Woodford: Petrol in a Diesel Car? How Worried to Be
Not started? Then less anxious than you probably are. Everyone around Woodford begins with "have I ruined it?", and the honest answer is usually no: petrol left inside the tank of a diesel stays harmless until priming, cranking or driving draws it into your fuel system. Petrol in Diesel is who to call, with the motor kept off, and so it remains that way. Further down, this page sets out what petrol actually does inside a diesel, how far it may have gone, and how to stop it happening to you again.
Inside the engine: what petrol in a diesel really does
On its way through a motor, diesel does two tasks. It ignites, and before it ignites it lubricates. What stops the surfaces inside the high-pressure pump and your injectors from wearing against each other is the diesel passing through them — they're built to very fine gaps.
The second job petrol doesn't do at all, and the first it does poorly. The knocking comes from petrol being made not to ignite when compressed, which is exactly the opposite of what a diesel needs. On top of that it is runnier than diesel, it strips away the protection the fuel system depends on, and any pump running on that mixture is wearing itself.
Put these together and the shape of the issue is clear. Not contact but circulation is what causes the damage. A tank with petrol in it is a fuel fault. A pump that has been running on that petrol is the beginning of a mechanical one, and whatever a worn pump sheds moves on with your fuel to the fuel injectors downstream.
How bad is petrol in a diesel? Four scenarios we see around Woodford
The best case there can be: filled up, never started. Only your tank has petrol in it, and so the job is emptying your tank and cleaning your system where your car sits. This scenario can only really take place at a forecourt: anywhere else, your car got there by being driven.
Ignition on, or started for a moment. The fuel system has been primed, so petrol has left the tank and got to the filter and the lines. Still normally straightforward: drain, flush, and the fuel filter dealt with.
Driven a few miles. Now the pump has spent those miles working on thinned fuel. The work is your tank, the fuel lines, the fuel filter and clean fuel through the entire system, and how hard the vehicle was driven counts as much as how far. On the school run to Woodford Church of England Primary School, the miles is small. A run as far as Peterborough, under 25 miles away, is a very different job. So is any distance on the A1(M), under 15 miles from Woodford, where the motor has been running hard throughout.
Driven till it protested. Clattering, a lumpy idle, smoke, a throttle that has gone soft. The fuel isn't behaving like diesel any more, and the motor is telling you. Pull up where you are — on the verge on Newtown, at the kerb, down a side road — and do not move it. The drain and flush still come first, and they sort it out: if a vehicle is pulled over soon, long-term damage is very uncommon.
What Woodford motorists get told about petrol in a diesel — and the stories in it
False: topping up to full with diesel makes it fine. It does not. Nothing comes out when you top up — that tank just carries more of the same mixture. Percentage isn't how a diesel fuel system copes with petrol, whatever that guidance assumes.
Myth: "A modern diesel can cope with a bit of petrol." If anything, the reverse. A modern diesel runs its fuel through more precise parts at higher pressure, which makes the protection matter more, not less.
Myth: "If it starts and drives, there's no damage done." This is the one that catches motorists. Diesel in a petrol motor usually means a motor that won't run: the motorist stops, and so does the damage. Whilst the pump grinds, a diesel fed petrol usually starts, idles and drives away as though nothing has happened. Being able to run isn't proof of being fine.
"A drop of two-stroke oil and drive it home" — a myth, borrowed from a fix from years ago, for a distinct problem. It does not put back what petrol has weakened, and the petrol is still inside your tank later.
"Drain it yourself and you're sorted" — a myth, since after it has been running some petrol lies in the filter and lines, ready to go straight back through the pump on the next start. Emptying petrol into a can at the kerbside is also truly hazardous.
Why petrol in a diesel takes place so easily
Carelessness is seldom the cause, and nobody means to do it. The petrol nozzle is the narrower 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 easy, and the slip runs one way.
The moment routine stops matching the vehicle is the moment it happens. Courtesy car, hire car, or a car that's new. A household with one petrol and one diesel on the driveway. Your normal forecourt, its pumps suddenly in a different order. Racing for a train outside Kettering and fuelling up en route. Exactly the moment it occurs: after years of driving petrol, a driver fills a diesel Volkswagen Golf for the first time. Throw in a back seat full of kids, a phone call or rain, and the hand takes over.
Woodford people: how to prevent petrol in a diesel next time
- Green for unleaded and black for diesel is usual in the UK — yet not on each forecourt. Go by the label on the pump, not the colour.
- Inspect your filler flap. Many cars state the fuel inside the flap or on the cap. Look at it on the first fill in any vehicle you do not know.
- A coloured fob or a sticker on your diesel's key achieves more than any amount of intending to remember, in a household with one of each.
- "Diesel", out loud, before you pick up the nozzle — each time a new or hired car gets its first tank. It sounds daft. It does the trick.
- Mistakes like this are straightforward to make while your mind is on something else, so the phone goes away.
- Some diesels come with a filler built to refuse a petrol nozzle, and devices you can add are available for vehicles without one. Such an inhibitor helps up to a point — as a safety net, not a reason to stop reading the petrol pump.
- Stop as soon as you realise. Less petrol in that tank makes a weaker blend. It does not change what to do next, which is still not to start it.
Put petrol in a diesel around Woodford already?
Short and simple, then, is the order of things. Don't switch the ignition on, and do not fire it up. If you're still on the forecourt — Shell or anywhere else — ask the staff to help you roll the vehicle away from the petrol pump. What got in, roughly how much, and if it's run: tell us those when you phone 0333 366 0103.
AdBlue, not petrol? Then say so. Corrosion and deposits, not lost protection, make it a different fault — Woodford has a page all for it.
One petrol in a diesel job in Woodford costs more than another. Why?
The four situations above are four different jobs, so no single quote for this would be honest. Which of them you're in is mostly what moves the cost. Where your car is matters too, and whether a van can safely get to it, after the ratio of petrol to what that tank holds.
What went in, how much, and whether it has run — three questions our engineers ask before costing, not after, for precisely that reason. A range turns into a straight figure once those answers are in.
About Woodford
Woodford is a small village in Northamptonshire, situated on the riverside at an elevation of 72 metres. Woodford lies near the villages of Woodwell, Twywell, and Great Addington, and is approximately 19 miles from Peterborough. Woodford has a population of around 569 people and features predominantly terraced housing. Woodford is also within commuting distance of London, being around 64 miles from the capital.
Woodford and Northamptonshire: where our fuel drain cover reaches
Woodwell, Twywell and Great Addington are among the places around Woodford that the same vans work, answering petrol in diesel call-outs across Woodford's NN postcode area. The car doesn't need to move to us: our technicians drain at a forecourt, in a supermarket car park, in a lay-by or on your own drive.
At the petrol pump in Woodford, right now?
Then you're in the best place there can be. Call, and leave it turned off.