How Worried Should You Be? Petrol in a Diesel Car in Trethomas
Far less than you probably are, provided it hasn't been started. "Have I wrecked it?" is the first thing anyone around Trethomas asks, and the honest answer is normally no, since petrol left in a diesel tank harms nothing until it is drawn through your fuel system — by ignition priming, by the starter, or by being driven. To leave the vehicle that way, call Petrol in Diesel then leave your motor alone. What petrol really does to a diesel, how to judge how far it has gone, and how to keep it from happening again: that's the remainder of this page.
The motor and petrol in a diesel: what happens
Diesel does two tasks on its way through a motor. Before it burns, it protects — then it combusts. What stops the surfaces inside your high-pressure pump and your injectors from wearing down against one another is your diesel passing through them — they are built to extremely fine gaps.
Petrol does the first job poorly and the second not at all. Being less viscous than diesel, petrol strips out the protection a fuel system needs, leaving a pump on that mixture to wear itself. It's also made to resist igniting under compression — the reverse of what a diesel needs — which is why you hear the clattering.
Put those together and the shape of the trouble is plain. Not contact but movement is what causes the harm. Where that petrol is only inside that tank, it is an issue of fuel, nothing more. Whatever a worn pump sheds goes on, borne by your fuel, into the injectors downstream, which is why a pump that has worked on that petrol is where a mechanical fault begins.
Four scenarios we see around Trethomas: how bad is petrol in a diesel?
Filled but not once started — the best it gets. Only the tank has petrol in it, so the job is clearing that tank and rinsing your system where your car sits. This scenario can only really happen on a forecourt: anywhere else, your car got there by being driven.
Your ignition switched on, or a short start. Petrol has come out of that tank and reached the filter and the lines, because the fuel system has been bled. Still normally easy: drain, flush, and the fuel filter dealt with.
Those few miles driven were the pump running on fuel thinned by petrol throughout. How hard your car was driven matters as much as how far, and this work takes in the tank, your lines, the filter and fresh fuel through the whole circuit. A school run to the local primary school is one thing. Very different a second time: a run to Cardiff, under 10 miles away, or some distance on the M4, under 5 miles from Trethomas, where the engine kept running hard.
It complained, and only then stopped: smoke, clattering, a throttle gone soft, an idle gone uneven. The engine is telling you the fuel is no longer acting like diesel. Whether it is the verge on Navigation Street, a lay-by or a side street, stop there and leave it. The drain and flush still go first, and they sort it out: when a car is pulled over early, long-term damage is very rare.
Trethomas motorists and that petrol in a diesel stories they hear
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. The assumption behind that advice — that diesel fuel systems accept petrol by percentage — is wrong.
"Modern diesels can handle a little petrol" — a myth, and nearer the opposite. Lubrication counts more on modern diesels, not less, because their fuel runs through finer components at higher pressure.
This is the one that catches people. Myth: "If it starts and drives, there's no harm done." Any petrol engine fed diesel normally won't run, so the motorist stops and the harm stops with them. While the pump wears, a diesel fed petrol normally starts, ticks over and drives away as though nothing has happened. Running is not the same as fine.
Myth: "Add some two-stroke oil and drive the vehicle home." It is an old workaround for a different fault. What petrol has thinned stays thinned, and that petrol remains inside that tank anyway.
If the engine has run, part of the petrol sits in your filter and the fuel lines, not your tank — and the next start pulls it right back through the pump. So the myth "drain that tank yourself and you are sorted" doesn't stand up. It's also really hazardous to drain petrol into a can at the roadside.
How does petrol in a diesel happen?
A careless moment is rarely the cause, and nobody means to do it. The forecourt makes it easy: the petrol nozzle is the narrower of the two, and it slips into a diesel filler with almost no resistance, while a diesel nozzle mostly will not go into a petrol car at all. So the slip runs one way.
Routine and your car getting out of step is how it takes place. A new car, a hire car or a courtesy car. One petrol and one diesel on the drive of the same household. The pumps in a different order on a well-known forecourt. Fuelling up in a hurry en route to Caerphilly, with a train to catch. The first fill-up in a diesel Renault Megane after years of driving petrol is exactly the moment it happens. Throw in rain, a phone call or a back seat full of children, and your hand repeats what it has always done.
Next time you fill up in Trethomas: avoiding petrol in a diesel
- Read the pump, not the colour. In the UK unleaded handles are normally green and diesel black, but not on every forecourt. The label is what counts.
- Plenty of vehicles state the fuel type inside the filler flap or on the cap, and so check there on your first fill in any vehicle you don't know.
- One of each in the household? Mark your key to your diesel — a coloured fob, a sticker — because 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 take the nozzle off the petrol pump. It seems silly and it does the trick.
- Thinking about something else is when this becomes an easy mistake to make, and so put the phone away and keep your mind on the pump.
- A misfuel inhibitor helps, up to a point. Some diesels have a filler designed to block a petrol nozzle, and add-on devices exist for vehicles without one. They are a backstop, not a reason to stop reading the pump.
- As soon as you realise, stop: less petrol inside that tank makes a thinner mixture. What to do next does not change, though — still do not fire it up.
Already put petrol in a diesel around Trethomas?
Then, there is just a short order of things. Do not switch the ignition on, and don't fire it up. If you are still on the forecourt — Esso or anywhere else — ask the attendants to help you push the vehicle clear of the petrol pump. Next, 0333 366 0103. We'll ask what was put in, about how much, and if it's been run.
Corrosion and deposits are the problem with AdBlue, where petrol means lost protection. So if it was AdBlue, say so — and see its own Trethomas page.
What makes one Trethomas petrol in a diesel job cost more than the next
The four situations above are four separate tasks, so no single price for this would be fair. Which of them you are in is mostly what moves your cost. Two things follow that: the petrol that got in as a proportion of what the tank holds, then the place your car is in and whether our van can reach it safely.
Before any quote, not after, we ask three queries: your fuel that went in, the volume, and whether it's run. That is why. A range becomes a straight figure when the answers are in.
About Trethomas, briefly
Trethomas is a village in Caerphilly, situated on the banks of the Rhymney River at an elevation of 62 metres. Trethomas lies approximately 7.7 miles north of Cardiff and is surrounded by nearby places such as Bedwas and Craig-y-Rhacca. Trethomas has a mix of housing types, but is predominantly characterized by terraced homes. Trethomas is located within commuting distance of Cardiff, making it a residential area for people working in the city.
Fuel drain cover across Caerphilly, Trethomas included
Our vans answering petrol in diesel call-outs around Trethomas, in the CF postcode area, equally work the spots around it, Bedwas, Mornington Meadows and Craig-y-Rhacca among them. No need to move the vehicle to us. Forecourts, supermarket car parks, lay-bys and your own drive are all locations where we work.
At the pump around Trethomas, right now?
No position is better: that is where you now are. Ring, and keep it switched off.