tank after that.
"Drain it yourself and you're sorted" — a myth, because after it has been running some petrol sits in your filter and lines, waiting to go right back through your pump on your next start. Besides, petrol cleared into a can by the road is genuinely hazardous.
Why petrol in a diesel occurs at all
No one does it on purpose, and it's seldom carelessness. One way only, since of the nozzles: the smaller petrol one slips inside a diesel filler with almost no resistance, whereas the diesel one mostly will not fit a petrol car. That is how the forecourt makes it easy.
The moment habit stops matching the vehicle is the moment it occurs. Hire cars, courtesy cars, and motors that are new to you. Two motors on one driveway, one petrol, one diesel. A forecourt you know, but with its pumps laid out differently. Fuelling up in a rush on the way to Chepstow, with a train to catch. Exactly the moment it happens: after years of driving petrol, someone fills a diesel Volkswagen Golf for the first time. The hand repeats what it has always done when you add rain, a phone call or a back seat full of kids.
Mynydd-bach motorists: how to prevent petrol in a diesel next time
- The label is what counts, and so check the pump rather than the colour: in the UK unleaded handles are usually green and diesel black, yet not on each forecourt.
- Inspect the filler flap. Many motors state the fuel type inside the flap or on the cap. Look at it on the first fill in a car you do not know.
- One of each in the home? Label the key to the diesel — a bright fob, a sticker — because meaning to remember isn't enough.
- Say it before you lift it. On your first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the pump. It sounds silly and it works.
- Thinking about something else is when this becomes an easy mistake to make, so tuck the phone away and keep your mind on the petrol pump.
- Some diesels come with a filler built to block a petrol nozzle, and devices you can add exist for vehicles without one. Such an inhibitor helps up to a point — as a backstop, not a reason to stop checking the petrol pump.
- Stop the moment you realise. Less petrol inside the tank means a thinner mix. It doesn't change what to do next, which is still not to crank it.
Petrol already in a diesel around Mynydd-bach? Begin here
Then the order of things is short. Do not turn the ignition on, and don't start it. Yet on the forecourt, at Texaco or anywhere else? Ask the attendants to help you push the vehicle clear of the pump. Next, 0333 366 0103. We'll ask what was put in, about how much, and if it's been run.
If it was AdBlue instead of petrol, say so. That is a separate issue — corrosion and deposits instead of lost protection — and it has its own page for Mynydd-bach.
Mynydd-bach petrol in a diesel jobs: why one costs more than another
An honest single quote does not exist here: four situations, four separate tasks. Which of them you are in is mostly what moves your cost. After that it is the ratio — how much petrol got in against how much the tank holds — and then where the vehicle is and whether a technician can get to it safely.
Three queries come first, before quoting, rather than after — what went in, how much, has it run — and that is the reason. The replies change a range into a straight figure.
About Mynydd-bach
Mynydd-bach is a small village in Monmouthshire, situated on the banks of a river at an elevation of around 141 metres. Mynydd-bach lies near the communities of Shirenewton, Itton, and Itton Common, and is approximately 11 miles from the city of Newport. Mynydd-bach has a population of around 668 people, predominantly living in detached houses. Mynydd-bach is located in a rural area, with the nearest major city being Newport and the capital city London being around 113 miles away.
Mynydd-bach and Monmouthshire: where our fuel drain cover runs
Petrol in diesel call-outs around Mynydd-bach — part of the NP postcode area — go to teams that also cover the spots about it, Shirenewton, Itton and Itton Common included. A forecourt, a supermarket car park, a lay-by or your own driveway are all places our technicians work, and the car does not need to move to us.
In Mynydd-bach and still at the petrol pump?
The best spot there is, then — that's where you are. Leave the car turned off, then ring.