Crossroads: Petrol in a Diesel Car? How Worried to Be
Provided it has not been started, you can worry less than you likely are. Everyone around Crossroads starts with "have I ruined it?", and the fair answer is usually no: petrol left inside that tank of a diesel stays harmless till priming, cranking or driving pulls it into the system. Petrol in Diesel is who to ring, with the engine left off, and so it remains that way. The remainder of this page sets out 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 the engine
Two tasks fall to diesel on its journey through a motor. Before it ignites, it protects — then it combusts. Extremely fine tolerances are built into the fuel injectors and your high-pressure pump, and only the diesel running through keeps their surfaces from grinding together.
Petrol does the first task badly and the second not at all. A pump working on a petrol mixture is wearing itself out, because petrol is runnier than diesel and strips out the protection the fuel system depends on. Petrol is also designed to resist igniting under compression — the opposite of what a diesel wants from its fuel — and that is where the clattering starts.
Combined, those leave the shape of the issue plain. Not contact but movement is what causes the damage. A tank with petrol in it is a fuel problem. Whatever a worn pump sheds travels on, borne by your fuel, into your injectors further along, which is why a pump that has worked on that petrol is where a mechanical problem begins.
How bad is petrol in a diesel? Four scenarios our engineers see in Crossroads
Fuelled up, never started. The best case there is. Just that tank has petrol in it, so the job is emptying the tank and cleaning the system where your car stands. This situation can only truly take place on a forecourt: anywhere else, the car got there by being driven.
The ignition turned on, or a brief start. With the fuel system primed, the lines and filter now hold petrol that has come from your tank. Still generally simple: drain, flush, and the fuel filter seen to.
A couple of miles driven, and your pump has spent every one of them labouring on diluted fuel. Distance is only half of that; how hard it was driven counts just as much. Either way this work runs to your tank, your lines, the filter and fresh fuel through the whole system. Short trips, such as the school run to the local primary school, are one thing. Very different again: a trip to Glasgow, under 25 miles away, or any miles on the M77, under 10 miles from Crossroads, where the engine never stopped working hard.
Rattling, a lumpy idle, smoke, a throttle gone soft: driven till it complained. That's the engine telling you its fuel has stopped acting like diesel. Whether it's the verge on A719, a lay-by or a side road, stop where you are then leave it. The drain and flush-through still go first, and they deal with it: when a car is stopped soon, long-term damage is very uncommon.
Petrol in a diesel myths Crossroads people hear
Myth: "Fill it the rest of the way with diesel and it'll be fine." It will not. A fuller tank of the same mix is all you get, since topping up removes nothing. That advice assumes a diesel fuel system handles petrol by percentage. It doesn't.
"Modern diesels can handle a little petrol" — a myth, and closer to the reverse. A newer diesel runs its fuel through more precise components at higher pressure, which makes the lubrication matter more, not less.
Myth: "If it starts and drives, there's no damage done." This is the one that catches motorists. The driver of a petrol motor fed diesel normally has no choice but to stop, since it will not run, and the harm stops there. Nothing looks to have happened when a diesel fed petrol fires up, ticks over and drives away, as it usually will; meanwhile your pump grinds. Fine and running aren't the same thing.
Myth: "Add some two-stroke oil and drive it home." It's an outdated workaround for a different problem. None of what petrol weakened is put back by it — and later, your tank still holds that petrol.
The next start draws petrol back round the pump out of the lines and the fuel filter, where some of it sits once your engine has been run. That's why a DIY tank drain does not sort it — a myth. And, a can of petrol drained by the roadside is truly dangerous.
How it happens: petrol in a diesel
It's rarely carelessness, and no one does it on purpose. 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 slip runs one way.
Habit and the vehicle falling out of step is when it happens. Any car that's new to you — hired, on loan as a courtesy car, or just bought. A household with one petrol and one diesel on the drive. A well-known forecourt with the pumps in a different order. Rushing for a train outside Kilmarnock and fuelling up en route. A new-to-you diesel Volkswagen Golf, its first fuelling, and a motorist used to petrol: exactly when it occurs. Add rain, a phone call or a back seat full of kids, and your hand does what it has always done.
Crossroads motorists: how to prevent petrol in a diesel next time
- Not every forecourt in the UK follows green for unleaded and black for diesel, though most generally do. Check the petrol pump's label; the colour is only a hint.
- Plenty of cars state the fuel inside the filler flap or on the cap, so check there on the first fill in a car you don't know.
- Label your key. In a house 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", out loud, before you lift the nozzle — every time a new or hired car gets its first tank. It seems daft. It works.
- Thinking about something else is when this becomes an easy mistake to make, and so put the phone away then keep your mind on the pump.
- Some diesels have a filler designed to reject a petrol nozzle, and devices you can add are available for motors without one. Such an inhibitor helps up to a point — as a safety net, not a reason to stop checking the petrol pump.
- The moment you notice, stop: less petrol in that tank makes a thinner mix. What to do next does not change, though — still don't start it.
Petrol already in a diesel in Crossroads? Begin here
Then the order of things is short. Don't fire it up, and do not switch the ignition on. If you are still on the forecourt — the nearest petrol station or anywhere else — ask the attendants to help you push it away from the pump. Next, 0333 366 0103. We'll ask what went in, about how much, and if it's been run.
If it was AdBlue rather than petrol, say so. That's a different fault — corrosion and deposits and not lost lubrication — and it has its own page for Crossroads.
What makes one Crossroads petrol in a diesel job cost more than the next
An honest single quote does not exist here: four scenarios, four separate tasks. What shifts your cost is chiefly which of them you are in. Two things follow that: the petrol that was put in as a proportion of what your tank holds, then the spot the car is in and whether a technician can get to it safely.
Three questions come first, before quoting, rather than after — what was put in, how much, has it run — and this is the reason. The answers turn a range into a straight figure.
About Crossroads, for a moment
Crossroads is a small hamlet situated on the riverside in East Ayrshire, with a relatively low elevation of 57 metres. Crossroads is located near the communities of Hurlford, Bellfield, and Crookedholm, and is approximately 21 miles from Glasgow. Crossroads has a population of around 535 people, and is characterised by predominantly detached housing. Crossroads is also a significant distance from London, at around 332 miles away.
Crossroads and East Ayrshire: where our fuel drain cover reaches
The teams answering petrol in diesel calls around Crossroads, in the KA postcode area, equally work the spots around it, Hurlford, Bellfield and Crookedholm among them. A forecourt, a supermarket car park, a lay-by or your own drive are all places we work, and the vehicle does not need to come to us.
Still at the petrol pump in Crossroads?
The best position there is, then — that is where you are. Keep it switched off, then ring.