In the UK, something like 150,000 people a year put the wrong fuel in their cars. That sounds high – until you realise how easy it is to do. And, if there was a show of hands among us, we’d certainly have a couple pop up around here, too…
We’ve borrowed and hired cars, campers, and all sorts in our time. And we’ve definitely made the mistake! But how bad is putting the wrong fuel in your car, and what should you do about it? It all depends on which way around you’ve misfuelled, and how much fuel you’ve put in.
ACCIDENTALLY PUT PETROL IN A DIESEL CAR
There’s this weird myth that it’s worse to put petrol in a diesel car – but in our experience, it’s the other way around (unless you absolutely fill the tank from empty). It’s still not at all good for the car either way you do it, but diesel engines just tend to be hardier (to a point).
This is the reason why a petrol pump nozzle will fit into a diesel fuel filler port, but not the other way around.
Diesel and petrol will mix, and petrol acts as a solvent. If there’s still enough diesel in the mix, it can still ignite under pressure and the engine will run. It won’t run well, but it will run. But if you stop it… It might not start again. DISCLAIMER – there is of course a risk of damage here, and the official advice is to not run the engine, roll the car to a safe area in neutral, and get rescued by your roadside assistance cover.
But here’s the stuff nobody talks about – a small amount of petrol in a diesel engine is practically speaking, a non-issue.
The petrol will dissolve into the diesel, and mix into it. If you spot the mistake quickly, keep the engine running, and fill to the brim with more diesel, you should be able to sort it out with minimal issues. It’s not perfect, but nine times out of ten, a little bit of petrol in the diesel is not a monumental cock up. Just keep driving it and filling it with diesel until it goes smooth again, using ‘pay at the pump’ with the engine running.
The other thing is, you are very likely to notice the mistake almost immediately, because the petrol will be splashing out over your hands as you fill up. The nozzle is narrower, rattles about in the fuel port, and lets fuel out of the sides! Petrol also has a distinct smell, and the nozzles are bright green. You have so many more signs that you’re doing it wrong this way than the other, that it’s fairly hard to get to the point where you’ve ruined it.
All that said, a full-to-the-brim tank of petrol in a diesel car is a pretty big problem.
At a certain point, the engine can’t ignite the petrol; petrol engines use spark plugs and diesels use injection pressure. Whatever diesel’s left in the lines will start the motor, inject petrol into the system, and seize it up. And you’ll need a mechanic in a garage setting to drain it out, flush it, replace everything (including the injector) and start again. That process isn’t particularly cheap.
But this is a worst case scenario, and while we agree with all the other advice out there that it’s not a good thing, we’re going out on a limb to say that it’s less likely to ruin your week for all the reasons above – and if anything, getting it wrong the other way around is worse.
But it’s also a LOT harder to do.
ACCIDENTALLY PUT DIESEL IN A PETROL CAR
So unlike the situation above, a petrol engine will run on a full tank of diesel. It will be loud, it will knock and wobble, it will smoke to high hell – but it will run. Extremely poorly, may we add.
And this is a danger, because the engine will become more and more damaged with every revolution as it goes out of time and vibrates out of expected parameters. Even a small amount of diesel dissolved into petrol can cause issues, with an effect similar to burning oil in the engine but about ten times worse.
You can try syphoning the tank and cramming in petrol, but this is almost certainly a roadside rescue and garage job. Avoid starting the engine, get somewhere safe, and get the AA, RAC, or Green Flag on the case.
However – you’ll have a hard job getting the diesel in the tank in the first place.
That’s because the nozzle for diesel is bigger than a petrol one. It physically won’t fit in the fuel hole. So, if you manage to actually get it in at the pump, we’d be impressed!
The only real situation you might encounter an opportunity to misfuel is with a jerry can or emergency refuel, where you put the wrong fuel in your car because you don’t have the additional safety nets of different colour or nozzle size.
This is the main reason we don’t see diesel in a petrol car as big of a threat – it’s just so much harder to get it wrong this way around.
But, the fact remains that neither is a good thing for your car, and ideally, you need to get to a local garage as soon as possible after a misfuel. Master Tech Autos is a garage in Eastleigh that can help you out if you’re in the local area. If you’ve put the wrong fuel in your car, give us a call on 023 8061 1161 or contact us for help!
