Should You Worry? Petrol in a Diesel Car in Cyntwell
Less than you probably are, if it hasn't been started. "Have I ruined it?" is the first thing anyone around Cyntwell asks, and the truthful answer is normally no, because petrol left in a diesel tank hurts nothing until it's drawn through the system — by ignition priming, by your starter, or by being driven. Leave the engine off then call Petrol in Diesel — that keeps it that way. The remainder of this page covers what petrol really does to a diesel, how to judge how far it has got, and how to make sure it does not happen to you again.
What petrol in a diesel does to your engine
Two tasks fall to diesel on its journey through a motor. It lubricates to begin with, and only then does it burn. Built to extremely fine tolerances, the injectors and the pump rely on your diesel flowing through them to keep their surfaces from wearing against one another.
Petrol handles the first job badly and the second not at all. It's runnier than diesel and removes the protection the system hinges on, 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 clattering comes from.
Put those together and the shape of the issue is clear. It's movement that does the damage, not contact. Where that petrol is only in your tank, it is a problem of fuel, nothing more. The start of a mechanical problem is a pump that has been working on that petrol — and whatever a worn pump sheds travels on with the fuel to the injectors beyond it.
How bad is petrol in a diesel? Four situations our engineers see in Cyntwell
Fuelled, and not started: no case is better. That petrol is inside your tank and only your tank, and the job is a drain and a flush where the vehicle sits. This is the one situation that can only truly occur on a forecourt, since anywhere else the vehicle has been driven to get there.
The ignition turned on, or a short start. Petrol has left that tank and reached the filter and your lines, since the fuel system has been primed. Normally nothing more than emptying, purging and sorting your filter, so still easy.
Your pump now spent a couple of miles on diluted fuel — that is what driving it meant. Miles is only half of that; how hard it was driven matters just as much. Either way the work includes that tank, your lines, the filter and fresh fuel through the whole system. Short trips, like the school run to the local primary school, are one matter. Very different once more: a run to Cardiff, under 5 miles away, or some mileage on the M4, under 5 miles from Cyntwell, where your engine kept labouring hard.
It complained, and only then stopped: smoke, clattering, a throttle gone soft, an idle gone ragged. The fuel isn't acting like diesel any more, and the engine is saying so. Leave it where it stops: a lay-by, a side street, the verge on Clos Tan y Ffarm. Long-term damage is very uncommon if a car is pulled over soon, and the fuel drain and flush-through that still go first deal with it.
What Cyntwell motorists get told about petrol in a diesel — and the stories in it
Myth: topping up to full with diesel makes it fine. It does not. A fuller tank of the same mixture is all it gives, because topping up takes out nothing. That guidance presumes a diesel fuel system handles petrol by percentage. It doesn't.
If anything, the opposite holds. Myth: "A newer diesel can cope with a bit of petrol." Smaller components and higher pressure are how a newer diesel runs its fuel, and that makes the lubrication matter more, not less.
This is the one that catches people. Myth: "If it starts and drives, there is no damage done." A petrol motor fed diesel generally won't run, so the driver stops and the harm stops with them. A diesel fed petrol normally starts, ticks over and drives away as though nothing has happened, whilst your pump wears. Fine and running are not the same at all.
An old workaround for a separate problem is behind this one. Myth: "Add some two-stroke oil and drive the vehicle home." The petrol is still in the tank after that, and what it has weakened is not put back.
"Drain it yourself and you are sorted" — a myth, since after it has been running some petrol sits in the fuel filter and lines, ready to go straight back through the pump on your next start. It's also really hazardous to drain petrol into a can at the roadside.
Why petrol in a diesel happens so easily
Nobody does it on purpose, and it is seldom carelessness. The forecourt makes it simple: the petrol nozzle is the narrower of the two, and it slides into a diesel filler with almost no resistance, while a diesel nozzle mostly won't go into a petrol motor at all. So the mistake runs one way.
Routine and the car falling out of step is how it occurs. A new vehicle, a hire car or a courtesy car. One petrol and one diesel on the driveway of the same household. A familiar forecourt with the pumps in a different order. A hurried fill on the way to Waun-Gron Park with a train to catch. A new-to-you diesel Volkswagen Tiguan, its first fuelling, and a motorist accustomed to petrol: precisely when it happens. Throw in rain, a phone call or a back seat full of children, and your hand repeats what it has always done.
Avoiding petrol in a diesel next time you fill up in Cyntwell
- Not every forecourt in the UK follows green for unleaded and black for diesel, even if most usually do. Read the pump's label; the colour is only a clue.
- In a car you don't know, look inside the flap over the filler or at the cap before the first fill: plenty of motors print the fuel there.
- One of each in the household? Mark your key to the diesel — a bright fob, a sticker — because meaning to remember is not enough.
- Before the nozzle comes off the pump the first time you fill a hired or new vehicle, say "diesel" out loud. Silly, yes, yet it does the trick.
- Tuck your phone away. This is an easy mistake to make while you are thinking about something else.
- Add-on devices exist for cars without one, and some diesels come with a filler that won't take a petrol nozzle. Helpful — but a safety net only, never a reason to stop checking the label.
- Less petrol means a weaker mixture, and so stop as soon as you realise — though remembering that your next stage is unchanged. It's still not to fire it up.
Already put petrol in a diesel around Cyntwell?
Short and simple, then, is the order of things. Do not switch the ignition on, and don't start it. The staff on the forecourt — whether at Tesco itself or wherever you filled up — will normally help push it clear of the petrol pump if you ask. Then ring 0333 366 0103 and tell us what was put in, about how much, and if it's run.
If it was AdBlue instead of petrol, say so. That is a different problem — corrosion and deposits rather than lost lubrication — and it has its own page for Cyntwell.
Why one petrol in a diesel job in Cyntwell costs more than another
The four scenarios above are four separate tasks, so no single price for this would be honest. Mostly, the cost moves with which situation you are in. After that it's the ratio — how much petrol got in against how much the tank holds — and then where the vehicle is and whether our van can reach it safely.
Before any quote, not after, come three queries: the fuel that got in, its amount, and whether it's run. That is why. Their replies are what turn a range into one firm number.
About Cyntwell itself
Cyntwell is a locality situated inland in Cardiff, approximately 3.7 miles from the city centre, with an elevation of around 51 metres. Cyntwell is characterised by its predominantly semi-detached housing stock, reflecting the residential development of the area. Cyntwell is close to nearby places such as Culverhouse Cross, Michaelston-super-Ely, and Caerau, and is home to a population of around 3788 people.
Fuel drain cover in Cyntwell and across Cardiff
Petrol in diesel jobs in Cyntwell — part of the CF postcode area — go to vans that likewise cover the locations around it, Culverhouse Cross, Michaelston-super-Ely and Caerau among them. A forecourt, a supermarket car park, a lay-by or your own driveway are all spots our engineers drain, and your vehicle doesn't need to come to us.
In Cyntwell and still at the pump?
The best place there is, then — that is where you are. Keep it off. Ring.