Should You Worry? Petrol in a Diesel Car in Mottingham
Probably less than you already are — provided nobody has started the car. Petrol sitting in a diesel tank does nothing to the motor till something pulls it through the system — your ignition priming the pump, your starter, or the drive itself. So the honest answer to the first question anyone in Mottingham asks, "have I ruined it?", is usually no. To leave the car like that, call Petrol in Diesel and leave the engine switched off. What petrol actually does to a diesel, how to gauge how far it has gone, and how to keep it from happening again: that's the remainder of this page.
What petrol in a diesel does to your engine
Passing through an engine, diesel is doing two jobs at the same time. Before it combusts, it lubricates — and then it combusts. The high-pressure pump and your injectors are built to very fine gaps, and your diesel passing through them is what stops those surfaces from grinding against each other.
The second task petrol doesn't do at all, and the first it does poorly. It is thinner than diesel and takes away the protection the system depends on, 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 is where the knocking comes from.
The shape of the problem is plain once you put those together. The harm is done by movement, not by contact. Petrol in the tank is a fuel fault. Once your pump has been running on that petrol, a mechanical fault has started: whatever is shed by a pump that is wearing is carried onward to the injectors with the fuel.
Four situations our engineers see around Mottingham: how serious is petrol in a diesel?
Filled up, and not started: no case is better. Only your tank has petrol in it, and so the task is emptying your tank and cleaning the fuel system where the vehicle sits. This scenario can only really occur at a forecourt: anywhere else, the vehicle got there by being driven.
Started for a moment, or only your ignition on. Once primed, your system has drawn petrol out of that tank as far as your filter and the lines. That is normally still an easy job — a drain, a flush, and the filter seen to.
A few miles driven, and the pump has spent every one of them working on diluted fuel. The work is your tank, your lines, the fuel filter and clean fuel through the whole circuit, and how hard your car was driven counts as much as how far. On the school run to St Vincent's Catholic Primary School, the miles is small. Any miles on the M20, under 10 miles from Mottingham, means the engine working hard throughout, and a run to London, under 10 miles away, makes a very different task as well.
Driven until it complained. Clattering, a lumpy idle, smoke, a throttle that has gone soft. The fuel is not acting like diesel any more, and your engine is saying so. Stop on the spot — on the verge on Dunkery Road, at the kerb, down a side road — and do not move it again. The fuel drain and flush still come first, and they deal with it: when a car is pulled over quickly, permanent damage is very unusual.
What Mottingham motorists get told about petrol in a diesel — and the myths in it
It will not be fine. The myth: "Fill it the rest of the way with diesel and it'll be." Nothing comes out when you top up — the tank simply holds more of the same blend. That guidance supposes a diesel fuel system tolerates petrol by percentage. It does not.
Myth: "A newer diesel can cope with a bit of petrol." If anything, the reverse. Protection matters more on modern diesels, not less, because their fuel runs through smaller parts at higher pressure.
Myth: "If it starts and drives, there's no damage done." This is the one that catches motorists. A petrol motor fed diesel generally will not run, so the motorist stops and the harm stops with them. A diesel fed petrol usually starts, idles and drives away as if nothing has happened, while the pump wears. Fine and running are not the same at all.
Myth: a two-stroke additive lets you to drive home. It was an old fix, meant for another problem. It does not put back what petrol has diluted, and that petrol is still inside your tank later.
Myth: "Drain your tank yourself and you are sorted." After the engine has been run, some of that petrol is no longer inside that tank: it is in the filter and the fuel lines, and it goes straight back through your pump on the next start. It is also really hazardous to drain petrol into a can by the kerbside.
How petrol ends up: petrol in a diesel
On purpose? Never. Through carelessness? Rarely, if ever. The petrol nozzle is the smaller of the two and slides into a diesel filler with almost no resistance; a diesel nozzle generally will not go into a petrol motor at all. The forecourt makes it simple, and the error runs one way.
When routine and your car stop matching, it takes place. Hire cars, courtesy cars, and motors that are new to you. A household with one petrol and one diesel on the driveway. A familiar forecourt with the pumps in a different order. A train to catch, and a rushed fill on the way to the nearest station. The first fill-up in a diesel Renault Megane after years of driving petrol is precisely the moment it takes place. Throw in a back seat full of kids, a phone call or rain, and the hand takes over.
How to avoid petrol in a diesel on your next fill-up around Mottingham
- The label is what counts, so read the pump rather than the colour: in the UK unleaded handles are usually green and diesel black, yet not on each forecourt.
- The first fill in a new vehicle is the time to check the filler flap — inside it, or on the cap, plenty of vehicles state which fuel to use.
- Good intentions count for less than a sticker or a fob in a vivid colour on your diesel's key when a house has one of each. So label it.
- Say it before you lift it. On the first fill in a new or hired car, say "diesel" aloud before you lift the nozzle off the petrol pump. It seems silly and it works.
- Put the phone away. This is an easy mistake to make whilst you are thinking about something else.
- Add-on devices are available for cars without one, and some diesels are built with a filler that won't accept a petrol nozzle. Useful — but a backstop at best, never a reason to stop checking the label.
- Less petrol makes a thinner mix, so stop the moment you realise — though remembering that your next stage is unchanged. It is still not to crank it.
Put petrol in a diesel in Mottingham already?
Short and simple, then, is the order of things. Do not switch your ignition on, and do not start it. If you are still on the forecourt — BP or anywhere else — ask the staff to help you roll the vehicle away from the pump. What went in, roughly how much, and if it has run: tell us those when you phone 0333 366 0103.
If it was AdBlue and not petrol, say so. That's a separate issue — corrosion and deposits rather than lost lubrication — and it has its own page for Mottingham.
One petrol in a diesel task around Mottingham costs more than another. Why?
Four different jobs sit behind the four scenarios above — which is why there is no fair single price. What shifts the cost is chiefly which of them you are in. The ratio matters next, meaning how much of your tank is petrol, followed by the position of the vehicle and whether reaching it is safe for a van.
What got in, how much, and if it has run — three questions we ask before quoting, not after, for exactly that reason. Those replies are what change a range into one firm number.
About Mottingham, for a moment
Mottingham is a suburban area in southeast London, situated near the River Ravensbourne at an elevation of around 57 metres. Mottingham lies approximately 9 miles from central London and is bordered by nearby places such as Chinbrook, Grove Park, and Elmstead. Mottingham has a population of around 10235 people and is predominantly composed of terraced housing. Mottingham is characterized by its residential streets and local amenities.
Mottingham and Greater London: where our fuel drain cover reaches
Our teams answering petrol in diesel jobs around Mottingham, in the SE postcode area, equally work the places near it, Chinbrook, Grove Park and Elmstead included. The vehicle doesn't need to come to us: our engineers drain on a forecourt, in a supermarket car park, in a lay-by or on your own driveway.
Mottingham: still at the petrol pump?
In that case, you couldn't be in a better position. Turned off it remains; then call.