Petrol in a Diesel Car in Swiss Cottage: How Worried Should You Be?
Not started? Then less anxious than you probably feel. Petrol resting in a diesel tank does nothing to the engine till something draws it through the fuel system — the ignition priming the pump, your starter, or the drive itself. So the fair answer to the first query everyone around Swiss Cottage asks, "have I ruined it?", is generally no. To leave the vehicle that way, call Petrol in Diesel and leave the engine alone. The rest of this page sets out what petrol actually does to a diesel, how to tell how far it has got, and how to make sure it doesn't happen to you again.
Inside the motor: what petrol in a diesel really does
On its way through a motor, diesel does two jobs. It combusts, and before it combusts it protects. Built to extremely fine gaps, your injectors and the pump rely on the diesel flowing through them to stop their surfaces wearing against one another.
Of those two tasks, petrol handles the first badly and the second not at all. It's runnier than diesel and takes away the protection the system depends on, and 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 opposite of what a diesel needs from its fuel. That's where the rattling comes from.
Seen together, they make clear the shape the issue takes. Not contact but circulation is what causes the damage. Where the petrol is only in that tank, it's a fault of fuel, nothing more. Once your pump has been working on that petrol, a mechanical issue has begun: whatever is shed by a pump that is wearing down travels downstream to the injectors with the fuel.
Four situations we see around Swiss Cottage: how serious is petrol in a diesel?
Fuelled up, never started. The best case there is. The petrol is inside the tank and only the tank, and the task is a drain and a flush where the vehicle sits. This is the one situation that can only really happen at a forecourt, since anywhere else the vehicle has been driven to get there.
A brief spell of running, or merely your ignition on. Once primed, the system has pulled petrol out of your tank up to the filter and your lines. Drain, flush, see to the filter: still usually straightforward.
Driven a few miles. Now your pump has spent these miles running on weakened fuel. How hard your car was driven matters as much as how far, and the work takes in your tank, the lines, the fuel filter and clean fuel through the whole system. On the school run to Holy Trinity CofE Primary School, NW3, the mileage is small. Driving as far as Westminster, under 5 miles away, makes a very different job — as does any mileage on the M1, under 5 miles from Swiss Cottage, with the motor labouring hard all the way.
Driven on until it complained — knocking, smoke, a lumpy idle, a dead throttle. What you are hearing is your motor reporting fuel that no longer behaves like diesel. Stop on the spot — on the verge on Avenue Road, at the kerb, down a side road — and don't move it again. A car stopped early very seldom has long-term damage, as the drain and the flush-through, which still come first, take care of it.
Swiss Cottage motorists and the petrol in a diesel stories they hear
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 fuller tank of the same mix. The idea behind that advice — that diesel fuel systems accept petrol by percentage — is wrong.
"Newer diesels can handle a little petrol" — a myth, and closer to the reverse. Lubrication counts more on current diesels, not less, since their fuel runs through more precise parts at higher pressure.
The myth that catches motorists out: if it starts and drives, it must be unharmed. Diesel in a petrol motor usually means a motor that won't run: the driver stops, and so does the damage. Whilst the pump grinds, a diesel fed petrol usually starts, ticks over and drives away as though nothing has happened. Fine and running are not the same thing.
An outdated workaround for a distinct problem is behind this one. Myth: "Add some two-stroke oil and drive the car home." The petrol is still in your tank after that, and what it has weakened is not put back.
The next start draws petrol back round the pump out of the fuel lines and the filter, where some of that ends up once your engine has run. That's why draining your tank yourself doesn't leave you sorted — a myth. Also, a can of petrol drained at the roadside is genuinely risky.
How petrol ends up: petrol in a diesel
On purpose? Never. Through carelessness? Rarely, if ever. The forecourt makes it easy: the petrol nozzle is the slimmer of the two, and it slips into a diesel filler with almost no resistance, whilst a diesel nozzle mostly will not go into a petrol car at all. So the error runs one way.
It occurs when routine and the car stop matching. Hire cars, courtesy cars, and cars that are new to you. A household with one petrol and one diesel on the drive. The pumps in a different order on a familiar forecourt. A train to catch, and a hurried fill en route to South Hampstead. Years of petrol vehicles behind you, a diesel Renault Megane in front of you at the forecourt for its first tank — that is when. Throw in a back seat full of children, a phone call or rain, and habit wins.
How to prevent petrol in a diesel on your next fill-up in Swiss Cottage
- Check the petrol pump, not the colour. In the UK unleaded handles are normally green and diesel black, yet not on every forecourt. The label is what counts.
- In a vehicle you don't know, look inside the flap over your filler or at the cap before the first fill: plenty of motors print the fuel type there.
- Label your key. In a home with one of each, a coloured fob or a sticker on your diesel's key does more than any amount of meaning to remember.
- "Diesel", aloud, before you lift the nozzle — every time a new or hired car gets its first tank. It sounds silly. It works.
- While you're thinking about something else, this is an easy mistake to make — so tuck your phone away.
- Some diesels have a filler built to block a petrol nozzle, and devices you can add exist for vehicles without one. Such an inhibitor helps up to a point — as a backstop, not a reason to stop checking the petrol pump.
- Less petrol makes a thinner mix, so stop as soon as you realise — while remembering that the next stage is unchanged. It's still not to start the car.
Petrol already in a diesel in Swiss Cottage? Start here
Short and simple, then, is the order of things. Don't crank it, and do not switch your ignition on. Ask the staff to help you push it away from the pump if you are still on the forecourt, whether that's BP or somewhere else. Phone 0333 366 0103 next, with three answers — your fuel, the rough quantity, and whether it has run.
AdBlue, not petrol? Then say so. Corrosion and deposits, not lost protection, make it a separate problem — Swiss Cottage has a page just for it.
Why one petrol in a diesel job in Swiss Cottage costs more than another
The four scenarios above are four different tasks, so no single price for this would be honest. The situation itself does most of the work of moving the cost. The ratio matters next, meaning how much of the tank is petrol, and then the position of the vehicle and whether getting to it is safe for a van.
That is why our engineers ask three queries before pricing rather than after: what was put in, how much, and if it's run. Given the replies, there is a straight figure rather than a range.
About Swiss Cottage
Swiss Cottage is a suburban area in northwest London, situated near the River Thames and the Regent's Canal. Swiss Cottage sits at an elevation of 59 metres, close to the city centre and neighbouring areas such as South Hampstead and Belsize Park. Swiss Cottage is predominantly composed of flat dwellings, accommodating a population of approximately 17,250 residents. Swiss Cottage is well connected to central London, being just 3.1 miles from the city centre and 3.5 miles from Westminster.
Swiss Cottage and Greater London: where our fuel drain cover runs
Swiss Cottage sits in the NW postcode area, and our vans that answer petrol in diesel calls there work the places around it as well — South Hampstead, Belsize Park and St Johns Wood among them. We drain where your vehicle is — your own driveway, a lay-by, a supermarket car park, a forecourt — so it never needs to come to us.
At the petrol pump around Swiss Cottage, right now?
Then you're in the best spot there can be. Keep the vehicle switched off, then ring.