Brentry: Petrol in a Diesel Car? How Worried to Be
Far less than you likely are, provided it has not been started. "Have I wrecked it?" is the first thing anyone in Brentry asks, and the honest answer is generally no, since petrol resting in a diesel tank harms nothing until it's drawn through the system — by ignition priming, by the starter, or by being driven. Petrol in Diesel is who to ring, with your motor left off, and so it remains that way. The remainder 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 a second time.
The engine and petrol in a diesel: what happens
On its way through a motor, diesel has two tasks. Before it ignites, it lubricates — and then it ignites. Extremely fine gaps are built into the fuel injectors and your high-pressure pump, and just your diesel running through stops their surfaces grinding together.
Of those two tasks, petrol handles the first badly and the second never. A pump running on a petrol mixture is wearing itself out, since petrol is thinner than diesel and strips out the lubrication the fuel system relies on. Petrol is also designed to resist igniting under compression — the reverse of what a diesel needs from its fuel — and that is where the rattling starts.
Seen together, they make clear the shape the trouble takes. The damage is done by movement, not by contact. All a tank with petrol in it amounts to is a fuel fault. Whatever a worn pump sheds travels on, borne by your fuel, into your injectors further along, which is why a pump that has been working on that petrol is where a mechanical problem starts.
How serious is petrol in a diesel? Four scenarios our technicians see in Brentry
The best case there can be: filled up, not once started. Being at a forecourt is what makes this the case: elsewhere, the vehicle has been driven there to begin with. With that petrol held inside that tank alone, it's drained and rinsed right where it stands, and nothing more is needed.
Your ignition switched on, or a brief start. Petrol has left the tank and got to your filter and the lines, since your system has been primed. Drain, flush, deal with the filter: still usually straightforward.
Driven a few miles. Now your pump has spent those miles running on thinned fuel. How hard your car was driven counts as much as how far, and this work takes in the tank, the lines, the filter and fresh fuel through the whole circuit. Driving the kids to Brentry Primary School is one sort of trip. Driving as far as Bristol, under 5 miles away, makes a very different job — as does any distance on the M5, under 3 miles from Brentry, with the engine working hard the whole time.
Driven until it complained. Knocking, an uneven idle, smoke, a throttle that has gone soft. The motor is telling you the fuel is no longer acting like diesel. Whether it's the verge on Forest Drive, a lay-by or a side road, stop there and leave it. What comes first is still the fuel drain and flush, and that takes care of it, with long-term damage very rare when a car is stopped quickly.
Brentry people and the petrol in a diesel myths they hear
"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 bigger tank, still the same mixture. The assumption behind the advice — that diesel fuel systems accept petrol by percentage — is wrong.
"Modern diesels can handle a little petrol" — a myth, and nearer the reverse. At higher pressure, through finer parts: that's how a modern diesel handles its fuel, and so the lubrication matters more, not less.
"It starts, it drives, and so no harm done" — the myth that trips up more motorists than any other. The motorist of a petrol engine fed diesel usually has no choice but to stop, because it will not run, and the damage stops there. A diesel fed petrol normally starts, idles and drives away as if nothing has happened, while the pump wears. Running doesn't mean fine.
Myth: "Add some two-stroke oil and drive the vehicle home." It is an old workaround for a separate issue. None of what petrol weakened is put back by it — and later, your tank still holds that petrol.
"Drain it yourself and you're sorted" — a myth, since after it has been running some petrol lies in your filter and lines, ready to go straight back through your pump on the next start. It is also truly hazardous to drain petrol into a can at the kerbside.
Why petrol in a diesel happens so easily
It's rarely carelessness, and nobody does it on purpose. A diesel nozzle generally will not go into a petrol car, yet the smaller petrol nozzle goes into the filler of a diesel almost without resistance — so forecourts make this one mistake easy, and just in one direction.
It occurs when habit and your car stop matching. A new motor, a hire car or a courtesy car. A household with one petrol and one diesel on the drive. A well-known forecourt with the pumps in a different order. Racing for a train outside Filton Abbey Wood and fuelling up en route. A new-to-you diesel Volkswagen Passat, its first fuelling, and a motorist used to petrol: precisely when it happens. Add a back seat full of children, a phone call or rain, and habit wins.
How to prevent petrol in a diesel on your next fill-up around Brentry
- Green for unleaded and black for diesel is usual in the UK — yet not on every forecourt. Go by the label on the pump, not the colour.
- Plenty of cars state the fuel type inside your filler flap or on the cap, and so check it on the first fill in any vehicle you don't know.
- One of each in the household? Mark the key to the diesel — a coloured fob, a sticker — since meaning to remember is not enough.
- It seems silly and it works: on your first fill in a new or hired car, say "diesel" aloud before you take the nozzle off the pump.
- Thinking about something else is when this becomes an easy mistake to make, so tuck the phone away and keep your mind on the pump.
- Add-on devices exist for vehicles without one, and some diesels come with a filler that will not accept a petrol nozzle. Useful — but a backstop at best, never a reason to stop checking the label.
- Stop as soon as you notice. Less petrol in that tank makes a weaker mix. It does not change what to do next, which is still not to fire it up.
Put petrol in a diesel in Brentry already?
Short and simple, then, is the order of things. Do not start it, and do not turn the ignition on. If you are still on the forecourt — Andy's Gas or anywhere else — ask the staff to help you push it away from the pump. What was put in, roughly how much, and whether it's run: tell us that when you ring 0333 366 0103.
If it was AdBlue rather than petrol, say so. That's a different problem — corrosion and deposits rather than lost protection — and it has its own page for Brentry.
What makes one Brentry petrol in a diesel job cost more than the next
A fair single quote doesn't exist here: four situations, four separate tasks. What shifts the cost is mostly which of them you are in. The proportion matters next, meaning how much of the tank is petrol, and then the position of your car and whether reaching it is safe for a technician.
That is why our technicians ask three queries before costing rather than after: what went in, how much, and whether it's run. Those replies turn a range into a straight figure.
About Brentry and its setting
Brentry is a suburban area located inland, approximately 4 miles north of Bristol city centre, with an elevation of around 64 metres. Brentry sits near the suburbs of Southmead, Catbrain, and Henbury, and is characterized by its predominantly semi-detached housing stock. Brentry is home to a population of around 7351 residents, and is situated within the larger Bristol urban area. Brentry is also relatively close to major transport links, being around 106.5 miles west of London.
Fuel drain cover across Bristol, Brentry included
Brentry waits in the BS postcode area, and our vans that answer petrol in diesel calls there work the spots near it as well — Southmead, Catbrain and Henbury among them. A forecourt, a supermarket car park, a lay-by or your own driveway are all places we drain, and your vehicle doesn't need to come to us.
At the petrol pump around Brentry, right now?
No position is better: that's where you now are. Turned off it remains; then call.