Should You Worry? Petrol in a Diesel Car in The Knab
Probably less than you already are — provided no one has started the vehicle. Everyone in The Knab begins with "have I ruined it?", and the honest answer is usually no: petrol left inside that tank of a diesel stays harmless until priming, cranking or driving draws it into the fuel system. Leave your engine off then phone Petrol in Diesel — that is how you keep it that way. The remainder of this page explains what petrol actually does to a diesel, how to judge how far it has got, and how to make sure it does not happen to you a second time.
Petrol in a diesel: what does it do to your engine?
On its way through an engine, diesel does two jobs. It ignites, and before it ignites it protects. Extremely fine clearances are built into the injectors and the high-pressure fuel pump, and only the diesel flowing through stops those surfaces grinding together.
Badly at the first, not at all at the second: that's petrol. It is runnier than diesel and takes away the protection the fuel system depends on, and so a pump running on a petrol blend 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 is where the knocking comes from.
Together, those make the shape of the problem clear. It is circulation that does the damage, not contact. Where that petrol is only in the tank, it's a problem of fuel, nothing more. The beginning of a mechanical problem is a pump that has been running on that petrol — and whatever a worn pump sheds travels on with the fuel to the injectors beyond it.
How serious is petrol in a diesel? Four situations our technicians see around The Knab
The best case there is: filled up, not once started. At a forecourt, and genuinely only there, that petrol is still limited to your tank — because everywhere else the vehicle was driven to reach it. The job is emptying and purging on the spot.
Started for a moment, or only the ignition on. Once primed, your system has pulled petrol out of that tank up to the filter and the fuel lines. Still generally straightforward: drain, flush, and the fuel filter dealt with.
Your pump has spent a few miles on thinned fuel — that is what driving the car meant. How hard the car was driven counts as much as how far, and this work takes in the tank, the fuel lines, the filter and fresh fuel through the whole circuit. A school run to the local primary school is one matter. Driving as far as Swansea, under 5 miles away, makes a very different job — as does any distance on the M4, under 10 miles from The Knab, with your engine running hard all the way.
Knocking, a lumpy idle, smoke, a throttle gone soft: driven till it complained. That's the engine telling you its fuel no longer is behaving like diesel. Pull up on the spot — on the verge on Clifton Terrace, at the kerb, down a side road — and don't move it. A car stopped soon very seldom has lasting damage, as the drain and the flush-through, which still come first, take care of it.
What The Knab motorists get told about petrol in a diesel — and the stories in it
Myth: "Fill it the rest of the way with diesel and it will be fine." It will not. Topping up takes out nothing; it gives you a fuller tank of the same mixture. A diesel fuel system doesn't accept petrol by percentage in the way that advice assumes.
Not so: a newer diesel copes with a bit of petrol. If anything, the opposite. At higher pressure, through finer components: that's how a current diesel handles its fuel, so the lubrication counts more, not less.
This is the one that catches motorists. Myth: "If it starts and drives, there is no harm done." Feed diesel to a petrol motor and it usually will not run, and so the driver stops — and the harm stops with them. Whilst the pump grinds, a diesel fed petrol usually starts, idles and drives away as though nothing has happened. Running is not the same as fine.
Myth: "Add some two-stroke oil and drive it home." It is an old fix for a distinct fault. The petrol is still inside that tank afterwards, and whatever it has thinned isn't put back.
The next start sends petrol back round the pump from your lines and the fuel filter, where some of it sits after the engine has run. That's why a DIY tank drain doesn't sort it — a myth. Besides, petrol emptied into a can by the road is truly risky.
Petrol in a diesel: how does it happen?
On purpose? Never. Through carelessness? Seldom, if ever. One way only, because of the nozzles: the slimmer petrol one slips inside a diesel filler with almost no resistance, while the diesel one mostly won't fit a petrol motor. That's how the forecourt makes it straightforward.
When routine and your car stop matching, it happens. Any motor that's new to you — hired, on loan as a courtesy car, or just bought. Two motors on one driveway, one petrol, one diesel. The pumps in a different order on a familiar forecourt. A rushed fill on the way to Swansea with a train to catch. Precisely the moment it occurs: after years of driving petrol, a driver fills a diesel Renault Megane for the first time. Throw in a back seat full of kids, a phone call or rain, and the hand takes over.
How to avoid petrol in a diesel at your next fill-up in The Knab
- Not every forecourt in the UK follows green for unleaded and black for diesel, even if most generally do. Check the pump's label; the colour is just a hint.
- The first fill in a new motor is the time to check your filler flap — on it, or on the cap, plenty of cars state which fuel to use.
- One of each in the house? Mark your key to your diesel — a bright fob, a sticker — since intending to remember is not enough.
- It seems silly and it does the trick: on your first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the petrol pump.
- While you are thinking about something else, this is an easy mistake to make — so tuck the phone away.
- Add-on devices are available for vehicles without one, and some diesels come with a filler that will not accept a petrol nozzle. Useful — but a safety net only, never a reason to stop checking the label.
- The moment you realise, stop: less petrol inside that tank means a weaker blend. What to do next does not change, though — still do not fire it up.
The Knab: already put petrol in a diesel?
Then, there's only a short order of things. Don't start it, and do not switch the ignition on. The staff on the forecourt — whether at Newton Garage itself or wherever you filled up — will usually help roll it away from the petrol pump if you ask. Then call 0333 366 0103 and tell us what got in, about how much, and whether it's run.
Say so if it was AdBlue rather than petrol: it is a different problem, with its own page for The Knab, since AdBlue brings corrosion and deposits rather than lost lubrication.
What makes one The Knab petrol in a diesel task cost more than the next
Four different jobs lie behind the four situations above — which is why there is no honest single price. Mostly, the cost shifts with which one you're in. Where your car is counts too, and whether a van can safely get to it, after the share of petrol to what the tank takes.
What went in, how much, and if it has run — three queries our engineers ask before pricing, not after, for precisely that reason. The replies turn a range into a straight figure.
About The Knab, briefly
The Knab is a coastal locality in Swansea, situated near Mumbles Hill and overlooking the Bristol Channel. The Knab sits at an elevation of 71 metres, with detached housing being the predominant type of accommodation in the area. The Knab is close to nearby places such as Limeslade and Thistleboon, and is approximately 4 miles from the city of Swansea.
Fuel drain cover across Swansea, The Knab included
Petrol in diesel call-outs around The Knab — part of the SA postcode area — go to teams that likewise cover the places round it, Mumbles Hill, Limeslade and Thistleboon among them. A forecourt, a supermarket car park, a lay-by or your own driveway are all places our technicians drain, and your car does not need to come to us.
The Knab: still at the petrol pump?
The best position there is, then — that is where you are. Keep the vehicle turned off, and ring.