Ambrosden: Petrol in a Diesel Car? How Worried to Be
Not started? Then less worried than you likely are. "Have I ruined it?" is the first thing anyone around Ambrosden asks, and the honest answer is normally no, because petrol left in a diesel tank harms nothing till it is dragged through the fuel system — by ignition priming, by the starter, or by being driven. Petrol in Diesel is who to phone, with your engine left off, and so it remains that way. The rest of this page sets out what petrol really does to a diesel, how to tell how far it has got, and how to make sure it does not happen to you a second time.
Inside the engine: what petrol in a diesel really does
Passing through a motor, diesel is doing two things at the same time. Before it combusts, it protects — then it burns. What stops the surfaces inside your high-pressure pump and your injectors from wearing down against one another is the diesel flowing through them — they are built to extremely fine gaps.
Petrol handles the first task badly and the second not at all. Being less viscous than diesel, petrol strips away the lubrication a fuel system needs, so a pump running on that mixture wearing itself. It is also made to resist igniting under compression — the opposite of what a diesel needs — which is why you hear the knocking.
Seen together, they show the shape the problem takes. The damage is done by movement, not by contact. Petrol in the tank is a fuel issue. Once the pump has been labouring on that petrol, a mechanical problem has started: anything shed by a pump that's grinding is carried onward to the injectors with your fuel.
How bad is petrol in a diesel? Four situations our technicians see in Ambrosden
Fuelled, and not started: no case is better. On a forecourt, and truly only there, that petrol is still confined to that tank — since everywhere else the vehicle was driven to get to it. The job is draining and cleaning on the spot.
Started for a moment, or only the ignition on. With the system primed, the lines and filter now contain petrol that has come from the tank. Usually nothing more than draining, purging and seeing to the fuel filter, and so still straightforward.
Your pump has worked a couple of miles on diluted fuel — that's what driving it meant. How hard the vehicle was driven counts as much as how far, and the work covers that tank, the lines, your filter and clean fuel through the whole system. The school run to Five Acres Primary School is one thing. Driving as far as Oxford, under 15 miles away, makes a very different job — as does any mileage on the M40, under 3 miles from Ambrosden, with your engine running hard the whole time.
Driven till it complained. Rattling, an uneven idle, smoke, a throttle that has gone soft. The fuel isn't behaving like diesel any more, and your engine is telling you. Leave it wherever it has stopped: a lay-by, a quiet street, the verge on Birch Road. The fuel drain and flush-through still come first, and they deal with it: if a vehicle is pulled over early, long-term damage is very rare.
What Ambrosden people get told about petrol in a diesel — and the myths in it
False: topping up to full with diesel makes it fine. It doesn't. Nothing is removed by topping up — you're left with a bigger tank, still the same mixture. The assumption behind the guidance — that diesel fuel systems accept petrol by percentage — is wrong.
If anything, the reverse is true. Myth: "A modern diesel can cope with a bit of petrol." Smaller parts and higher pressure are how a current diesel runs its fuel, and that makes the protection matter more, not less.
This is the one that fools people. Myth: "If it starts and drives, there is no harm done." Diesel in a petrol motor usually leaves a motor that won't run: the motorist stops, and so does the harm. Feed petrol to a diesel and it usually fires up, idles and pulls away like nothing has happened — all while its pump is wearing. Being able to run isn't proof of being fine.
"A drop of two-stroke oil and drive it home" — a myth, taken from a fix from years ago, for a distinct fault. What petrol has weakened stays thinned, and that petrol remains inside the tank regardless.
Myth: "Drain that tank yourself and you're sorted." After the engine has been run, some of that petrol is no longer inside your tank: it's in the fuel filter and your lines, and it goes straight back through the pump on the next start. It is also truly dangerous to drain petrol into a can at the kerbside.
How it happens: petrol in a diesel
No one does it on purpose, and it's seldom carelessness. The forecourt makes it simple: the petrol nozzle is the narrower of the two, and it slips into a diesel filler with almost no resistance, whilst a diesel nozzle mostly won't go into a petrol vehicle at all. So the slip runs one way.
It happens when routine and your car stop matching. Courtesy car, hire car, or a vehicle that is brand new. Two vehicles on one drive, one petrol, one diesel. Your usual forecourt, its pumps now in a different order. A train to catch, and a hurried fill on the way to Bicester Village. The first fill-up in a diesel Renault Megane after years of driving petrol is exactly the moment it takes place. With rain, a call arriving, or children to deal with, your hand just does what it always has.
How to avoid petrol in a diesel at your next fill-up in Ambrosden
- Green for unleaded and black for diesel is usual in the UK — but not on each forecourt. Trust the label on the petrol pump, not the colour.
- Many cars state the fuel type inside the filler flap or on the cap, so check it on the first fill in any car you don't know.
- A coloured fob or a sticker on your diesel's key does more than any amount of meaning to remember, for a house with one of each.
- It seems daft and it works: on your first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the pump.
- While you are thinking about something else, this is an easy mistake to make — so tuck your phone away.
- Add-on devices are available for motors without one, and some diesels come with a filler that won't take a petrol nozzle. Useful — but a safety net at best, never a reason to stop checking the label.
- Less petrol makes a thinner mixture, and so stop as soon as you realise — while remembering that the next step is unchanged. It's still not to start the vehicle.
Ambrosden: already put petrol in a diesel?
The order of things is brief, then. Leave it unstarted, with your ignition switched off too. The staff on the forecourt — whether at Shell itself or wherever you filled up — will normally help roll it away from the pump if you ask. Then phone 0333 366 0103 and tell us what went in, roughly how much, and if it's run.
If it was AdBlue instead of petrol, say so. That is a different problem — corrosion and deposits rather than lost protection — and it has its own page for Ambrosden.
Why one petrol in a diesel task in Ambrosden costs more than another
The four scenarios above are four separate tasks, so no single price for this would be honest. The situation itself does most of the work of moving your cost. Beyond that it's the proportion — how much petrol went in against how much the tank holds — and then where the car is and whether our van can reach it safely.
Before any quote, not after, come three queries: your fuel that went in, its amount, and whether it's run. That is why. A range turns into a straight figure once those replies are in.
About Ambrosden and where it is
Ambrosden is a village in Oxfordshire, situated approximately 10 miles from Oxford and 49.5 miles from London, with a population of around 1388 residents. Ambrosden sits in a low-lying area at an elevation of 68 metres, surrounded by nearby places such as Lower Arncott, Blackthorn, and Upper Arncott. Ambrosden has a mix of housing types, with semi-detached homes being predominant. Ambrosden remains a relatively small and quiet community in the South East region.
Fuel drain cover across Oxfordshire, Ambrosden included
Ambrosden waits in the OX postcode area, and the teams that answer petrol in diesel jobs there work the locations around it as well — Lower Arncott, Blackthorn and Upper Arncott among them. You need not move your vehicle to us. Forecourts, supermarket car parks, lay-bys and your own driveway are all locations we drain.
At the petrol pump in Ambrosden, right now?
In that case, you couldn't be in a better spot. Turned off it remains; then call.