Stibb: Petrol in a Diesel Car? How Worried to Be
Less than you probably are, provided it hasn't been started. "Have I ruined it?" is the first question anyone in Stibb asks, and the honest answer is usually no, because petrol left in a diesel tank harms nothing till it is dragged through the system — by ignition priming, by the starter, or by driving. Keep it that way: leave the motor off then ring Petrol in Diesel. The remainder of this page covers what petrol really does to a diesel, how to tell how far it has got, and how to make sure it does not happen to you a second time.
Petrol in a diesel: what does it do to the engine?
On its way through an engine, diesel has two jobs. It lubricates first, and after that does it burn. The high-pressure pump and the fuel injectors are made to very fine tolerances, and the diesel passing through them is what stops those surfaces from grinding against each other.
Of these two tasks, petrol handles the first badly and the second never. Being less viscous than diesel, petrol strips out the lubrication a fuel system needs, leaving a pump running 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 get the knocking.
Put these together and the shape of the problem is clear. Not contact but movement is what causes the harm. Petrol in the tank is a fuel problem. The start of a mechanical one is a pump that has been labouring on that petrol — and whatever a worn pump sheds travels on with your fuel to your injectors beyond it.
Four scenarios our technicians see in Stibb: how bad is petrol in a diesel?
Filled but not once started — as good as it gets. Being on a forecourt is what makes this the case: elsewhere, the vehicle has been driven there to begin with. With that petrol kept inside the tank alone, it's emptied and cleaned on the spot, and nothing more is needed.
Started for a second, or only the ignition on. Once primed, the system has drawn petrol from that tank up to your filter and the lines. Still normally easy: drain, flush, and the filter sorted.
Driven a few miles. Now the pump has spent those miles labouring on thinned fuel. How hard the car was driven counts as much as how far, and this work takes in that tank, the fuel lines, your filter and fresh fuel through the whole system. Driving the children to Kilkhampton Junior and Infant School is one sort of trip. Any distance on the M5, under 50 miles from Stibb, means the motor working hard throughout, and a run to Plymouth, under 50 miles away, is a very different task too.
Driven on till it complained — knocking, smoke, a ragged idle, a dead throttle. That is your engine telling you its fuel has stopped acting like diesel. Stop on the spot — on the verge on Stibb Levels, at the kerb, down a side road — and don't move it again. What goes first is still the drain and flush-through, and that takes care of it, with permanent damage very uncommon if a car is stopped soon.
Stibb drivers and the petrol in a diesel myths they hear
"Top it up the rest of the way with diesel and it will be fine" — a myth. It will not. Nothing is removed by topping up — you are left with a fuller tank, still the same mix. That guidance assumes a diesel fuel system handles petrol by percentage. It does not.
If anything, the opposite holds. Myth: "A newer diesel can cope with a bit of petrol." A modern diesel runs its fuel through smaller components at higher pressure, which makes the lubrication count more, not less.
One myth that catches motorists out: if it starts and drives, it must be unharmed. The driver of a petrol engine fed diesel usually has no choice but to stop, because it will not run, and the harm stops with them. Whilst the pump grinds, a diesel fed petrol usually starts, idles and drives away as if nothing has happened. Running is not 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 distinct problem. None of what petrol weakened gets put back by it — and afterwards, the tank still holds that petrol.
"Drain it yourself and you are sorted" — a myth, since after it has been running some petrol lies in the filter and lines, ready to go right back through your pump at your next start. Emptying petrol into a can at the kerbside is also truly hazardous.
How petrol ends up: petrol in a diesel
It's seldom carelessness, and no one does it on purpose. One way only, because of the nozzles: the smaller petrol one slips inside a diesel filler with almost no resistance, whereas the diesel one generally won't fit a petrol motor. That's how the forecourt makes it simple.
When habit and the car stop matching, it happens. Courtesy car, hire car, or a motor that is new. Two vehicles on one drive, one petrol, one diesel. A forecourt you know, but with its pumps arranged differently. A train to catch, and a hurried fill en route to the nearest station. The first fill-up in a diesel BMW 3 Series after years of driving petrol is precisely the moment it takes place. The hand does what it has always done once you add rain, a phone call or a back seat full of children.
How to avoid petrol in a diesel on your next fill-up in Stibb
- The label is what matters, and so read the petrol pump rather than the colour: in the UK unleaded handles are usually green and diesel black, but not on every forecourt.
- The first fill in a new motor is the time to check your filler flap — inside it, or on the cap, plenty of cars state which fuel they take.
- A coloured fob or a sticker on your diesel's key achieves more than any amount of meaning to remember, for a home with one of each.
- It sounds daft and it works: on your first fill in a new or hired car, say "diesel" aloud before you take the nozzle off the pump.
- Whilst you are thinking about something else, this is an easy mistake to make — so put your phone away.
- Up to a point, a misfuel inhibitor helps: some diesels have a filler designed to refuse a petrol nozzle, and add-on devices are available for the remainder. Count it as a backstop, and keep checking the petrol pump.
- As soon as you notice, stop: less petrol in the tank means a weaker mixture. What to do next doesn't change, though — still do not start it.
Petrol already in a diesel in Stibb? Start here
Then the order of things is short. Don't crank it, and don't turn the ignition on. The attendants on the forecourt — whether at the nearest petrol station itself or wherever you filled up — will usually help roll it clear of the petrol pump if you ask. What went in, roughly how much, and whether it has run: tell us that when you call 0333 366 0103.
AdBlue, not petrol? Then tell us. Corrosion and deposits, not lost protection, make it a separate problem — Stibb has a page all for it.
What makes one Stibb petrol in a diesel task cost more than the next
There's no fair single price for this, since the four situations above are four different tasks. Which of them you're in is mostly what shifts your cost. Two things come after that: the petrol that got in as a share of what your tank holds, then the spot your car is in and whether our van can reach it safely.
That's why our engineers ask three questions before pricing rather than after: what got in, how much, and if it's run. With the answers, you get a straight figure rather than a range.
About Stibb
Stibb is a small coastal hamlet in Cornwall, situated near the villages of Collery, Forda, and Coombe. Stibb sits at an elevation of 155 metres, overlooking the surrounding countryside. Stibb is characterized by its predominantly detached housing stock, reflecting its rural setting. Stibb is located approximately 38 miles from Plymouth, the nearest city.
Across Cornwall and in Stibb: fuel drain cover
Our vans answering petrol in diesel calls in Stibb, in the EX postcode area, also work the locations around it, Collery, Forda and Coombe included. We drain where your vehicle is — your own driveway, a lay-by, a supermarket car park, a forecourt — so it does not have to come to us.
Still at the pump in Stibb?
Then you are in the best spot there is. Keep it off. Call.