Thatcham: Petrol in a Diesel Car? How Worried to Be
Likely less than you are now — as long as no one has started the vehicle. Until your ignition primes the pump, your starter cranks, or your vehicle is driven, petrol in a diesel tank simply waits there and does no harm. Which makes the truthful reply to that first Thatcham question — "have I ruined it?" — usually no. Leave the engine off and call Petrol in Diesel — that is how you keep it like that. What petrol actually does to a diesel, how to gauge how far it has travelled, and how to keep it from happening a second time: that's the remainder of this page.
Inside the motor: what petrol in a diesel really does
Passing through an engine, diesel does two things at the same time. Before it ignites, it protects — then it ignites. Designed to very fine clearances, the fuel injectors and the pump rely on the diesel flowing through them to stop their surfaces wearing against one another.
Of these two tasks, petrol manages the first poorly and the second not at all. The knocking comes from petrol being made not to ignite when compressed, which is exactly the opposite of what a diesel needs. On top of that it is thinner than diesel, it strips out the protection the fuel system depends on, and any pump running on that mix is wearing itself.
Put these together and the shape of the problem is plain. Contact does not do the harm — circulation does. A tank with petrol in it is a fuel problem. After the pump has been labouring on that petrol, a mechanical problem has started: whatever is shed by a pump that is wearing travels downstream to the injectors with the fuel.
Four situations our technicians see in Thatcham: how bad is petrol in a diesel?
Fuelled up, never started. The best case there can be. At a forecourt, and truly only there, that petrol is still limited to your tank — because everywhere else the vehicle was driven to get to it. The job is emptying and purging on the spot.
Ignition on, or started for a moment. With the system primed, your lines and filter now contain petrol that came from the tank. Still usually simple: drain, flush, and the fuel filter dealt with.
The pump now worked a few miles on weakened fuel — that's what driving it meant. How hard the vehicle was driven counts as much as how far, and this work covers that tank, your lines, your filter and fresh fuel through the entire system. Short trips, such as the school run to Thatcham Park CofE Primary, are one thing. A run as far as Winchester, under 25 miles away, is a very different task. So is any mileage on the M4, under 5 miles from Thatcham, where the engine has been working hard throughout.
Rattling, an uneven idle, smoke, a throttle gone soft: driven till it complained. The engine is telling you the fuel is no longer behaving like diesel. Whether it is the verge on The Broadway, a lay-by or a side street, stop there and leave it. A vehicle stopped quickly very seldom has lasting damage, because the drain and the flush, which still go first, take care of it.
Myths Thatcham people hear about petrol in a diesel
Myth: "Fill it the rest of the way with diesel and it'll be fine." It will not. Topping up takes out nothing; it gives you a bigger tank of the same mixture. Percentage isn't how a diesel fuel system tolerates petrol, however much that advice presumes.
"Current diesels can handle a little petrol" — a myth, and nearer the opposite. Lubrication matters more on modern diesels, not less, because their fuel passes through smaller components at higher pressure.
Myth: "If it starts and drives, there is no damage done." This is the one that fools drivers. Diesel in a petrol motor usually means a motor that will not run: the motorist stops, and so does the damage. Feed petrol to a diesel and it generally fires up, ticks over and drives away as if nothing has happened — all while its pump is wearing. Running isn't the same as fine.
"A drop of two-stroke oil and drive it home" — a myth, borrowed from a fix from years ago, for a different fault. It does not put back what petrol has weakened, and the petrol is still inside the tank later.
The next start draws petrol back round the pump from your lines and your filter, where some of it sits once the engine has been run. That is why draining the tank yourself doesn't leave you sorted — a myth. Emptying petrol into a can by the roadside is also genuinely risky.
Petrol in a diesel: how does it happen?
Carelessness is rarely the cause, and no one means to do it. The petrol nozzle is the narrower of the two and slides into a diesel filler with almost no resistance; a diesel nozzle mostly will not go into a petrol vehicle at all. The forecourt makes it easy, and the slip runs one way.
Routine and the car getting out of step is when it takes place. Hire cars, courtesy cars, and cars that are new to you. A household with one petrol and one diesel on the drive. A forecourt you know, only with its pumps laid out differently. A train to catch, and a rushed fill on the way to the nearest station. Years of petrol vehicles behind you, a diesel Volkswagen Passat in front of you at the forecourt for its first tank — that's when. Add rain, a phone call or a back seat full of children, and your hand does what it has always done.
Avoiding petrol in a diesel next time you fill up in Thatcham
- Not every forecourt in the UK keeps to green for unleaded and black for diesel, even if most generally do. Read the pump's label; the colour is just a clue.
- In a motor you do not know, look inside the flap over your filler or at the cap before the first fill: plenty of vehicles print the fuel there.
- Good intentions do less than a sticker or a fob in a vivid colour on your diesel's key if a household has one of each. So label it.
- Before the nozzle leaves the petrol pump the first time you fill a hired or new motor, say "diesel" out loud. Daft, yes, yet it does the trick.
- Whilst you're thinking about something else, this is an easy mistake to make — so put the phone away.
- Add-on devices are available for cars without one, and some diesels are built with a filler that won't accept a petrol nozzle. Helpful — but a safety net at best, never a reason to stop checking the label.
- Less petrol means a thinner mixture, and so stop as soon as you realise — though remembering that your next stage is unchanged. It is still not to start the car.
Petrol already in a diesel in Thatcham? Begin here
The order of things is brief, then. Starting it is out, and so is turning on the ignition. If you're still at the forecourt — Esso or anywhere else — ask the staff to help you push it away from the pump. What went in, about how much, and whether it has run: tell us those when you phone 0333 366 0103.
AdBlue, not petrol? Then say so. Corrosion and deposits, not lost protection, make it a different issue — Thatcham has a page just for it.
What makes one Thatcham petrol in a diesel job cost more than the next
Four separate jobs lie behind the four scenarios above — which is why there's no fair single price. Chiefly, the cost moves with which situation you are in. Next it's the share — how much petrol went in against how much that tank holds — and after that where your car is and whether a technician can get to it safely.
What was put in, how much, and if it's run — three questions we ask before costing, not after, for exactly that reason. Their answers are what change a range into one firm number.
About Thatcham
Thatcham is a town in Berkshire, situated inland at an elevation of 79 metres, approximately 49 miles west of London. Thatcham lies near the towns and villages of Colthrop, Cold Ash, and Mount Pleasant, and is around 24 miles east of Winchester. Thatcham has a population of around 23,612 people, predominantly living in semi-detached housing. Thatcham is a significant urban centre in the area, with a range of local amenities and services.
Thatcham and Berkshire: where our fuel drain cover runs
Petrol in diesel call-outs around Thatcham — part of the RG postcode area — go to teams that likewise cover the locations about it, Colthrop, Cold Ash and Mount Pleasant among them. A forecourt, a supermarket car park, a lay-by or your own driveway are all places our technicians work, and your car doesn't need to move to us.
Still at the pump in Thatcham?
Then you're in the best spot there can be. Turned off it stays; then phone.