Fleet-Downs: Petrol in a Diesel Car? How Worried to Be
Likely less than you are now — provided nobody has started the vehicle. "Have I ruined it?" is the first question anyone in Fleet-Downs asks, and the truthful answer is normally no, since petrol left in a diesel tank hurts nothing until it's dragged through your fuel system — by ignition priming, by your starter, or by driving. Petrol in Diesel is who to phone, with the motor kept off, so it remains that way. Further down, this page sets out what petrol really does inside a diesel, how far it may have gone, and how to stop it happening to you again.
Your engine and petrol in a diesel: what happens
Diesel has two jobs on its way through an engine. Before it ignites, it protects — and then it burns. Very fine clearances are built into your injectors and the high-pressure pump, and just the diesel running through keeps those surfaces from grinding together.
Petrol does the first job badly and the second not at all. It's thinner than diesel and strips away the lubrication the fuel system rests on, so a pump running on a petrol mixture 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 clattering comes from.
Combined, those make the shape of the issue clear. Contact doesn't do the damage — circulation does. Petrol in a tank is a fuel fault. A pump that has been working on that petrol is the beginning of a mechanical one, and whatever a worn pump sheds travels on with your fuel to your injectors further on.
How bad is petrol in a diesel? Four scenarios we see around Fleet-Downs
The best case there is: filled up, not once started. On a forecourt, and truly only there, the petrol is still confined to your tank — since anywhere else the vehicle was driven to reach it. The job is emptying and purging on the spot.
Your ignition switched on, or a brief start. Petrol has come out of your tank and reached the filter and the lines, because your system has been primed. Normally little more than draining, purging and sorting your filter, so still easy.
Driven a few miles. Now the pump has spent these miles labouring on diluted fuel. Tank, lines, filter, then clean fuel run through the whole circuit: that is this work, and the way the car was driven matters as much as the miles. On the school run to Fleetdown Primary Academy, the mileage is small. Any miles on the M25, under a mile from Fleet-Downs, means your engine working hard throughout, and a run to London, under 25 miles away, is a very different task as well.
Knocking, an uneven idle, smoke, a throttle gone soft: driven until it complained. The fuel isn't acting like diesel any more, and the motor is saying so. Stop where you are — on the verge on Hesketh Avenue, at the kerb, down a side road — and do not move it. Permanent damage is very rare if a vehicle is stopped early, and the fuel drain and flush-through that still come first take care of it.
Petrol in a diesel stories Fleet-Downs drivers hear
Myth: "Fill it the rest of the way with diesel and it'll be fine." It will not. Nothing is removed by topping up — you're left with a fuller tank, still the same mixture. Percentage is not how a diesel fuel system tolerates petrol, however much that guidance presumes.
Myth: "A modern diesel can cope with a bit of petrol." If anything, the opposite. Protection counts more on current diesels, not less, because their fuel runs through smaller parts at higher pressure.
Myth: "If it starts and drives, there is no harm done." This is the one that fools motorists. The driver of a petrol motor fed diesel normally has no choice but to stop, since it will not run, and the damage stops with them. A diesel fed petrol usually starts, idles and drives away as though nothing has happened, while your pump wears. Fine and running are not the same thing.
An old workaround for a distinct fault lies behind this one. Myth: "Add some two-stroke oil and drive the car home." None of what petrol diluted is put back by it — and afterwards, your tank still contains the petrol.
Myth: "Drain the tank yourself and you're sorted." After your engine has been run, some of the petrol is no longer in that tank: it is in the filter and the fuel lines, and it goes right back through the pump on the next start. Emptying petrol into a can at the kerbside is also truly dangerous.
Petrol in a diesel: how does it happen?
Carelessness is rarely the cause, and no one means to do it. A diesel nozzle mostly will not fit a petrol motor, yet the slimmer petrol nozzle goes into the filler of a diesel nearly without resistance — so forecourts make this one error straightforward, and just in that direction.
When habit and your car stop matching, it takes place. A new car, a hire car or a courtesy car. A household with one petrol and one diesel on the driveway. Your normal forecourt, its pumps now in a different order. A train to catch, and a rushed fill en route to Dartford. Years of petrol motors behind you, a diesel Skoda Octavia in front of you at the forecourt for its first tank — that's when. Throw in a back seat full of kids, a phone call or rain, and habit wins.
How to avoid petrol in a diesel on your next fill-up in Fleet-Downs
- Read the petrol pump, not the colour. In the UK unleaded handles are usually green and diesel black, but not on every forecourt. The label is what matters.
- The first fill in a strange car is the time to check the filler flap — on it, or on the cap, many motors state the fuel they take.
- Label the key. In a house with one of each, a coloured fob or a sticker on your diesel's key does more than any amount of intending to remember.
- Before the nozzle comes off the pump the first time you fill a hired or new vehicle, say "diesel" aloud. Daft, yes, yet it works.
- While you are thinking about something else, this is an easy mistake to make — so put your phone away.
- A misfuel inhibitor helps, up to a point. Some diesels have a filler built to block a petrol nozzle, and add-on devices exist for motors without one. They are a safety net, not a reason to stop checking the petrol pump.
- The moment you realise, stop: less petrol inside the tank means a weaker mix. What to do next does not change, though — still don't start the car.
Petrol already in a diesel in Fleet-Downs? Start here
Short and simple, then, is the order of things. Don't switch your ignition on, and do not crank it. Ask the staff to help you roll it clear of the pump if you're still on the forecourt, whether that's BP or anywhere else. What was put in, about how much, and if it has run: tell us that when you call 0333 366 0103.
Say so if it was AdBlue instead of petrol: it is a separate problem, with its own page for Fleet-Downs, since AdBlue brings corrosion and deposits instead of lost lubrication.
One petrol in a diesel job in Fleet-Downs costs more than another. Why?
An honest single price does not exist here: four situations, four separate jobs. Which of them you're in is mostly what moves your cost. The proportion matters next, meaning how much of your tank is petrol, followed by the position of your car and whether getting to it is safe for our van.
That's why our engineers ask three queries before quoting instead of after: what got in, how much, and whether it's run. The replies turn a range into a straight figure.
About Fleet-Downs
Fleet-Downs is a suburban area in Kent, situated inland at an elevation of 48 metres, and is located approximately 16.6 miles from London. Fleet-Downs forms part of the wider commuter belt around London, with nearby places including Darenth Valley, New Town, and Brooklands. Fleet-Downs has a population of around 6268 people, predominantly living in semi-detached housing.
Fuel drain cover across Kent, Fleet-Downs included
Petrol in diesel jobs around Fleet-Downs — part of the DA postcode area — go to teams that likewise cover the locations round it, Darenth Valley, New Town and Brooklands among them. You need not move your car to us. Forecourts, supermarket car parks, lay-bys and your own drive are all locations we drain.
Still at the pump around Fleet-Downs?
The best position there is, then — that's where you are. Ring, and leave it turned off.