Freestones Corner: Petrol in a Diesel Car? How Worried to Be
Not started? Then less worried than you likely feel. Everyone around Freestones Corner starts with "have I ruined it?", and the fair answer is usually no: petrol left inside your tank of a diesel stays harmless till priming, cranking or driving draws it into the fuel system. Leave the motor off and call Petrol in Diesel — that keeps it that way. The remainder of this page sets out what petrol really does to a diesel, how to judge how far it has got, and how to make sure it doesn't happen to you a second time.
Petrol in a diesel: what does it do to the motor?
Diesel does two jobs on its way through a motor. Before it burns, it lubricates — and then it burns. The high-pressure fuel pump and the injectors are built to extremely fine gaps, and the diesel passing through them is what keeps those surfaces from wearing against each other.
The second task petrol doesn't do at all, and the first it does poorly. The clattering comes from petrol being designed not to ignite when compressed, which is precisely the reverse of what a diesel needs. On top of that it is runnier than diesel, it strips out the protection the system hinges on, and any pump working on that mix is wearing itself.
Taken together, they make clear the shape the issue has. Contact doesn't do the damage — movement does. Petrol in the tank is a fuel fault. Whatever a worn pump sheds travels on, borne by your fuel, into your injectors further along, which is why a pump that has been working on that petrol is where a mechanical problem begins.
How serious is petrol in a diesel? Four scenarios our technicians see around Freestones Corner
Filled but never started — as good as it gets. On a forecourt, and really only there, the petrol is still confined to that tank — because anywhere else the vehicle was driven to reach it. The task is draining and cleaning on the spot.
Ignition on, or started for a second. Once primed, your system has pulled petrol from your tank as far as the filter and the fuel lines. Still usually straightforward: drain, flush, and your filter seen to.
A few miles driven, and the pump has spent each of them labouring on weakened fuel. Distance is only half of it; how hard it was driven counts just as much. Either way the work includes that tank, the lines, the filter and clean fuel through the whole circuit. Short trips, such as the school run to Great and Little Shelford CofE (Aided) Primary School, are one thing. Getting as far as Cambridge, under 5 miles away, makes a very different job — as does any miles on the M11, under 3 miles from Freestones Corner, with the motor labouring hard all the way.
Driven on until it complained — clattering, smoke, an uneven idle, a dead throttle. What you are hearing is the engine reporting fuel that no longer runs like diesel. Leave it where it has stopped: a lay-by, a side street, the verge on Tunwells Lane. Long-term damage is very unusual if a vehicle is pulled over soon, and the fuel drain and flush that still come first sort it out.
Petrol in a diesel myths Freestones Corner motorists hear
Myth: topping up to full with diesel makes it fine. It does not. Nothing is removed by topping up — you're left with a fuller tank, still the same mixture. That advice supposes a diesel fuel system handles petrol by percentage. It does not.
Not so: a modern diesel copes with a bit of petrol. If anything, the reverse. Protection matters more on current diesels, not less, because their fuel runs through finer components at higher pressure.
"It starts, it drives, so no harm done" — the myth that catches more people than any other. A petrol motor fed diesel generally will not run, and so the driver stops and the damage stops with them. Feed petrol to a diesel and it generally starts, ticks over and drives away like nothing has happened — all while its pump wears. Fine and running are not the same at all.
Myth: a two-stroke additive makes it safe to drive home. It is an old fix, supposed for another issue. It doesn't put back what petrol has thinned, and the petrol is still inside your tank afterwards.
"Drain it yourself and you're sorted" — a myth, because after it has been running some petrol lies in the filter and lines, waiting to go right back through the pump at your next start. Also, a can of petrol filled by the roadside is truly hazardous.
How does petrol in a diesel happen?
A careless moment is seldom the cause, and no one means to do it. A diesel nozzle generally will not fit a petrol motor, but the smaller petrol nozzle goes into the filler of a diesel almost without resistance — so forecourts make this one mistake easy, and just in that direction.
When habit and the car stop matching, it happens. Any motor that's new to you — hired, on loan as a courtesy car, or brand new. One petrol and one diesel on the drive of the same household. Your normal forecourt, its pumps now in a different order. Racing for a train outside Shelford and fuelling up en route. A new-to-you diesel Volkswagen Passat, its first fuelling, and a motorist accustomed to petrol: precisely when it happens. Add rain, a phone call or a back seat full of children, and the hand does what it has always done.
Avoiding petrol in a diesel next time you fill up in Freestones Corner
- Green for unleaded and black for diesel is usual in the UK — but not on each forecourt. Go by the label on the petrol pump, not the colour.
- The first fill in an unfamiliar motor is the time to check your filler flap — on it, or on the cap, plenty of cars state the fuel they take.
- Good intentions count for less than a sticker or a fob in a vivid colour on the diesel's key if a house has one of each. So label it.
- Before the nozzle leaves the petrol pump the first time you fill a hired or new vehicle, say "diesel" aloud. Daft, perhaps, but it works.
- Thinking about something else is when this becomes an easy mistake to make, and so tuck your phone away then keep your mind on the pump.
- Add-on devices are available for motors without one, and some diesels come with a filler that will not take a petrol nozzle. Helpful — but a safety net only, never a reason to stop checking the label.
- Less petrol makes a weaker mix, and so stop as soon as you notice — while remembering that the next stage is unchanged. It's still not to crank it.
Petrol already in a diesel in Freestones Corner? Start here
Then, there's only a short order of things. Do not fire it up, and do not switch the ignition on. If you are still at the forecourt — Esso or anywhere else — ask the staff to help you roll the car clear of the pump. Then ring 0333 366 0103 and tell us what was put in, about how much, and whether it has run.
If it was AdBlue rather than petrol, say so. That's a separate issue — corrosion and deposits instead of lost protection — and it has its own page for Freestones Corner.
Freestones Corner petrol in a diesel jobs: why one costs more than another
The four scenarios above are four different jobs, so no single quote for this would be fair. What moves the cost is chiefly which of them you're in. Next it's the share — how much petrol went in against how much the tank holds — and after that where the car is and whether a technician can reach it safely.
That is why we ask three questions before pricing instead of after: what got in, how much, and whether it's run. Those answers turn a range into a straight figure.
About Freestones Corner
Freestones Corner is a locality situated on the riverside in Cambridgeshire, with an elevation of around 21 metres. Freestones Corner lies near the city of Cambridge, approximately 4.1 miles from the city centre, and is close to the villages of Great Shelford, Stapleford, and Little Shelford. Freestones Corner has a population of around 2428 people and is characterised by predominantly detached housing.
Freestones Corner and Cambridgeshire: where our fuel drain cover runs
Petrol in diesel jobs around Freestones Corner — part of the CB postcode area — go to teams that also cover the places round it, Great Shelford, Stapleford and Little Shelford among them. The car does not need to come to us: we drain on a forecourt, in a supermarket car park, in a lay-by or on your own drive.
Freestones Corner: still at the petrol pump?
Nothing beats it: that's where you now are. Keep it off. Ring.