Britwell: Petrol in a Diesel Car? How Worried to Be
Far less than you probably are, if it has not been started. Everyone in Britwell starts with "have I ruined it?", and the honest answer is generally no: petrol resting inside the tank of a diesel stays harmless till priming, cranking or driving draws it into your fuel system. Keep it that way: leave your engine off then ring Petrol in Diesel. The remainder of this page explains what petrol actually does to a diesel, how to tell 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 motor?
Diesel has two tasks on its way through an engine. It lubricates to begin with, and only then does it burn. Designed to extremely fine tolerances, your injectors and the pump rely on the diesel moving through them to keep their surfaces from wearing against one another.
Poorly at the first, not at all at the second: that is petrol. The knocking comes from petrol being made not to ignite when compressed, which is precisely the reverse of what a diesel needs. On top of that it is thinner than diesel, it strips out the protection your system rests on, and any pump running on that mix is wearing itself.
Taken together, they make clear the shape the trouble has. It is circulation that causes the harm, not contact. A tank with petrol in it is a fuel problem. A pump that has been running on that petrol is the start of a mechanical one, and whatever a worn pump sheds travels on with your fuel to the injectors further on.
How serious is petrol in a diesel? Four scenarios our engineers see around Britwell
Filled up, not once started. The best case there can be. At a forecourt, and genuinely only there, that petrol is still limited to your tank — since anywhere else the car was driven to get to it. The job is emptying and purging on the spot.
Started for a moment, or just your ignition on. Petrol has left your tank and reached the filter and the lines, since your system has been bled. That is usually still an easy job — a drain, a flush, and your filter dealt with.
The pump now spent a couple of miles on diluted fuel — that is what driving the car does. How hard the car was driven counts as much as how far, and this work takes in your tank, the lines, the filter and clean fuel through the whole circuit. The school run to Lynch Hill School Primary Academy is one thing. A run as far as St Albans, under 25 miles away, is a very different task. So is any distance on the Huntercombe Spur, under 3 miles from Britwell, where your engine has been running hard throughout.
Driven until it protested. Knocking, a ragged idle, smoke, a throttle that has gone soft. What you are hearing is your engine signalling fuel that no longer behaves like diesel. Leave it where it stops: a lay-by, a quiet street, the verge on Pemberton Road. Long-term damage is very uncommon if a car is stopped soon, and the fuel drain and flush-through that still go first deal with it.
Myths Britwell motorists hear about petrol in a diesel
It will not be fine. The myth: "Fill it the rest of the way with diesel and it'll be." Topping up takes out nothing; it gives you a bigger tank of the same mix. The assumption behind the advice — that diesel fuel systems tolerate petrol by percentage — is wrong.
Myth: "A modern diesel can cope with a bit of petrol." If anything, the reverse. Protection counts more on current diesels, not less, since their fuel runs through more precise components at higher pressure.
One myth that catches drivers out: if it starts and drives, it must be unharmed. Feed diesel to a petrol motor and it normally won't run, so the driver stops — and the harm stops with them. A diesel fed petrol usually starts, ticks over and drives away as though nothing has happened, whilst the pump wears. Running doesn't mean fine.
Myth: "Add some two-stroke oil and drive the vehicle home." It's an outdated fix for a different fault. It does not put back what petrol has diluted, and that petrol is still inside your tank after that.
The next start sends petrol back round the pump from the fuel lines and the filter, where some of that ends up after the engine has run. That is why draining the tank yourself does not sort it — a myth. On top of that, petrol drained into a can by the road is genuinely hazardous.
Why petrol in a diesel takes place at all
On purpose? Never. Through carelessness? Rarely, if ever. A diesel nozzle generally will not fit a petrol motor, but the narrower petrol nozzle goes into the filler of a diesel nearly without resistance — so forecourts make this one error easy, and only in one direction.
The moment habit stops matching your car is the moment it occurs. Courtesy car, hire car, or a vehicle that is new. One petrol and one diesel on the drive of the same household. A familiar forecourt with the pumps in a different order. Rushing for a train at Burnham and fuelling up en route. Exactly the moment it happens: after years of driving petrol, a driver fills a diesel BMW 3 Series for the first time. Throw in a back seat full of children, a phone call or rain, and the hand wins.
Next time you fill up around Britwell: avoiding petrol in a diesel
- Read the petrol pump, not the colour. In the UK unleaded handles are normally green and diesel black, but not on each forecourt. The label is what counts.
- Inspect your filler flap. Plenty of vehicles state the fuel type inside the flap or on the cap. Look at it on the first fill in any motor you don't know.
- Good intentions count for less than a sticker or a fob in a vivid colour on your diesel's key when a household has one of each. So label it.
- It seems daft and it does the trick: on your first fill in a new or hired car, say "diesel" aloud before you lift the nozzle off the petrol pump.
- Put the phone away. This is an easy mistake to make whilst you are thinking about something else.
- A misfuel inhibitor helps, up to a point. Some diesels have a filler designed to reject a petrol nozzle, and add-on devices exist for vehicles without one. They are a backstop, not a reason to stop reading the petrol pump.
- Less petrol means a thinner mixture, so stop the moment you notice — though remembering that your next step is unchanged. It's still not to fire it up.
Britwell: already put petrol in a diesel?
The order of things is brief, then. Leave it unstarted, with your ignition off too. Yet at the forecourt, at Shell or anywhere else? Ask the attendants to help you push it away from the petrol pump. What got in, roughly how much, and if it's run: tell us those when you call 0333 366 0103.
If it was AdBlue and not petrol, say so. That is a different issue — corrosion and deposits instead of lost lubrication — and it has its own page for Britwell.
What makes one Britwell petrol in a diesel job cost more than the next
There is no honest single price for this, since the four situations above are four different jobs. What moves your cost is chiefly which of them you're in. Two things come after that: the petrol that was put in as a proportion of what your tank holds, then the spot the vehicle is in and whether a technician can reach it safely.
Before any quote, not after, come three queries: your fuel that was put in, the volume, and if it has run. That's why. With those replies, there is a straight figure rather than a range.
About Britwell
Britwell is a suburban area in Berkshire, situated inland at an elevation of 44 metres, and is located approximately 22 miles west of London. Britwell is characterized by its predominantly terraced housing and is near several other suburban areas, including Farnham Royal and Burnham. Britwell sits within the wider commuter belt of London, with nearby cities like St Albans also influencing the local economy. Britwell is part of a network of suburban communities in the South East of England, with its own distinct residential identity.
Britwell and Berkshire: where our fuel drain cover runs
Britwell rests in the SL postcode area, and the vans that answer petrol in diesel calls there work the places near it as well — Farnham Royal, Burnham and East Burnham among them. A forecourt, a supermarket car park, a lay-by or your own drive are all places our technicians work, and the vehicle does not need to move to us.
Still at the pump around Britwell?
The best place there is, then — that is where you are. Keep it off. Phone.