Should You Worry? Petrol in a Diesel Car in Downside
If it has not been started, you can worry far less than you likely are. Everyone in Downside begins with "have I ruined it?", and the fair answer is generally no: petrol resting inside the tank of a diesel remains harmless till priming, cranking or driving draws it into your fuel system. To keep the car that way, ring Petrol in Diesel and leave the motor switched off. What petrol actually does to a diesel, how to judge how far it has travelled, and how to stop it from happening again: that is the remainder of this page.
Your engine and petrol in a diesel: what happens
On its way through an engine, diesel has two jobs. It combusts, and before it burns it lubricates. The high-pressure pump and the injectors are designed to extremely fine gaps, and your diesel passing through them is what keeps those surfaces from wearing against one another.
Of those two jobs, petrol manages the first poorly and the second never. A pump working on a petrol mix is wearing itself out, because petrol is runnier than diesel and strips out the lubrication your fuel system relies on. Petrol is also made to resist igniting under compression — the reverse of what a diesel needs from its fuel — and that's where the rattling starts.
Combined, those leave the shape of the issue clear. The damage is done by circulation, not by contact. Any tank with petrol in it is a fuel issue. Whatever a worn pump sheds goes on, borne by the fuel, into your injectors further along, which is why a pump that has been working on the petrol is where a mechanical problem starts.
Four scenarios our technicians see around Downside: how bad is petrol in a diesel?
Filled but not once started — the best it gets. Being at a forecourt is what makes this possible: elsewhere, the car has been driven there to begin with. With the petrol kept inside the tank alone, it's emptied and flushed right where it stands, and nothing more is needed.
A brief spell of running, or just the ignition on. Once primed, the fuel system has pulled petrol out of your tank as far as the filter and the lines. That's normally still a straightforward task — a drain, a flush, and your filter dealt with.
A couple of miles driven, and your pump has spent each of them working on thinned fuel. Miles is only half of that; how hard it was driven counts just as much. Either way this work includes your tank, the fuel lines, the filter and fresh fuel through the entire circuit. The school run to St Mary's Catholic Primary School is one matter. A run as far as St Albans, under 15 miles away, is a very different job. So is any miles on the M1, under 3 miles from Downside, where your engine has been labouring hard throughout.
It complained, and only then stopped: smoke, clattering, a throttle gone soft, an idle gone ragged. What you are hearing is the motor signalling fuel that no longer runs like diesel. Pull up where you are — on the verge on Morcom Road, at the kerb, down a side road — and don't move it again. A vehicle pulled over early very rarely has lasting damage, as the fuel drain and the flush, which still come first, take care of it.
Myths Downside drivers hear about petrol in a diesel
Myth: "Fill it the rest of the way with diesel and it will be fine." It will not. A bigger tank of the same mixture is all it gives, because topping up removes nothing. That guidance supposes a diesel fuel system tolerates petrol by percentage. It doesn't.
If anything, the reverse is true. Myth: "A current diesel can cope with a bit of petrol." Protection counts more on newer diesels, not less, because their fuel runs through finer parts at higher pressure.
"It starts, it drives, and so there is no harm" — the myth that catches more motorists than any other. Feed diesel to a petrol engine and it usually will not run, so the motorist stops — and the damage stops with them. Feed petrol to a diesel and it generally fires up, ticks over and drives away as if nothing happened — all the while its pump wears. Being able to run is not proof of being fine.
Myth: "Add some two-stroke oil and drive the car home." It is an old fix for a distinct problem. It does not put back what petrol has diluted, and that petrol is still inside the tank after that.
Myth: "Drain your tank yourself and you are sorted." Once your engine has been run, some of the petrol is no longer in your tank: it's in the filter and the lines, and it goes right back through the pump on the next start. And, a can of petrol filled at the roadside is truly risky.
Why petrol in a diesel takes place at all
On purpose? Never. Through carelessness? Seldom, if ever. The forecourt makes it easy: the petrol nozzle is the smaller of the two, and it slips into a diesel filler with almost no resistance, while a diesel nozzle generally will not go into a petrol vehicle at all. So the slip runs one way.
When habit and the vehicle stop matching, it happens. Any motor that's new to you — a hire car, on loan as a courtesy car, or just bought. Households running one of each, petrol and diesel. Your ordinary forecourt, its pumps suddenly in a different order. A hurried fill en route to Leagrave with a train to catch. Years of petrol motors behind you, a diesel Renault Megane in front of you at the forecourt for its first tank — that is when. Throw in rain, a phone call or a back seat full of children, and your hand repeats what it has always done.
Downside motorists: how to prevent petrol in a diesel next time
- A green handle for unleaded and black for diesel is normal in the UK — but not on every forecourt. Go by the label on the pump, not the colour.
- The first fill in an unfamiliar car is the time to check the filler flap — on it, or on the cap, plenty of vehicles state the fuel to use.
- One of each in the household? Mark your key to the diesel — a bright fob, a sticker — since meaning to remember is not enough.
- Say it before you lift it. On your first fill in a new or hired car, say "diesel" out loud before you lift the nozzle off the pump. It seems daft and it does the trick.
- Mistakes like this are straightforward to make whilst your mind is on something else, and so the phone goes away.
- A misfuel inhibitor helps, up to a point. Some diesels come with a filler designed to refuse a petrol nozzle, and add-on devices exist for cars without one. They are a safety net, not a reason to stop checking the pump.
- Stop the moment you notice. Less petrol in that tank makes a thinner mixture. It doesn't change what to do next, which is still not to fire it up.
Petrol already in a diesel around Downside? Start here
In that case, there is just a short order of things. Don't start the vehicle, and don't switch the ignition on. Still on the forecourt, at BP or anywhere else? Ask the attendants to help you push it away from the pump. Next, 0333 366 0103. We will ask what went in, roughly how much, and if it has been run.
If it was AdBlue and not petrol, say so. That is a different fault — corrosion and deposits rather than lost protection — and it has its own page for Downside.
One petrol in a diesel job in Downside costs more than another. Why?
An honest single quote does not exist here: four scenarios, four separate jobs. Chiefly, the cost moves with which situation you are in. Where your car is matters too, and whether our van can safely reach it, after the proportion of petrol to what your tank holds.
Before any quote, not after, come three queries: your fuel that got in, the amount, and whether it has run. That's why. A range becomes a straight figure once those answers are in.
About Downside, briefly
Downside is a suburban area in Bedfordshire, situated approximately 11 miles from St Albans and 30 miles northwest of London. Downside occupies a relatively high elevation of 165 metres, with nearby settlements including Stipers Hill, Churchend, and Dunstable. Downside has a population of around 4989 residents, predominantly living in terraced housing. Downside is characterised by its inland setting, typical of the Eastern region of England.
Fuel drain cover across Bedfordshire, Downside included
The vans answering petrol in diesel calls in Downside, in the LU postcode area, equally work the locations around it, Stipers Hill, Churchend and Dunstable among them. A forecourt, a supermarket car park, a lay-by or your own driveway are all locations we drain, and your car doesn't need to move to us.
Still at the pump in Downside?
The best place there is, then — that's where you are. Turned off it remains; then ring.