Johnstone: Petrol in a Diesel Car? How Worried to Be
Less than you probably are, provided it hasn't been started. Till your ignition primes the pump, your starter cranks, or your car is driven, petrol in a diesel tank simply sits there and does nothing. Which makes the truthful reply to that first Johnstone question — "have I wrecked it?" — normally no. Keep it that way: leave the engine off then ring Petrol in Diesel. Below, you will find what petrol actually does inside a diesel, how far it might have got, and how to stop it happening to you again.
Petrol in a diesel: what does it do to the engine?
Two jobs fall to diesel on its route round a motor. Before it combusts, it protects — and then it ignites. Your high-pressure pump and the fuel injectors are built to extremely fine clearances, and your diesel passing through them is what keeps those surfaces from grinding against each other.
Badly at the first, not at all at the second: that's petrol. A pump working on a petrol blend is wearing itself out, because petrol is runnier than diesel and takes away the protection your fuel system relies on. Petrol is also designed to resist igniting under compression — the opposite of what a diesel wants from its fuel — and that is where any knocking comes.
Put these together and the shape of the issue is plain. Not contact but circulation is what does the harm. All a tank with petrol in it amounts to is a fuel issue. Whatever a worn pump sheds travels on, carried by your fuel, into your injectors downstream, which is why a pump that has worked on that petrol is where a mechanical problem begins.
How serious is petrol in a diesel? Four situations our technicians see around Johnstone
Filled up, never started. The best case there is. Just the tank has petrol in it, so the job is clearing your tank and purging the system where your car sits. This situation can only truly take place at a forecourt: anywhere else, your car got there by being driven.
Started for a moment, or just the ignition on. Petrol has left your tank and reached the fuel filter and the lines, since your system has been primed. That is usually still an easy job — a drain, a flush, and the fuel filter dealt with.
Some miles driven, and your pump has spent each of them running on diluted fuel. The work is your tank, the lines, the filter and clean fuel through the entire circuit, and how hard the vehicle was driven counts as much as how far. On the school run to the local primary school, the mileage is small. A run as far as Glasgow, under 15 miles away, is a very different job. So is any distance on the M8, under 3 miles from Johnstone, where the engine has been labouring hard the whole time.
Clattering, a ragged idle, smoke, a throttle gone soft: driven till it complained. That is the engine telling you its fuel no longer is acting like diesel. Pull up wherever you are — the verge on High Street, a lay-by, a side street — then leave the vehicle where it is. A vehicle stopped quickly very rarely has lasting damage, as the drain and the flush, which still come first, sort it out.
Johnstone people and that petrol in a diesel stories they hear
Myth: "Fill it the rest of the way with diesel and it'll be fine." It will not. Nothing comes out when you top up — that tank just holds more of the same mixture. The idea behind that advice — that diesel fuel systems tolerate petrol by percentage — is wrong.
If anything, the reverse holds. Myth: "A current diesel can cope with a bit of petrol." Protection matters more on current diesels, not less, since their fuel runs through more precise parts at higher pressure.
This is the one that catches drivers. Myth: "If it starts and drives, there is no damage done." Any petrol engine fed diesel usually will not run, so the driver stops and the harm stops with them. A diesel fed petrol normally starts, idles and drives away as though nothing has happened, while the pump grinds. Being able to run isn't the same as being fine.
Myth: a two-stroke additive lets you to drive home. It is an old fix, supposed for another issue. That petrol is still in your tank afterwards, and whatever it has diluted is not put back.
"Drain it yourself and you are sorted" — a myth, because after it's been running some petrol lies in the fuel filter and lines, waiting to go right back through your pump at your next start. And, a can of petrol filled at the roadside is genuinely hazardous.
How does petrol in a diesel happen?
It's rarely carelessness, and nobody does it on purpose. The petrol nozzle is the smaller of the two and slips into a diesel filler with almost no resistance; a diesel nozzle generally will not go into a petrol car at all. The forecourt makes it easy, and the slip runs one way.
Habit and the car getting out of step is how it takes place. A new vehicle, a hire car or a courtesy car. One petrol and one diesel on the drive of the same household. A forecourt you know, but with its pumps laid out differently. A rushed fill en route to the nearest station with a train to catch. Precisely the moment it occurs: after years of driving petrol, a driver fills a diesel Volkswagen Golf for the first time. Throw in rain, a phone call or a back seat full of children, and the hand repeats what it has always done.
Avoiding petrol in a diesel next time you fill up around Johnstone
- Read the pump, not the colour. In the UK unleaded handles are usually green and diesel black, but not on every forecourt. The label is what counts.
- Plenty of cars state the fuel type inside your filler flap or on the cap, and so check there on your first fill in a vehicle you don't know.
- Good intentions count for less than a sticker or a fob in a bright colour on the diesel's key if a home has one of each. So label it.
- Before the nozzle leaves the pump the first time you fuel a hired or new vehicle, say "diesel" out loud. Silly, perhaps, yet it works.
- Put your phone away. This is an easy mistake to make whilst you're thinking about something else.
- 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 motors without one. They are a safety net, not a reason to stop checking the petrol pump.
- Less petrol means a weaker mixture, so stop as soon as you realise — though remembering that the next stage is unchanged. It's still not to fire it up.
Put petrol in a diesel around Johnstone already?
Short and simple, then, is the order of things. Don't start the car, and do not switch your ignition on. If you're still at the forecourt — Morrisons or anywhere else — ask the staff to help you roll it away from the petrol pump. What went in, about how much, and if it's run: tell us those when you phone 0333 366 0103.
Say so if it was AdBlue rather than petrol: it is a separate problem, with its own page for Johnstone, because AdBlue brings corrosion and deposits rather than lost lubrication.
Why one petrol in a diesel job in Johnstone costs more than another
A fair single price does not exist here: four scenarios, four separate tasks. What moves your cost is mostly which of them you are in. Where the car is counts too, and whether a technician can safely reach it, after the share of petrol to what that tank holds.
That is why we ask three questions before quoting instead of after: what went in, how much, and if it's run. The replies turn a range into a straight figure.
About Johnstone
Johnstone is a town situated in a riverside setting within Renfrewshire, in the western part of Scotland's central low-lying area. It is part of a larger historic county that shares its name. The town's location in this region provides a unique character. Johnstone is nestled in a scenic area near a river.
Fuel drain cover across Renfrewshire, Johnstone included
Quarrelton, Cartside and Howwood Road are among the places around Johnstone that the same vans work, answering petrol in diesel jobs across Johnstone's PA postcode area. No need to move your car to us. Forecourts, supermarket car parks, lay-bys and your own driveway are all spots we drain.
In Johnstone and still at the pump?
Then you're in the best place there is. Keep it turned off, then ring.