Little Heath: Petrol in a Diesel Car? How Worried to Be
Less than you likely are, if it hasn't been started. "Have I ruined it?" is the first thing anyone around Little Heath asks, and the honest answer is usually no, since petrol left in a diesel tank harms nothing till it is drawn through the system — by ignition priming, by the starter, or by driving. Keep it like that: leave your motor off then call Petrol in Diesel. 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 doesn't happen to you again.
Petrol in a diesel: what does it do to your engine?
Diesel has two tasks on its way through a motor. It protects to begin with, and after that does it burn. Designed to extremely fine clearances, the fuel injectors and your pump rely on the diesel flowing through them to keep their surfaces from wearing against one another.
Petrol does the first job poorly and the second not at all. It's thinner than diesel and removes the lubrication your fuel system hinges on, so a pump running on a petrol blend is a pump wearing itself. And petrol is made to resist igniting under compression, which is the opposite of what a diesel needs from its fuel. That's where the rattling comes from.
Seen together, they show what shape the trouble has. Contact does not do the damage — movement does. Any tank with petrol in it 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 travels on with the fuel to your injectors downstream.
How bad is petrol in a diesel? Four situations we see in Little Heath
Fuelled, and not started: nothing beats it. Only that tank has petrol in it, so the job is emptying the tank and cleaning your system where the car stands. This situation can only really happen at a forecourt: anywhere else, the car got there by being driven.
Started for a second, or just the ignition on. Once primed, your system has drawn petrol from your tank up to the fuel filter and your lines. Usually nothing more than clearing, rinsing and seeing to your filter, and so still straightforward.
Your pump has worked a couple of miles on diluted fuel — that's what driving it meant. The work is the tank, your lines, the fuel filter and fresh fuel through the whole system, and how hard the vehicle was driven counts as much as how far. On the school run to Little Heath Primary School, the distance is small. Driving as far as St Albans, under 10 miles away, makes a very different job — as does any mileage on the M25, under 3 miles from Little Heath, with the motor working hard the whole time.
Driven until it protested. Rattling, an uneven idle, smoke, a throttle that has gone soft. That's the motor telling you its fuel has stopped behaving like diesel. Pull up on the spot — on the verge on Church Road, at the kerb, down a side road — and do not move it. A vehicle stopped soon very rarely has long-term damage, because the drain and the flush-through, which still go first, sort it out.
Myths Little Heath drivers hear about petrol in a diesel
"Top it up the rest of the way with diesel and it will be fine" — a myth. It will not. A bigger tank of the same mixture is all it gives, since topping up removes nothing. Percentage is not how a diesel fuel system tolerates petrol, whatever that guidance presumes.
Myth: a current diesel copes with a bit of petrol. Rather, the reverse. More precise components and higher pressure are the way a current diesel runs its fuel, and that makes the lubrication matter more, not less.
The myth that catches drivers out: if it starts and drives, it must be unharmed. Diesel in a petrol motor usually leaves a motor that won't run: the driver stops, and so does the damage. A diesel fed petrol normally starts, idles and drives away as if nothing has happened, while the pump wears. Fine and running are not the same at all.
An old fix for a different fault is behind this one. Myth: "Add some two-stroke oil and drive it home." What petrol has thinned remains weakened, and the petrol stays inside the tank regardless.
If the engine has run, part of that petrol sits in your filter and the lines, not your tank — and your next start sends it right back through your pump. So the myth "drain the tank yourself and you're sorted" does not stand up. It is also really risky to drain petrol into a can at the kerbside.
How it happens: petrol in a diesel
It is rarely carelessness, and nobody does it on purpose. 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, while a diesel nozzle generally won't go into a petrol vehicle at all. So the mistake runs one way.
The moment habit stops matching your car is the moment it occurs. Hire cars, courtesy cars, and cars that are new to you. One petrol and one diesel on the drive of the same household. A well-known forecourt with the pumps in a different order. Fuelling up in a hurry en route to Potters Bar, 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 a back seat full of children, a phone call or rain, and the hand wins.
Avoiding petrol in a diesel next time you fill up around Little Heath
- Read the pump, not the colour. In the UK unleaded handles are usually green and diesel black, yet not on each forecourt. The label is what counts.
- The first fill in an unfamiliar motor is the time to check the filler flap — on it, or on the cap, many vehicles state the fuel to use.
- One of each in the house? Label your key to the diesel — a bright fob, a sticker — because intending to remember is not enough.
- Before the nozzle comes off the petrol pump the first time you fill a hired or new vehicle, say "diesel" out loud. Silly, perhaps, but it does the trick.
- Tuck the phone away. This is an easy mistake to make while you're thinking about something else.
- Add-on devices exist for cars without one, and some diesels are built with a filler that won't accept a petrol nozzle. Helpful — but a safety net at best, never a reason to stop checking the label.
- As soon as you realise, stop: less petrol inside that tank makes a weaker mix. What to do next doesn't change, though — still don't crank it.
Already put petrol in a diesel in Little Heath?
Then, there's just a short order of things. Do not switch the ignition on, and don't crank it. If you're still at the forecourt — BP or anywhere else — ask the attendants to help you push the vehicle away from the pump. What went in, about how much, and if it's run: tell us those when you ring 0333 366 0103.
AdBlue, not petrol? Then tell us. Corrosion and deposits, not lost lubrication, make it a separate fault — Little Heath has a page all for it.
One petrol in a diesel task around Little Heath costs more than another. Why?
The four situations above are four different tasks, so no single price for this would be honest. Which of them you are in is mostly what moves your cost. The proportion matters next, meaning how much of that tank is petrol, followed by the position of the car and whether reaching it is safe for a technician.
That's why we ask three queries before quoting instead of after: what was put in, how much, and whether it's run. Those answers change a range into a straight figure.
About Little Heath and its setting
Little Heath is a suburban area in Hertfordshire, situated approximately 13.7 miles from London and 7.6 miles from St Albans. Little Heath sits at an elevation of around 125 metres, with detached housing being the predominant type in the area. Little Heath is located near other places such as Swanley Bar, Potters Bar, and Northaw, and has a population of around 3290 residents.
Little Heath and Hertfordshire: where our fuel drain cover runs
The teams answering petrol in diesel jobs around Little Heath, in the EN postcode area, equally work the locations around it, Swanley Bar, Potters Bar and Northaw included. You need not bring your vehicle to us. Forecourts, supermarket car parks, lay-bys and your own driveway are all locations where we work.
Still at the pump around Little Heath?
In that case, you couldn't be in a better place. Call, and keep it switched off.